THE OPEN UNIVERSITY OF TANZANIA
FACULTY OF SCIENCE, TECHNOLOGY AND ENVIRONMENTAL STUDIES
OEV 205 (3 UNIT COURSE)
FUNDAMENTALS OF ENVIRONMENTAL CHEMISTRY
JOSEPHAT ALEXANDER SARIA
PREFACE
Environmental chemistry is becoming an increasingly popular subject in tertiary education. All over
the world, all Universities, courses in chemistry, environmental science, civil engineering,
environmental studies, public health and environmental engineering all have to include
environmental chemistry in their syllabuses to a greater or lesser extent. Many textbooks have
appeared in recent years aiming to fulfill this requirement; however, most of these deal mainly with
theoretical aspects of the subject. This manual aims to supplement the existing environmental
textbooks by providing detailed, in understanding the chemistry of the environment.
This module is about environmental issues (atmospheric, water, soil etc) and the chemistry behind
them. It aims to apply knowledge of chemistry to understand and solve environmental issues.
Treatment of general and analytical chemistry was considered to be outside the scope of this
manual. It is assumed that the student would be familiar with basic chemical theory. Anyway, many
good textbooks dealing with these topics are available and the student is referred to these books in
the text where appropriate.
It is my assumption you will enjoy reading this book and good-luck in your examinations.
Learning Outcomes
On completion of this course, students should have knowledge and understand:
Inter-relationships between water, soil and atmospheric environments, their physical, chemical
and biochemical properties.
The chemistry that govern environment degradation.
Application of chemistry in solving environmental problems.
Types and sources of air, soil and ground and surface water pollution.
Mode of Assessment
Timed tests
30%
Final examination
70%
Table of Content
9
List of Figures ………………………………………………………………………………..vi
List of Tables ………………………………………………………………………………. vii
1: LECTURE ONE 1
1.1:
The Meaning of Environmental Chemistry
1
2: LECTURE TWO 4
2.1:
Science of the Surrounding of the Earth 4
2.2:
Earth – Air – Water 4
2.1:
Science of the Surrounding of the Earth 5
2.2.1:Water and Hydrosphere 5
2.2.2:Earth’s Surface 6
2.2.3:Earth’s Water 7
3: LECTURE THREE 8
3.1:
The Chemistry of the Light and Atmosphere
3.1:
Light 8
4: LECTURE FOUR 12
4.1:
Atmospheric Structure
4.2:
Troposphere 13
4.3:
Stratosphere 13
4.4:
Mesosphere 14
4.5:
Thermosphere
14
4.5:
Exosphere 14
8
12
5: LECTURE FIVE
17
5.1:
Ozone layer 17
5.2:
Ozone Formation 18
5.3:
Importance of Ozone to our Life
18
6: LECTURE SIX
20
6.1:
Destruction of Ozone Layer and Mechanisms of Depletion
6.2:
Destruction of Ozone Layer 20
6.3:
Mechanism of Ozone Destruction 22
7: LECTURE SEVEN 25
7.1:
Ozone Layer Depletion
25
7.2:
Consequences of ozone Layer Depletion
20
25
8: LECTURE EIGHT 32
8.1:
Climate: Global Warming, Green House Effect and Radiation Balance
8.2:
Greenhouse Gases 32
10
32
9: LECTURE NINE 36
9.1:
The Consequences of Green House Gases Imbalance
9.2:
Global Warming
36
10:
LECTURE TEN
40
10.1:How Does the Earth Balance the Radiations
10.2:Radiation Balance 40
36
40
11:
LECTURE ELEVEN
44
11.1:Components of the Atmosphere: Clouds and Aerosols44
11.2: Aerosols and Clouds
45
11.2.1: Clouds 45
11.2.2:Aerosols 47
11.3: The importance of Clouds and Aerosols to Climate Change 47
11.4: Healthy Hazards of the Aerosols
49
12:
LECTURE TWELVE
51
12.1:Air Pollution and Management 51
12.2: Air Pollutants
51
12.2.1: Air Major Primary Pollutants
12.2.2:Air Major Secondary Pollutants
12.2.3:Air Minor pollutants 56
52
54
13:
LECTURE THIRTEEN
57
13.1: Water Pollution and Management
57
13.2:Water in the Atmosphere 57
13.2:Physical Properties of Water in the Atmosphere
13.3: Chemical Properties of Water in the Atmosphere
14:
LECTURE FOURTEEN 61
14.1: Acid rain and the Effect to the Environment 61
14.2: Acid Rain and its Source 62
14.2.1: Human Activity 63
14.2.2: Chemical Processes
63
14.3: The Effect of Acid Rain in the Environment 63
14.3.1: Effect on Surface Waters and Aquatic Animals
14.3.2: Effect on Soils 64
14.3.4: Effect on Human Health
65
14.3.5: Other Adverse Effects 65
15:
15.1:
15.2:
15.3:
15.3.1:
15.3.2:
58
59
64
LECTURE FIFTEEN
67
Soil and Ground Water Pollution Management
67
Soil and Groundwater 67
Contamination of Water and Soil 69
Groundwater Pollution
69
Surface and Ground Water Quality Situation: Tanzania Case
11
70
15.3.3: Pollution of Water Resources 71
15.3.4: Organic Water Pollutants: 72
15.3.5: Inorganic Water Pollutants: 72
15.3.6: Macroscopic Pollution 72
15.3.7: Thermal Pollution
73
15.7: Waste Management as Source of water Pollution Tanzania Case 73
15.4: Soil Pollution 75
16:
Lecture 16 77
16.1: Health Problems of Water Pollution 77
16.2: Unsafe Water a Source of Waterborne Diseases
77
17:
LECTURE SEVENTEEN 82
17.1: Water Treatment 82
17.2:Various Processes Used in Drinking Water Treatment
17.3: Sewage Treatment
83
17.3.1: Pre-treatment 85
17.3.3: Primary Treatment - Sedimentation 86
17.3.4: Secondary Treatment 86
17.3.5: Activated Sludge
87
17.3.6: Surface Aerated Basins
87
17.3.7: Filter Beds (Oxidizing Beds) 87
17.3.8: Biological Aerated Filters
88
17.3.9: Membrane Bioreactors88
17.4: Secondary Treatments 89
17.4:1: Secondary Sedimentation
89
17.4.3: Tertiary Treatment
90
17.4.4: Filtration
90
17.4.7: Nitrogen Removal
91
17.4.8: Phosphorus Removal 91
17.4.8: Disinfection
92
17.4.9: Package Plants and Batch Reactors 93
17.4.10: Electricity Use
94
17.4.11: Sludge Treatment and Disposal 94
17.4.12: Anaerobic Digestion
94
17.4.13: Aerobic Digestion 94
17.4.14: Composting 95
17.4.15: Sludge Disposal
95
18:
19:
82
Additional Practice Questions and Model Answers …………………………………….97
Additional Reading List
103
12
LIST OF FIGURES
Fig. 1:
Water Cycle ……………………………………………………………………..6
Fig. 2:
The Different Regions of Earth’s Atmosphere ……………………………… 12
Fig 3:
Position of Ozone Layer …………………………………………………….
19
Fig 4:
Chemical Mechanism of the Bromine ………………………………………
21
Fig. 5:
Depth to which UV Radiation Penetrates in Human Skin …………………… 25
Fig. 6:
Picture of Skin Cancer ………………………………….………………...
28
Fig. 7:
The use of Pesticide in Tanzania ………………………………………...…
29
Fig. 8:
Radiation Balance…………………………………………….…….....
42
Fig. 9:
Picture of the Clouds ………………………………………………………..
44
Fig. 10:
Earth's Radiation Balance Between Incoming and Outgoing Radiation ……
48
Fig. 11:
Air Pollution …………………………………………………………………. 51
Fig. 12:
Effect of Acid Rain on a Forest, Jizera Mountains, Czech Republic ………… 64
Fig. 13:
Waste Management Structure of the Dar es Salaam Health Department ……
Fig. 14:
Trash Closer to Living Premises with Colored Discharge Find its Way to
74
Water/Soil System …………………………………………………….
75
Fig. 15:
Process Flow Diagram for a Typical Large-Scale Treatment Plant …………. 85
LIST OF TABLES
13
Table 1:
Types of Water Sources in Tanzania ……………………………………….... 70
Table 2:
Protozoal Infections …………………………………………………………. 78
Table 3:
Parasitic Infections (Kingdom Animalia) …………………………………..... 78
Table 4:
Bacterial Infections ………………………………………………................... 79
Table 5:
Viral Infections ………………………………………………………………. 80
14
1:
1.1:
LECTURE ONE
The Meaning of Environmental Chemistry
We shall begin this lecture by defining the term chemistry, environment and then the environmental
chemistry according to views of various scholars. From these definitions characteristics of the
environment will be summarized.
What does the term chemistry mean to you as an environmentalist? Most of you will refer back in
class room “form one” and define chemistry as a branch of science which deals with composition
and decomposition of matter. That is very right, because Chemistry is the scientific study of
interaction of chemical substances that are constituted of atoms or the subatomic particles: protons,
electrons and neutrons. Atoms combine to produce molecules or crystals. In The American heritage
Dictionary they define chemistry as the science of the composition, structure, properties and
reaction of matter, especially the atomic and molecular system. Chemistry is often called "the
central science" because it connects the other natural sciences such as astronomy, physics, material
science, biology, and geology. Holderness and Lambert they define chemistry as a study of the
nature and properties of all forms of matter-the substances which makeup our environment – and
various changes which these substances undergo in different conditions. Therefore the chemistry is
related with what happen to the environment. Now, what is the environment?
According to the American heritage dictionary defines the environment as the totality of
circumstances surrounding the organism or group of organisms, especially the combination of
external physical conditions affecting the nature of organism. From this we can now define the
environmental study as the systematic study of human interaction with their environment. That’s
where the science/chemistry comes. This is a broad field of study that includes the natural
environment, built environments, social environments, organizational environments, and the sets of
relationships between them. The discipline encompasses study in the basic principles of ecology
and environmental sciences as well as the associated subjects such as policy, politics, law,
economics, social aspects, planning, pollution control, natural resources, and the interactions of
human beings and nature. Current environmental problems have evolved into a complex set of
interdisciplinary issues involving ecological, political, economic, social, as well as physical and
15
biological considerations. Modern environmental studies must include the study of the urban
environment as well as the natural environment.
What is the environmental chemistry then? Mahan in 2000, tell us that in order to understand
environmental chemistry it is better to understand first environmental science. He defines this as
“the study of the earth, air, water, and living environments, and the effects of technology thereon”.
In the actual sense environmental science in its broadest sense in the science of the complex
interactions that occur among the terrestrial, atmospheric, aquatic, living, and anthropological
environments. It includes all the disciplines, such as chemistry, biology, ecology, sociology, and
government that affect or describe these interactions. Manahan define the environmental chemistry
as “the study of the sources, reactions, transport, effects, and fates of chemical species in water,
soil, air, and living environments, and the effects of technology thereon”. This should not be
confused with green chemistry, which aims to reduce potential pollution at its source.
Environmental chemistry is an interdisciplinary science that includes atmosphere, aquatic and soil
chemistry, as well as heavily relying on analytical chemistry and being related to environmental and
other areas of science.
Environmental chemistry involves first understanding how the uncontaminated environment works,
which chemicals in what concentrations are present naturally, and with what effects. Without this it
would be impossible to accurately study the effects humans have on the environment through the
release of chemicals.
In the past decades, massive amounts of chemicals have given humankind an unprecedented
standard of living and quality of life. However, this has also exerted a price of environmental
degradation. You can't go far without being reminded about pollution, global warming and the
ozone layer. Chemistry plays an intrinsic part in issues surrounding the environment; some people
would even blame “chemistry” for most of what's going wrong.
From all this we can think of what happens to the chemicals in an industrial cleaner after someone
pour it into the sink? When you see black smoke heavy out of the pipe (chimney) at an industrial
complex, what impact is it having on the atmosphere? These are the types of questions
environmental chemists seek to answer.
16
The fate of chemicals in the environment and their effects are matters of increasing concern to
specialists in environmental management. "The outcome" involves studying where chemicals show
up in streams, rivers, and air. Such pollution/contamination contains molecules that have not been
removed in water treatment plants, caught by the filters in industrial smokestacks, disposed of
properly, or successfully sealed in containers.
As concerns about geochemistry and the natural environment increase, environmental chemists also
study the processes that affect chemicals in the environment. Gases emitted by a pine forest may
create a mist when mixed with car exhaust, for example. In other instances, the environment may
have effects on chemicals that can be toxic. Environmental chemists examine the ways both
chemicals and the environment are changed by interacting.
Study questions 1. What is the chemistry of the environment?
2. Who is the environmentalist?
3. What is the relation ship of atom, proton, electron and neutron?
Class Activity
Chemists are not the only one affect the natural environment. Discuss this statement by citing
with vivid examples like what biologists have done to environment.
References
1. The American Heritage Dictionary of the English Language, 4th ed Houghton Mifflin Company,
2006.
2. Holderness, A. and Lambert, J. (1987), A New Certificate Chemistry, Heinemann Education
3. Manahan S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher.
17
2:
2.1:
LECTURE TWO
Science of the Surrounding of the Earth
Introduction
The earth is the third planet from the sun and the largest of the other planets in the solar system in
terms of diameter, mass and density. It is also referred to as the World. According to the American
heritage dictionary earth which is revolving around the sun in a distance of about 149 million km
has an average radius of 6,378 km and a mass of approximately 5.974 x 1024 kg. Let us think again
the definitions given in Lecture 1, it is now possible to consider environmental chemistry from the
viewpoint of the interactions among water, air, earth and organism life in relation to the
surrounding. These environmental “spheres” and the interrelationships among them are summarized
in this Lecture. Earth science generally recognizes four spheres, the lithosphere, the hydrosphere,
the atmosphere, and the biosphere. These correspond to rocks, water, air, and life. Some
practitioners include the cryosphere (ice) as a distinct portion of the hydrosphere and the
pedosphere (soil) as an active, intermixed sphere as part of Earth's spheres. We might prefer the
word ecosphere, as all encompassing of both biological and physical components of the planet. So it
is combining gasses, water, life and earth's crust. You have the ecosystem which is a natural unit
consisting of all plants, animals and micro-organisms in an area functioning together with all the
non-living physical factors of the environment.
2.2:
Earth – Air – Water
The environment encompasses three main zones or “spheres”: the earth's crust or simply “earth”,
the hydrosphere or the water on the surface of the earth like oceans, lakes, rivers and groundwater
and the atmosphere or the air around us. These three zones and their interactions with one another
are collectively “the environment”. So the chemistry of the environment is the chemistry of what
goes on in the three areas and their interfaces.
18
2.1:
Science of the Surrounding of the Earth
2.2.1: Water and Hydrosphere
The term hydrosphere means the water vapor in the earth’s atmosphere. Water, with a very simple
chemical formula of H2O, is a vitally important substance in all parts of the environment. Water
covers about 71% of Earth’s surface. It occurs in all spheres of the environment in the oceans as a
vast reservoir of saltwater, on land as surface water in lakes and rivers, underground as
groundwater, in the atmosphere as water vapor, in the polar icecaps as solid ice (top of mountain
Kilimanjaro), and in many segments of the anthrosphere such as in boilers or municipal water
distribution systems. Water is an essential part of all living systems and is the medium from which
life evolved and in which life exists. On earth, it is found mostly in oceans and other large water
bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds (formed
of solid and liquid water particles suspended in air), and precipitation, hold 97% of surface water,
glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%.
A very small amount of the Earth's water is contained within biological bodies and manufactured
products. Other water is trapped in ice caps, glaciers, aquifers, or in lakes, sometimes providing
fresh water for life on land.
Water moves continually through a cycle of evaporation or transpiration (evapo-transpiration),
precipitation, and runoff, usually reaching the sea. Winds carry water vapor over land at the same
rate as runoff into the sea. Over land, evaporation and transpiration contribute to the precipitation
over land. Figure 1 below shows the water circle.
19
Fig. 1: Water Cycle
Water has three states (solid, liquid and gaseous). Below freezing water is a solid (ice or
snowflakes), between freezing and boiling water is a liquid, and above its boiling point water is a
gas. Water changing from solid to liquid is said to be melting. When it changes from liquid to gas it
is evaporating. Water changing from gas to liquid is called condensation (An example is the 'dew'
that forms on the outside of a glass of cold soda). Frost formation is when water changes from gas
directly to solid form. When water changes directly from solid to gas the process is called
sublimation.
2.2.2: Earth’s Surface
Geochemists study the chemistry of the earth's crust. They define earths crust as a thin layer of rock,
some 40 km thick that covers the whole surface of the planet. In terms of elements, the crust is 46%
oxygen, 28% silicon, 8% aluminium, and 5% iron with smaller amounts of other elements. These
elements are combined into mainly silicon dioxide (silica) based rocks. The earth is divided into
layers, including the solid, iron-rich inner core, molten outer core, mantle, and crust. Environmental
science is most concerned with the lithosphere, which consists of the outer mantle and the crust.
The latter is the earth’s outer skin that is accessible to humans. It is extremely thin compared to the
diameter of the earth, ranging from 5 to 40 km thick. As such, it pertains mostly to the solid mineral
portions of Earth’s crust. But it must also consider water, which is involved in weathering rocks and
20
in producing mineral formations; the atmosphere and climate, which have philosophical effects on
the geosphere and interchange matter and energy with it; and living systems, which largely exist on
the geosphere and in turn have significant effects on it. Geological science uses chemistry in the
form of geochemistry to explain the nature and behavior of geological materials, physics to explain
their mechanical behavior, and biology to explain the mutual interactions between the geosphere
and the biosphere. Modern technology, for example the ability to move massive quantities of dirt
and rock around, has a profound influence on the geosphere. The most important part of the
geosphere for life on earth is soil formed by the disintegrative weathering action of physical,
geochemical, and biological processes on rock. It is the medium upon which plants grow, and
virtually all terrestrial organisms depend upon it for their existence. The productivity of soil is
strongly affected by environmental conditions and pollutants. That’s where the environmental
chemistry and environmental science plays.
2.2.3: Earth’s Water
The hydrosphere encompasses the water gathered in different places on the surface of the earth. The
huge majority of this water is found in the oceans and trapped beneath them in sediments. Only
about 0.5% of water is found on the continents in rivers, lakes, glaciers and ground waters. The
oceans play an important part in absorbing a transporting solar energy which controls climate. They
also are involved in the evaporation-precipitation cycles which so affect life on the planet. Water is
the universal solvent, therefore the chemistry of water is a huge subject and its ability as a solvent is
the basis of all life on earth.
Study Questions: 1) What is the difference between these terms: atmosphere, hydrosphere,
geosphere, and biosphere
2) How can you distinguish between evapo-transpiration and
sublimation?
References
1. The American Heritage Dictionary of the English Language, 4th ed Houghton Mifflin Company,
2006.
2. Water cycle http://ga.water.usgs.gov/edu/watercycleprint.html
3. Manahan, S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher.
21
3:
3.1:
LECTURE THREE
The Chemistry of the Light and Atmosphere
Introduction
The chemistry composition of the atmosphere is of importance for several reasons, but primarily
because of the interactions between the atmosphere and living organisms. The composition of the
Earth's atmosphere has been changed by human activity and some of these changes are harmful to
human health, crops and ecosystems. Examples of problems which have been addressed by
atmospheric chemistry include acid rain, photochemical smog and global warming. Atmospheric
chemistry seeks to understand the causes of these problems, and by obtaining a theoretical
understanding of them, allow possible solutions to be tested and the effects of changes in
government policy evaluated.
Instead of concentrating on atmospheric chemistry in isolation the focus is now on seeing it as one
part of a single system with the rest of the atmosphere, biosphere and geosphere. An especially
important driver for this is the links between chemistry and climate such as the effects of changing
climate on the recovery of the ozone hole and vice versa but also interaction of the composition of
the atmosphere with the oceans and terrestrial ecosystems.
3.1:
Light
Light plays an important role in atmospheric chemistry. The energy of the light in the Earth’s
atmosphere is often needed to initiate reactions. For example, without the energy of light coming
from the Sun, there would be no stratospheric ozone. The critical feature of light as a reactant is its
wavelength. Thus:
E = hυ =
hC
λ
Where: E = Energy in Joules
h = Planck’s Constant (6.63 x 10-34Js/molecule)
υ = Frequency of light in s-1
22
C = Speed of light (3 x 108 m/s)
λ = Wavelength (nm)
The following are the examples of electromagnetic radiation: microwaves, infrared and ultraviolet
light, X-rays and gamma-rays. Hotter, more energetic objects and events create higher energy
radiation than cool objects. Only extremely hot objects or particles moving at very high velocities
can create high-energy radiation like X-rays and gamma-rays.
These types of radiation in the electromagnetic spectrum can be arranged from the one with lowest
energy to highest as follows:
Radio wave: This is the same kind of energy that radio
stations emit into the air for your radio to capture. But
radio waves are also emitted by other things such as stars
and gases in space. They have a frequency ranging from
104 to 1010 s-1
Microwaves: They are used in cooking just in a few
minutes. Also astronomers in space are using to learn
about the structure of nearby galaxies.
They have a
frequency ranging from 109 to 1011 s-1
Infrared: Our skin emits infrared light, which is why we
can be seen in the dark by someone using night vision
goggles. In space, IR light maps the dust between stars.
They have a frequency ranging from 1011 to 1015 s-1
Visible: This is the part that our eyes see. Visible radiation
is emitted by everything from fireflies to light bulbs to
stars ... also by fast-moving particles hitting other
particles. They have wavelength ranging from 390 nm to
760 nm.
23
Ultraviolet: We know that the Sun is a source of
ultraviolet (or UV) radiation, because it is the UV rays
that cause our skin to burn! Stars and other "hot" objects
in space emit UV radiation. They have a frequency
ranging from 1014 to 1017 s-1
X-rays: Used by medical doctors to look at your bones and
dentist to look at your teeth. Hot gases in the Universe
also emit X-rays. They have a frequency ranging from
1017 to 1021 s-1
Gamma-rays: Radioactive materials (some natural and
others made by man in things like nuclear power plants)
can emit gamma-rays. Big particle accelerators that
scientists use to help them understand what matter is made
of can sometimes generate gamma-rays. But the biggest
gamma-ray generator of all is the Universe! It makes
gamma radiation in all kinds of ways. They have a
frequency ranging from 1019 to 1024 s-1
24
Study Question and Model Answer:
A wavelength of 242.4 nm is the longest wavelength that will bring about the photo-dissociation of
O2. What is the energy of (a) one photon and (b) a mole of photon of this light?
2.998 x1010 ms −1
Answer: (a) We know that : υ =
=
= 1.237 x1015 s −1
−9
λ
242.4 x10 m
C
E = hυ = 6.626 x 10-34 Js/photon x 1.237 x 1015 s-1 = 8.196 x 10-19J/photon
(b) E = 8.196 x 10-19J/photon x 6.022 x 1023 photon/mol = 4.936 x 105J/mol
Practice Question
The energy of a photon is higher when the frequency of the associated wave is higher (and the
wavelength shorter). Discuss.
Activity:
If gaseous dihydrogen (H2) at low pressure is subjected to electrical discharge, the
molecules are broken down into energetically excited atoms (H). These atoms emit
visible radiation which, when passed through a prism, show a discrete set of
wavelengths an atomic spectrum is observed. The emitted radiation is not a continuous
spectrum. Draw the Emission spectrum of hydrogen showing the position of distinct
colors in respect of the wavelength of each.
References:
1. Chang, R. (1991), Chemistry, 4th ed McGraw-Hill, Inc
2. Manahan S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher
3. Wright, J. (2005), Environmental Chemistry, Taylor & Francis e-Library. NY.
25
4:
4.1:
LECTURE FOUR
Atmospheric Structure
Introduction
The gaseous area surrounding the planet is divided into several concentric spherical strata separated
by narrow transition zones. The upper boundary at which gases disperse into space lies at an altitude
of approximately 1000 km above sea level. More than 99% of the total atmospheric mass is
concentrated in the first 40 km from Earth's surface.
Atmospheric layers are characterized by differences in chemical composition that produce
variations in temperature.
100
Thermosphere N2, O2+, NO+, O2
Mesopause
80
Mesosphere [N2, O2, O2+, NO+]
Stratopause
Altitude (Km)
60
Stratosphere [N2, O2, O3]
40
Ozone Layer
20
Tropospause
Troposphere [N2, O2, H2O, Ar] CO2]
-80
-60
-40
-20
0
20
40
Temperature (oC)-
Fig. 2: The Different Regions of Earth’s Atmosphere (Chang, R. 1991 pg 584)
26
4.2:
Troposphere
The troposphere is the atmospheric layer closest to the planet and contains the largest percentage of
the mass of the total atmosphere. It is characterized by the density of its air and an average vertical
temperature change of 6oC/km.Temperature and water vapor content in the troposphere decrease
rapidly with altitude. Water vapor plays a major role in regulating air temperature because it
absorbs solar energy and thermal radiation from the planet's surface. The troposphere contains 99 %
of the water vapor in the atmosphere. Water vapor concentrations vary with latitudinal position.
They are greatest above the tropics, where they may be as high as 3 %, and decrease toward the
polar regions.
All weather phenomena occur within the troposphere, although turbulence may extend into the
lower portion of the stratosphere. Troposphere means "region of mixing" and is so named because
of vigorous convective air currents within the layer.
The upper boundary of the layer ranges in height from 8 Km in high latitudes, to 18 Km above the
equator. Its height also varies with the seasons; highest in the summer and lowest in the winter. A
narrow zone called the tropopause separates the troposphere from the next highest layer called the
stratosphere. Air temperature within the tropopause remains constant with increasing altitude.
4.3:
Stratosphere
The stratosphere is the second major strata of air in the atmosphere. It resides between 10 and 50
km above the planet's surface. The air temperature in the stratosphere remains relatively constant up
to an altitude of 25 km. Then it increases gradually to 200-220 degrees Kelvin (K) at the lower
boundary of the stratopause (~50 Km), which is marked by a decrease in temperature. Because the
air temperature in the stratosphere increases with altitude, it does not cause convection and has a
stabilizing effect on atmospheric conditions in the region. Ozone plays the major role in regulating
the thermal regime of the stratosphere, as water vapor content within the layer is very low.
Temperature increases with ozone concentration. Solar energy is converted to kinetic energy when
ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere.
The ozone layer (27k jpeg) is located at an altitude between 20-30 km. Approximately 90% of the
ozone in the atmosphere resides in the stratosphere. Ozone concentration in the region is about 10
27
parts per million by volume as compared to approximately 0.04 parts per million by volume in the
troposphere. Ozone absorbs the bulk of solar ultraviolet radiation in wavelengths from 290 nm - 320
nm. These wavelengths are harmful to life because they can be absorbed by the nucleic acid in cells.
Increased penetration of ultraviolet radiation to the planet's surface would damage plant life and
have harmful environmental consequences. Appreciably large amounts of solar ultraviolet radiation
would result in a host of biological effects, such as a dramatic increase in cancers. Meteorological
conditions strongly affect the distribution of ozone. Most ozone production and destruction occurs
in the tropical upper stratosphere, where the largest amounts of ultraviolet radiation are present.
Dissociation takes place in lower regions of the stratosphere and occurs at higher latitudes than does
production.
4.4:
Mesosphere
The mesosphere, a layer extending from approximately 50 km to 80 km, is characterized by
decreasing temperatures, which reach 190-180 K at an altitude of 80 km. In this region,
concentrations of ozone and water vapor are negligible. Hence the temperature is lower than that of
the troposphere or stratosphere. With increasing distance from Earth's surface the chemical
composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched
with lighter gases. At very high altitudes, the residual gases begin to stratify according to molecular
mass, because of gravitational separation.
4.5:
Thermosphere
The thermosphere is located above the mesosphere and is separated from it by the mesopause
transition layer. The temperature in the thermosphere generally increases with altitude up to 10001500 K. This increase in temperature is due to the absorption of intense solar radiation by the
limited amount of remaining molecular oxygen. At an altitude of 100-200 km, the major
atmospheric components are still nitrogen and oxygen. At this extreme altitude gas molecules are
widely separated.
4.5:
Exosphere
The exosphere is the most distant atmospheric region from Earth's surface. The upper boundary of
the layer extends to heights of perhaps 960 to 1000 km and is relatively undefined. The exosphere is
28
a transitional zone between Earth's atmosphere and interplanetary space. Atmospheric chemists are
interested in understanding the chemical composition of the natural atmosphere, the way gases,
liquids, and solids in the atmosphere interact with each other and with the earth's surface and
associated biota, and how human activities may be changing the chemical and physical
characteristics of the atmosphere. This latter question is currently a driving force behind the
growing need for atmospheric chemists in the world. There are a number of critical environmental
issues associated with a changing atmosphere, including photochemical smog, global climate
change, toxic air pollutants, acidic deposition, and stratospheric ozone depletion. All of these issues
affect the world, and a great deal of research and development activity aimed at understanding and
hopefully solving some of these problems is underway. Much of the anthropogenic (human) impact
on the atmosphere is associated with our increasing use of fossil fuels as an energy source - for
things such as heating, transportation, and electric power production. Photochemical
smog/tropospheric ozone is one serious environmental problem associated with burning fossil fuels.
The importance of Atmospheric Chemistry has been acknowledged by The Royal Swedish
Academy of Sciences awarding the Nobel Prize in Chemistry 1995 to the Atmospheric Scientists P.
Crutzen, M. Molina and F. S. Rowland particularly concerning the formation and decomposition of
ozone.
29
Study question and model answer:
Flight engineers know much the use of Figure 2. They know that for
more fuel efficiency the airlines cannot reach the stratosphere. Discuss
this in detail.
Answer: Commercial airliners typically sail at altitudes of 9–12 km in temperate
latitudes, in the lower reaches of the stratosphere. They do this to optimize jet engine fuel
burn, mostly thanks to the low temperatures encountered near the tropopause. It also
allows them to stay above any hard weather, and avoid atmospheric turbulence from the
convection in the troposphere. Turbulence experienced in the cruise phase of flight is
often caused by convective overshoot from the troposphere below. Although a few gliders
have achieved great altitudes in the powerful thermals in thunderstorms, this is
dangerous. Most high altitude flights by gliders use lee waves from mountain ranges and
were used to set the current record of 15,447m.
Class activity: Why the pressure on this mountain is very low and the temperature is
colder than sea level. Suppose the summit of Mountain Kilimanjaro is flat; is it possible
to play football on top for 90 minutes?
References:
1. Chang, R. (1991), Chemistry, 4th ed McGraw-Hill, Inc.
2. Seinfeld, J. H., and Pandis, S. N. (2006), Atmospheric Chemistry and Physics: From Air
Pollution to Climate Change 2nd ed, Wiley, New Jersey.
30
5:
5.1:
LECTURE FIVE
Ozone layer
Introduction
Ozone or trioxygen (O3) is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope
of oxygen that is much less stable than the diatomic O2. Ground-level ozone is an air pollutant with
harmful effects on the respiratory systems of animals. The ozone layer in the upper atmosphere
filters potentially damaging ultraviolet light from reaching the Earth's surface. It is present in low
concentrations throughout the Earth's atmosphere. It has many industrial and consumer applications.
The chemistry of ozone can be considered as a powerful oxidizing agent, far better than dioxygen. It
is also unstable at high concentrations, decaying to ordinary diatomic oxygen (in about half an hour
in atmospheric conditions): 2 O3 → 3 O2. Also ozone can combine with metal or non metal to give
oxygen as shown here:
Ozone will oxidize metals (except gold, platinum, and iridium) to oxides of the metals in their
highest oxidation state:
2 Cu+ (aq) + 2 H3O+ (aq) + O3 (g) → 2 Cu2+ (aq) + 3 H2O (l) + O2 (g) or for non metal Ozone reacts
with carbon to form carbon dioxide, even at room temperature:
C + 2 O3 → CO2 + 2 O2 or another metal like potassium
2 KOH + 5 O3 → 2 KO3 + 5 O2 + H2O. Example of non-metal is the reaction of ozone with
ammonia. Ozone does not react with ammonium salts but it reacts with ammonia to form
ammonium nitrate:
2NH3 + 4O3 → NH4NO3 + 4O2 + H2O
The question is oxygen is very important in life as every one knows. What is the problem of having
this big source of oxygen? First we have to discuss the formation of this ozone. What is the
importance of Ozone to our life?
31
5.2:
Ozone Formation
When fossil fuels (e.g., gasoline) are burned, a variety of pollutants are emitted into the earth's
troposphere, i.e., the region of the atmosphere in which we live - from ground level up to about 15
km. Two of the pollutants that are emitted are hydrocarbons (e.g., unburned fuel) and nitric oxide
(NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their
interaction with sunlight in which the NO is converted to nitrogen dioxide (NO2). NO2 is a brown
gas and at sufficiently high levels can contribute to urban haze. However, a more serious problem is
that NO2 can absorb sunlight and break apart to produce oxygen atoms that combine with the O2 in
the air to produce ozone (O3). Oxygen also decomposes with the solar radiation at a wavelength
shorter than 260 and in effect can form ozone as shown in equations below:
O2 (g) + hυ → O(g) + O(g)
O(g) + O2 → O3(g)
5.3:
Importance of Ozone to our Life
The largest quantity of ozone is used in the preparation of pharmaceuticals, synthetic lubricants, as
well as many other commercially useful organic compounds, where it is used to sever carboncarbon bonds. It can also be used for bleaching substances and for killing microorganisms in air and
water sources. Many municipal drinking water systems kill bacteria with ozone instead of the more
common chlorine. Ozone has a very high oxidation potential. Ozone does not form organochlorine
compounds, nor does it remain in the water after treatment. Where electrical power is abundant,
ozone is a cost-effective method of treating water, since it is produced on demand and does not
require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or
odor in drinking water. Ozone is also called earth’s blanket. About 25 km above the earth is a layer
of this gas ozone. Ozone is produced naturally in the atmosphere. The ozone layer is very important
because it stops too many of the sun’s ‘ultra-violet rays’ (UV rays) getting through to the Earth these are the rays that cause burn to our skin especially people with low melanin. Too much UV can
cause skin cancer and will also harm all plants and animals. The following figure can summarize the
lecture.
32
Fig 3: Position of Ozone Layer
Class activity
Life on Earth could not exist without the protective shield of the ozone layer.
Discuss this statement in depth and give relevant consequences.
References
1.
Chang, R. (1991), Chemistry, 4th Ed, McGraw-Hill, Inc.
2.
Shriver, D. and Atkins, P. (2003), 3rd Ed. Inorganic Chemistry, W. H. Freeman
Company, NY.
3.
http://science.howstuffworks.com/global-warming.htm.
33
6:
6.1:
LECTURE SIX
Destruction of Ozone Layer and Mechanisms of Depletion
Introduction
We have learned from previous lecture that the stratosphere is the layer of our atmosphere just
above the one we breathe which includes a thin layer of ozone. This layer is thicker over the poles
than the equator. It might seem insignificant compared to the depth of the rest of the atmosphere,
but it does a very important job. Example of the job is to prevents much of the sun's ultraviolet
(UV) light from reaching the earth. UV light can cause skin cancer, cataracts and other disorders.
Ozone protects us from the sun by interacting with light. When ultraviolet light hits oxygen
molecules (O2) in the stratosphere, it splits the molecules into two atoms of oxygen (O). When this
atom encounters another oxygen molecule, the two combine to make ozone (O3). Ultraviolet light
also breaks ozone back down into an oxygen molecule and an oxygen atom. We can term this
process as ozone-oxygen cycle, and it converts UV radiation into heat, protecting the Earth. In this
lecture we will lean what destruct this important layer and disturb the circle so that our life is in
danger?
6.2:
Destruction of Ozone Layer
Ozone is a powerful oxidizing agent, and a toxic gas. It is believed that the natural level of ozone in
the clean troposphere is 10 to 15 parts-per-billion (ppb). Because of increasing concentrations of
hydrocarbons and NO in the atmosphere, scientists have found that ozone levels in "clean air" are
now approximately 30 ppb. A principal activity of atmospheric chemists is to study and determine
how we might reverse this trend.
Ozone depletion describes two distinct, but related observations: a slow, steady decline of about 4
percent per decade in the total volume of ozone in Earth's stratosphere (ozone layer) since the late
1970s, and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions
during the same period. The latter phenomenon is commonly referred to as the ozone hole. In
addition to this well-known stratospheric ozone depletion, there are also tropospheric ozone
depletion events, which occur near the surface in polar regions during spring. I know for us the
34
word spring could be very confusing because we are in Africa very close to equator where the
seasons are not very distinct. For those in Europe or America they can tell the differences, that we
start with winter (Very cold we can see snow here) to spring and then summer (very hot). Now we
know spring is not hot not cold. During this time in the Polar Regions, unique photochemistry
converts inert halide salt ions (e.g. Br-) into reactive halogen species (e.g. Br atoms and BrO) that
deplete ozone in the boundary layer to near zero levels. Since their discovery in the late 1980s,
research on these ozone depletion events (ODEs) has shown the central role of bromine
photochemistry. Due to the autocatalytic nature of the reaction mechanism, it has been called
bromine explosion. The following is the bromine explosion which shows what we have just learned
as the reaction mechanism.
Fig 4: Chemical mechanism of the bromine explosion (Adopted from
http://en.wikipedia.org/wiki/Tropospheric_ozone_depletion_events on 1st August 2009)
It is still not fully understood how salts are transported from the ocean and oxidized to become
reactive halogen species in the air. Other halogens (chlorine and iodine) are also activated through
mechanisms coupled to bromine chemistry. The main consequence of halogen activation is
chemical destruction of ozone, which removes the primary precursor of atmospheric oxidation, and
generation of reactive halogen atoms/oxides that become the primary oxidizing species. The
different reactivity of halogens as compared to OH and ozone has broad impacts on atmospheric
chemistry, including near complete removal and deposition of mercury, alteration of oxidation fates
35
for organic gases, and export of bromine into the free troposphere. Recent changes in the climate of
the Arctic and state of the Arctic sea ice cover are likely to have strong effects on halogen activation
and ODEs.
6.3:
Mechanism of Ozone Destruction
The detailed mechanism by which the polar ozone holes form is different from that for the midlatitude thinning, but the most important process in both trends is catalytic destruction of ozone by
atomic chlorine and bromine. The main source of these halogen atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds, commonly called freons, and of
bromofluorocarbon compounds known as halons. These compounds are transported into the
stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as
emissions of CFCs and halogens increased.
CFCs and other contributory substances are commonly referred to as ozone-depleting substances
(ODS). Since the ozone layer prevents most harmful UVB wavelengths (270–315 nm) of ultraviolet
light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in
ozone have generated worldwide concern leading to adoption of the Montreal Protocol banning the
production of CFCs and halons as well as related ozone depleting chemicals such as carbon
tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as
increases in skin cancer, damage to plants, and reduction of plankton populations in the ocean's
photic zone may result from the increased UV exposure due to ozone depletion.
Oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas
(O2 or diatomic oxygen), and ozone gas (O3 or triatomic oxygen). Ozone is formed in the
stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose
wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then
combines with O2 to create O3. Ozone molecules absorb UV light between 310 and 200 nm,
following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then
joins up with an oxygen molecule to regenerate ozone. This is a continuing process which
terminates when an oxygen atom "recombines" with an ozone molecule to make two O2 molecules:
O + O3 → 2 O2
36
The overall amount of ozone in the stratosphere is determined by a balance between photochemical
production and recombination. Ozone can be destroyed by a number of free radical catalysts, the
most important of which are the hydroxyl radical (OH·), the nitric oxide radical (NO·) and atomic
chlorine (Cl·) and bromine (Br·). All of these have both natural and anthropogenic (manmade)
sources; at the present time, most of the OH· and NO· in the stratosphere is of natural origin, but
human activity has dramatically increased the levels of chlorine and bromine. These elements are
found in certain stable organic compounds, especially chlorofluorocarbons (CFCs), which may find
their way to the stratosphere without being destroyed in the troposphere due to their low reactivity.
Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action
of ultraviolet light, e.g. ('h' is Planck's constant, 'ν' is frequency of electromagnetic radiation)
CFCl3 + hν → CFCl2 + Cl
The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the
simplest example of such a cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen
atom with it (forming ClO) and leaving a normal oxygen molecule. The chlorine monoxide (i.e., the
ClO) can react with a second molecule of ozone (i.e., O3) to yield another chlorine atom and two
molecules of oxygen. The chemical shorthand for these gas-phase reactions is:
Cl + O3 → ClO + O2
ClO + O3 → Cl + 2 O2
The overall effect is a decrease in the amount of ozone. More complicated mechanisms have been
discovered that lead to ozone destruction in the lower stratosphere as well.
A single chlorine atom would keep on destroying ozone (thus a catalyst) for up to two years (the
time scale for transport back down to the troposphere) were it not for reactions that remove them
from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate
(ClONO2). On a per atom basis, bromine is even more efficient than chlorine at destroying ozone,
but there is much less bromine in the atmosphere at present. As a result, both chlorine and bromine
contribute significantly to the overall ozone depletion. Laboratory studies have shown that fluorine
and iodine atoms participate in analogous catalytic cycles. However, in the Earth's stratosphere,
37
fluorine atoms react rapidly with water and methane to form strongly-bound HF, while organic
molecules which contain iodine react so rapidly in the lower atmosphere that they do not reach the
stratosphere in significant quantities. Furthermore, a single chlorine atom is able to react with
100,000 ozone molecules. This fact plus the amount of chlorine released into the atmosphere by
chlorofluorocarbons (CFCs) yearly demonstrates how dangerous CFCs are to the environment.
Study Question
1. What do you understand by the term ozone hole? Why do you think slow
or steady state is not referred as ozone hole?
2. Which one is most harmful UVA or UVB? Give reason.
3. What can you say about tropospheric ozone depletion events, which
occur near the surface in polar regions during spring.
To assist you please refer reference 2and 3 below.
References:
1. Simpson, W. R. , von Glasow, R. , Riedel, Anderson, K. , Ariya, P. , P. (2007), Halogens
and their role in polar boundary-layer ozone depletion, atmos. Chem. Phys., 7, 4375-4418.
2. http://ozonewatch.gsfc.nasa.gov/facts/hole.html
3. http://www.wunderground.com/education/ozone_skeptics.asp
38
7:
7.1:
LECTURE SEVEN
Ozone Layer Depletion
Introduction
We have seen in the previous lecture that life on Earth depends in part on a thin shell of gaseous
ozone that stretches from about 10 to 25 miles above our heads, encompassing the planet like an
invisible, protective shield. At this altitude, it lies well above the height at which normal
commercial aircraft fly, and far beneath the orbital paths of spacecraft. The ozone layer is the main
barrier between us and the hazardous ultraviolet radiation that streams toward the Earth, day in and
day out, from the burning surface of the Sun. Ozone--a form of oxygen--is selective in what it takes
from sunlight: screening out, through a process of atomic absorption, only the more energetic
ultraviolet rays while allowing the visible light and the warm infrared to pass through, untouched.
The remaining question is what are the scientific consequences of this layer exhausted? Let us start
by looking on the issues may happen.
7.2:
Consequences of ozone Layer Depletion
Let us start this lecture by using the following figure:
Fig. 5: Depth to which UV Radiation Penetrates in Human Skin
(adopted from http://www.gcrio.org/CONSEQUENCES/ summer95/fig2-2.html)
39
On figure 5, we can find UVA, UVB and UVC. They seem to be confusing. What are these? The
answer is simple because they are based on the wavelength.
•
UVC - 100 to 290 nm
•
UVB - 290 to 320 nm
•
UVA - 320 to 400 nm
UVA
UVA was once thought to have a minor effect on skin damage, but now studies are showing that
UVA is a major contributor to skin damage. UVA penetrates deeper into the skin and works more
efficiently. The intensity of UVA radiation is more constant than UVB without the variations during
the day and throughout the year. UVA is also not filtered by glass.
UVB
UVB affects the outer layer of skin, the epidermis, and is the primary agent responsible for
sunburns. It is the most intense between the hours of 10:00 am and 2:00 pm when the sunlight is
brightest. It is also more intense in the summer months accounting for 70% of a person's yearly
UVB dose. UVB does not penetrate glass.
UVC
UVC radiation is almost completely absorbed by the ozone layer and does not affect the skin. UVC
radiation can be found in artificial sources such as mercury arc lamps and germicidal lamps.
Since the ozone layer absorbs UVB ultraviolet light from the Sun, ozone layer depletion is expected
to increase surface UVB levels, which could lead to damage, including increases in skin cancer.
Although decreases in stratospheric ozone are well-tied to CFCs and there are good theoretical
reasons to believe that decreases in ozone will lead to increases in surface UVB, there is no direct
observational evidence linking ozone depletion to higher incidence of skin cancer in human beings.
This is partly due to the fact that UVA, which has also been implicated in some forms of skin
cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for lifestyle
changes in the populace.
40
While a minority constituent in the earth's atmosphere, is responsible for most of the absorption of
UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases
exponentially with the slant-path thickness/density of the layer. Correspondingly, a decrease in
atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface.
Because it is this same UV radiation that creates ozone in the ozone layer from O2 (regular oxygen)
in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical
production of ozone at lower levels (in the troposphere), although the overall observed trends in
total column ozone still show a decrease, largely because ozone produced lower down has a
naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a
level which would compensate for the ozone reduction higher up.
The main public concern regarding the ozone hole has been the effects of surface UV on human
health. So far, ozone depletion in most locations has been typically a few percent and, as noted
above, no direct evidence of health damage is available in most latitudes. Were the high levels of
depletion seen in the ozone hole ever to be common across the globe, the effects could be
substantially more dramatic. As the ozone hole over Antarctica has in some instances grown as
large as to reach southern parts of Australia and New Zealand, environmentalists have been
concerned that the increase in surface UV could be significant.
UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory
factor to skin cancer. In addition, increased surface UV leads to increased tropospheric ozone,
which is a health risk to humans. The increased surface UV also represents an increase in the
vitamin D synthetic capacity of the sunlight. The cancer preventive effects of vitamin D represent a
possible beneficial effect of ozone depletion. In terms of health costs, the possible benefits of
increased UV irradiance may outweigh the burden. Below is the picture of skin cancer adopted from
About.com Health's Disease and Condition content is reviewed by the Medical Review Board.
41
Fig. 6:
Picture of Skin Cancer
Not only that Ozone depletion has only effect on human beings. A decrease in the amount of ozone
in the upper atmosphere results an increase in the amount of ultraviolet (UV) radiation at the Earth's
surface. Additional UV-B (the most harmful wavelengths) is expected to have negative effects on
human health, health services, farm animals, crop production, forest production, fisheries and tourist
industries. Examples of likely effects include:
•
Reduced immune responses, which may increase the incidence of infectious disease and reduce
the effectiveness of vaccination programmes;
•
Disrupted growth processes in some plants, leading to reduced yields for certain crops and forest
trees; and hence hunger, death, poverty etc.
•
Disrupted development in fish, estimated to reduce ocean fish stocks by several million tones
per annum.
In agriculture we can discuss this better by giving up a story, especially those who have being using
pesticides to spray on their crops.
The figure below shows the use of DDT by farmer from Kilimanjaro, Old Moshi Tella, spray on his
coffee plantation.
42
Fig. 7:
The use of Pesticide in Tanzania
Most of the commonly used chemical especially in coffee is the pesticide. One of the main threats
to the Earth's ozone layer today is a powerful pesticide called methyl bromide that plays a pivotal
role in Southern cash-crop economies. This type was used mainly in Zimbabwe. Methyl bromide is
a highly toxic fungicide and the second most widely applied pesticide in the world. It is primarily
used on "high value" export crops such as tomatoes, peppers, grapes, strawberries, tobacco and
flowers. Methyl bromide is also used to protect stored grains. The pesticide is effective against a
wide range of pests including insects, worms, and pathogenic microorganisms. However, methyl
bromide has a dramatic environmental impact. After being sprayed on crops, the pesticide drifts into
the upper atmosphere where it damages the ozone layer, which blocks ultraviolet (UV) rays from
reaching the Earth's surface. Although a shorter-lived substance than chlorofluorocarbons (CFCs)
— a better-known family of ozone-depleting compounds — methyl bromide destroys ozone
molecules at 50 times the rate of CFCs. In a 1994 scientific assessment, the World Meteorological
Organization concluded that phasing out this chemical is the single largest step that governments
can take to protect the ozone layer. The question is what should we use to kill pests in our home or
in the farm?
43
Study Questions and Model Answers:
1. What is the ozone layer and why is it important?
Answer: The ozone layer is a concentration of ozone molecules in the stratosphere. About 90% of
the planet's ozone is in the troposphere layer. The stratosphere, the next higher layer, extends about
10-50 kilometers above the Earth's surface. Stratospheric ozone is a naturally-occurring gas that
filters the sun's ultraviolet (UV) radiation. A diminished ozone layer allows more radiation to reach
the Earth's surface. For people, over-exposure to UV rays can lead to skin cancer, cataracts, and
weakened immune systems. Increased UV can also lead to reduced crop yield and disruptions in the
marine food chain.
2. How does ozone depletion occur?
Answer:It is caused by the release of chlorofluorocarbons (CFCs), hydrofluorocarbons (HCFCs),
and other ozone-depleting substances (ODS), which were used widely as refrigerants, insulating
foams, and solvents. Although CFCs are heavier than air, they are eventually carried into the
stratosphere in a process that can take as long as 2 to 5 years. When CFCs and HCFCs reach the
stratosphere, the ultraviolet radiation from the sun causes them to break apart and release chlorine
atoms which react with ozone, starting chemical cycles of ozone destruction that deplete the ozone
layer. One chlorine atom can break apart more than 100,000 ozone molecules.
Other chemicals that damage the ozone layer include methyl bromide (used as a pesticide), halons
(used in fire extinguishers), and methyl chloroform (used as a solvent in industrial processes for
essential applications). As methyl bromide and halons are broken apart, they release bromine atoms,
which are 60 times more destructive to ozone molecules than chlorine atoms.
3. Will the ozone layer recover? Can we make more ozone to fill in the hole?
Provided that we stop producing ozone-depleting substances, ozone will be created through natural
processes that should return the ozone layer to normal levels by about 2050. It is very important that
the world comply with the Montreal Protocol; delays in ending production could result in additional
damage and prolong the ozone layer's recovery. More detail on these questions is provided here.
44
4.
How do we know the amount of CO2 in the atmosphere is linked to human Activity?
Answer: One way we know this is linked to human activity is by looking at historical records of
human activity. Fossil fuels have been burnt since the start of the industrial revolution, along with
forested land being cleared at unprecedented rates. These processes have been turning the organic
compounds into Carbon Dioxide. Enough carbon dioxide has been produced to raise the
atmospheric content to 500ppm. Fortunately, these levels haven't been reaching because the oceans
and biosphere have the ability to absorb some of this CO2. Unfortunately, because we are producing
CO2 faster than the oceans and biosphere can absorb them, we see the increased levels. Another
way, we know that the fossil fuels are causing CO2 increase in the atmosphere is by analyzing the
carbon isotopes, similar to carbon dating. More information can be found at Real Climate.
Class Activity
1. In late 80’s the government of Tanzania burn the use of pesticides. There was a pile of
stock in the country. Where are they now? Who destroy them? How did they be
destroyed?
2. What are the contents of the chemicals your relatives or your self have been spraying on
the plant like maize, coffee or any agricultural products?
References:
1. Gruijl, F. R. (1995), Impacts of a Projected Depletion of the Ozone Layer, The Nature and
Implications of Environmental Changes, V 1 &2.
2. http://www.gcrio.org/CONSEQUENCES/summer95/fig2-2.html.
3. http://archive.idrc.ca/books/reports/1997/14-02e.html
45
8:
8.1:
LECTURE EIGHT
Climate: Global Warming, Green House Effect and Radiation Balance
Introduction
We have talked about the atmosphere and the consequences of the destruction of the normal setup
on our life. Now it is time to discuss on these changes. We will start with the commonly used term
climate. You may be familiar with this word but probably you don’t know exactly what does this
mean. The word climate comes from the Greek word klima, meaning leaning or inclination. The
term is commonly defined as the weather averaged over a long period of time. The standard
averaging period is 30 years, but other periods may be used depending on the purpose. Climate also
includes statistics other than the average, such as the magnitudes of day-to-day or year-to-year
variations.
According to Intergovernmental Panel on Climate Change (IPCC) they define the climate as the
"average weather," or more rigorously, as the statistical description in terms of the mean and
variability of relevant quantities over a period of time ranging from months to thousands or millions
of years. The classical period is 30 years, as defined by the World Meteorological Organization
(WMO). These quantities are most often surface variables such as temperature, precipitation, and
wind. Climate in a wider sense is the state, including a statistical description, of the climate system.
Some people tend to use interchangeably the climate and weather. However these two terms are
different. The difference between climate and weather is usefully summarized by the popular phrase
"Climate is what you expect, weather is what you get." The climate is controlled by different
factors. One of the factor controlling the climate change is the green house gas. In this lecture we
will discuss the usefulness of this green house gas and the effect of missing these gases to our life.
8.2:
Greenhouse Gases
Greenhouse gases are gases in an atmosphere that absorb and emit radiation within the thermal
infrared range. This process is the fundamental cause of the greenhouse effect. Common greenhouse
gases in the Earth's atmosphere include water vapor, carbon dioxide, methane, nitrous oxide, ozone,
and chlorofluorocarbons.
46
In our solar system, the atmospheres of Venus, Mars and Titan also contain gases that cause
greenhouse effects. Greenhouse gases, mainly water vapor, are essential to helping determine the
temperature of the earth; without them this planet would likely be so cold as to be uninhabitable.
Although many factors such as the sun and the water cycle are responsible for the Earth's weather
and energy balance, if all else was held equal and stable, the planet's average temperature should be
considerably lower without greenhouse gases. In the absence of the greenhouse effect and an
atmosphere, the Earth's average surface temperature of 14 °C could be as low as −18 C. Could we
survive at this very low temperature? How do we counter balance the greenhouse gases? We will
come to this after learning the mechanism used by the earth to control his temperature.
We have learned that the energy is coming from the sun. The Earth receives energy from the Sun
mostly in the form of visible light. The atmosphere is almost transparent to visible light, so that
about 50% of the sun's energy reaches the Earth and is absorbed by the surface. Like all bodies with
a temperature above absolute zero the Earth's surface radiates energy in the infrared range.
Greenhouse gases absorb infrared radiation and pass the absorbed heat to other atmospheric gases
through molecular collisions. The greenhouse gases also radiate in the infrared range. Radiation is
emitted both upward, with part escaping to space, and downward toward Earth's surface. The
surface and lower atmosphere are warmed by the part of the energy that is radiated downward,
making our life on earth possible.
Earth's radiation balance is the equation of the incoming and outgoing thermal radiation which can
be illustrated in terms of mathematical model. Though it is complicated and beyond the scope of
this module, but it worth for understanding. That the incoming solar radiation is short wave,
therefore the equation below is called the short wave radiation balance Qs:
Qs = G - R = D + H - R or depending on the albedo (back-reflection to space): = G (1 - a)
•
G = global radiation
•
D = direct radiation
•
H = diffuse radiation
•
R = reflected portion of global radiation (ca. 4%)
•
a = albedo
47
The Earth's surface and atmosphere emits heat radiation (in the infrared spectrum). There is little
overlap between this and the solar spectrum. Since this is long wave radiation, this formula also is
known as the long wave radiation balance (Ql):
Ql = AE = AO - AG
•
AE = effective radiation
•
AO = radiation of the Earth's surface
•
AG = trapped radiation (radiation forcing, also known as the so called greenhouse effect)
From those two equations for incoming and outgoing radiation, the total amount of energy now can
be calculated (total radiation balance (Qt), net radiation):
Qt = Qs - Ql = G - R - AE
The difficulty is to accurately quantify a range of internal and external factors influencing the
radiation balance. Internal factors are all mechanisms affecting atmospheric composition
(volcanism, biological activity, land use change, human activities etc.). The main external factor is
solar radiation. It is interesting to note in this context that over its lifetime the sun's average
luminosity to date has increased by ca. 25%.
Human activities have an impact upon the levels of greenhouse gases in the atmosphere, which has
other effects upon the system, with their own possible repercussions. The 2007 assessment report
compiled by the IPCC observed that "changes in atmospheric concentrations of greenhouse gases
and aerosols, land cover and solar radiation alter the energy balance of the climate system", and
concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have
caused most of the increases in global average temperatures since the mid-20th century"
Recent increases in atmospheric carbon dioxide (CO2). Monthly CO2 measurements display small
seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the
Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as
plants remove some CO2 from the atmosphere.
48
The greenhouse effect was discovered by Joseph Fourier in 1824 and first investigated
quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of
infrared radiation by atmospheric gases warm a planet's lower atmosphere and surface. Existence of
the greenhouse effect as such is not disputed even by those who do not agree that the modern
temperature increase is attributable to human activity. The question is instead of the importance of
the greenhouse gas human activity increases the atmospheric concentrations of greenhouse gases.
Does this have problem?
Practice Question: List all examples of gases considered to be green house gases.
Do we consider O3 to be one of green house gas? Explain.
Class Activity
The external and internal factors are closely interconnected in disturbing the
earth radiation balance. Discuss this statement by giving vivid examples.
References
1. Fredlund, D.G.; Rahardjo, H. (1993), Soil Mechanics for Unsaturated Soils. WileyInterscience.
2. Thornthwaite, C. W. (1948), An Approach Toward a Rational Classification of
Climate, Geographical Review, 38:55-94.
49
9:
9.1:
LECTURE NINE
The Consequences of Green House Gases Imbalance
Introduction
Sometimes we will use the word "greenhouse effect". However the term can be a source of
confusion as actual greenhouses do not function by the same mechanism the atmosphere does.
Various materials at times imply incorrectly that they do, or do not make the distinction between the
processes of radiation and convection.
By definition from The American Heritage Dictionary, indicated this as the phenomenon whereby
the earth’s atmosphere traps solar radiations, caused by the presence in the atmosphere gases such
as carbon dioxide, water vapor, and methane that allow incoming sun light to pass through but
absorb heat radiated back from the earth’s surface.
The term 'greenhouse effect' originally came from the greenhouses used for gardening, but as
mentioned the mechanism for greenhouses operates differently. Many sources make the "heat
trapping" analogy of how a greenhouse limits convection to how the atmosphere performs a similar
function through the different mechanism of infrared absorbing gases. From this definition we can
now discuss the consequences of disturbing the natural balance of these gases. One of the biggest
problem is what it causes the global warming.
9.2:
Global Warming
Naturally occurring greenhouse gases have a mean warming effect of about 33 °C, without which
Earth would be uninhabitable. The major greenhouse gases are water vapor, which causes about 36–
70 percent of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–
26 percent; methane (CH4), which causes 4–9 percent; and ozone, which causes 3–7 percent.
We have discussed earlier that human activity is one of the internal factor which increase the levels
of the green house gases. The question is what is the problem? We know the bigger the blanket the
efficiency is the protection. Let us answer this question in detail.
50
While the greenhouse effect is an essential environmental prerequisite for life on Earth, there really
can be too much of a good thing.
The problems begin when human activities distort and accelerate the natural process by creating
more greenhouse gases in the atmosphere than are necessary to warm the planet to an ideal
temperature. The following are some of the human activities which accelerate levels of these green
house gases:
•
Burning natural gas, coal and oil —including gasoline for automobile engines—raises the level of
carbon dioxide in the atmosphere.
•
Some farming practices and land-use changes increase the levels of methane and nitrous oxide.
Taking practices of burning bushes during farming at your home.
•
Many factories produce long-lasting industrial gases that do not occur naturally, yet contribute
significantly to the enhanced greenhouse effect and “global warming” that is currently under way.
•
Deforestation also contributes to global warming. Trees use carbon dioxide and give off oxygen in
its place, which helps to create the optimal balance of gases in the atmosphere. As more forests
are logged for timber or cut down to make way for farming, however, there are fewer trees to
perform this critical function.
•
Population growth is another factor in global warming, because as more people use fossil fuels for
heat, transportation and manufacturing the level of greenhouse gases continues to increase. As
more farming occurs to feed millions of new people, more greenhouse gases enter the atmosphere.
Therefore, more greenhouse gases mean more infrared radiation trapped and held by the green
house gas, which gradually increases the temperature of the Earth’s surface and the air in the lower
atmosphere.
We should know that since the industrial revolution, there is an increased in the amount of
greenhouse gases in the atmosphere like CO2, methane, tropospheric ozone, CFCs and nitrous
oxide. Consider the number of cars in your city compared to ten years ago. Compare number of
industries in your country compared to those we were having ten years ago. Figure out the increase
in the concentration of gases like CO2. The concentrations of CO2 and methane have increased by
36% and 148% respectively since the mid-1700s. These levels are considerably higher than at any
51
time during the last 650,000 years, the period for which reliable data has been extracted from ice
cores. Less direct geological evidence indicates that CO2 values this high were last seen
approximately 20 million years ago. Fossil fuel burning has produced approximately three-quarters
of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to landuse change, in particular deforestation.
The CO2 concentrations are continuing to rise due to burning of fossil fuels and land-use change.
The future rate of rise will depend on uncertain economic, sociological, technological, and natural
developments. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2
scenarios, ranging from 541 to 970 ppm by the year 2100.
Global warming is the increase in the average temperature of the Earth's near-surface air and oceans
since the mid-twentieth century and its projected continuation. Global surface temperature increased
0.74 ± 0.18 °C during the last century. Great scientists believe that greenhouse gases are responsible
for most of the observed temperature increase since the middle of the twentieth century, and that
natural phenomena such as solar variation and volcanoes probably had a small warming effect from
pre-industrial times to 1950 and a small cooling effect afterward. These basic conclusions have been
endorsed by more than 40 scientific societies and academies of science, including all of the national
academies of science of the major industrialized countries.
Climate model projections summarized in the latest IPCC report indicate that global surface
temperature will probably rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first
century. The uncertainty in this estimate arises from the use of models with differing climate
sensitivity, and the use of differing estimates of future greenhouse gas emissions. Some other
uncertainties include how warming and related changes will vary from region to region around the
globe. Most studies focus on the period up to 2100. However, warming is expected to continue
beyond 2100, even if emissions stop, because of the large heat capacity of the oceans and the long
lifetime of carbon dioxide in the atmosphere.
Increasing global temperature will cause sea levels to rise and will change the amount and pattern of
precipitation, probably including expansion of subtropical deserts. The continuing retreat of glaciers
consider mountain Kilimanjaro, permafrost and sea ice is expected, with the Arctic region being
particularly affected. Other likely effects include shrinkage of the Amazon rainforest and Boreal
52
forests, increases in the intensity of extreme weather events, species extinctions and changes in
agricultural yields.
Study Questions
1.
If the temperature of the earth continue to increase what will happen to island of
Zanzibar and Mafia?
2. Discuss in detail the effect of global warming on agricultural sector.
3. Does global warming has anything to do with increase in population in the city
like Dar es Salaam? Discuss.
References
1.
http://environment.about.com/od/globalwarming/a/greenhouse.htm
2.
The American Heritage dictionary of the English language, 4th ed, Houghton Mifflin
Company.
3.
http://en.wikipedia.org/wiki/Joseph_Fourier
53
10:
LECTURE TEN
10.1: How Does the Earth Balance the Radiations
Introduction
The radiation equilibrium of the Earth system is an accounting of the received and leaving
components of radiation. These components are balanced over long time periods and over the Earth
as whole. If they weren't the Earth would be continually cooling or warming. However, over a short
period of time, radiant energy is unequally distributed over the Earth.
Sunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the
Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is clear as
daylight when the Sun is above the horizon. When the direct radiation is not blocked by clouds, it is
experienced as sunshine, a mixture of bright light and heat. Radiant heat directly produced by the
radiation of the sun is different from the increase in atmospheric temperature due to the radioactive
heating of the atmosphere by the sun's radiation. The World Meteorological Organization defines
sunshine as direct irradiance from the Sun measured on the ground of at least 120 W·m−2.
10.2: Radiation Balance
Direct sunlight has a luminous efficiency of about 93 lumens per watt of radiant flux, which
includes infrared, visible, and ultra-violet light. Sunlight is a key factor in the process of
photosynthesis, crucially important for life on Earth. The Earth receives energy from the Sun in the
form of radiation. Most of the energy is in visible wavelengths and in infrared wavelengths that are
near the visible range (often called "near infrared"). The Earth reflects about 30% of the incoming
solar radiation. The remaining 70% is absorbed, warming the land, atmosphere and ocean.
For the Earth's temperature to be in steady state so that the Earth does not rapidly heat or cool, this
absorbed solar radiation must be very closely balanced by energy radiated back to space in the
infrared wavelengths. Since the intensity of infrared radiation increases with increasing temperature,
54
one can think of the Earth's temperature as being determined by the infrared flux needed to balance
the absorbed solar flux. The visible solar radiation mostly heats the surface, not the atmosphere,
whereas most of the infrared radiation escaping to space is emitted from the upper atmosphere, not
the surface. The infrared photons emitted by the surface are mostly absorbed in the atmosphere by
greenhouse gases and clouds and do not escape directly to space.
The reason this warms the surface is most easily understood by starting with a simplified model of a
purely radiative greenhouse effect that ignores energy transfer in the atmosphere by convection
(sensible heat transport, Sensible heat flux) and by the evaporation and condensation of water vapor
(latent heat transport, Latent heat flux). In this purely radiative case, one can think of the
atmosphere as emitting infrared radiation both upwards and downwards. The upward infrared flux
emitted by the surface must balance not only the absorbed solar flux but also this downward
infrared flux emitted by the atmosphere. The surface temperature will rise until it generates thermal
radiation equivalent to the sum of the incoming solar and infrared radiation.
A more realistic picture taking into account the convective and latent heat fluxes is somewhat more
complex. But the following simple model captures the essence. The starting point is to note that the
opacity of the atmosphere to infrared radiation determines the height in the atmosphere from which
most of the photons are emitted into space. If the atmosphere is more opaque, the typical photon
escaping to space will be emitted from higher in the atmosphere, because one then has to go to
higher altitudes to see out to space in the infrared. Since the emission of infrared radiation is a
function of temperature, it is the temperature of the atmosphere at this emission level that is
effectively determined by the requirement that the emitted flux balance the absorbed solar flux.
But the temperature of the atmosphere generally decreases with height above the surface, at a rate of
roughly 6.5 °C per kilometer on average, until one reaches the stratosphere 10–15 km above the
surface. (Most infrared photons escaping to space are emitted by the troposphere, the region
bounded by the surface and the stratosphere, so we can ignore the stratosphere in this simple
picture.). A very simple model, but one that proves to be remarkably useful, involves the
assumption that this temperature profile is simply fixed, by the non-radiative energy fluxes. Given
the temperature at the emission level of the infrared flux escaping to space, one then computes the
surface temperature by increasing temperature at the rate of 6.5 °C per kilometer, the environmental
lapse rate, until one reaches the surface. The more opaque the atmosphere, and the higher the
emission level of the escaping infrared radiation, the warmer the surface, since one then needs to
55
follow this lapse rate over a larger distance in the vertical. While less intuitive than the purely
radiative greenhouse effect, this less familiar radiative-convective picture is the starting point for
most discussions of the greenhouse effect in the climate modeling literature.
In order to have a picture of what happen in the atmosphere and earth surface we can summarize the
process using the following picture.
SUN
1
9
5
4
8
7
2
3
10
6
EARTH
Fig. 8: Radiation Balance
Let us first have a look what happens to the sunlight.
(1)
The sun is the source of all radiation and energy coming to the Earth from the space.
(2)
A part of the sunlight reaches the Earth surface and all its different landscapes: forests, oceans,
deserts, savannah, cities, ice and snow
(3)
The Earth's surface does not take up all the sunlight, but sends a certain part of it directly back
(reflection). In particular very bright surfaces like ice and snow are excellent reflectors.
(4)
Reflection does not only occur at the Earth surface. Some light is already sent back from the
top side of the clouds and smaller amounts by aerosols.
(5)
Also the uptake of the light (we call it absorption) does not only take place at the Earth
surface. Also gas molecules and particles in the air absorb some sunlight. The portion of the
56
sunlight reaching the Earth warms up its surface. The Earth sends this warmth back as infrared
radiation.
(6)
The Earth's surface warmed up by the sun is a source of heat radiation (long wave infrared
radiation)
(7) A bit of the energy is needed to evaporate water. As you know from kettles, you need energy to
bring water from the liquid phase (e.g. oceans) to the vapour phase (water vapour in the air).
(8) Some infrared radiation goes directly back to the space.
(9) Clouds do not only reflect sunlight, they also absorb and re-emitted infrared radiation back
to the Earth. A cloudy sky keeps the Earth warmer, like a blanket.
(10)
Finally there are particles and gases in the air absorbing the infrared radiation. These gases
are called greenhouse gases. They keep the energy of this heat radiation near the ground.
All this interaction of light with the atmosphere and the surface happens in our climate system and
we have to take them into account, in order to understand the climate. But why do we call it a
greenhouse effect?
Study Questions
1. How do greenhouse gases influence earth's surface temperature?
2. The heat-trapping ability of a greenhouse is influenced by a number of factors
including the transparency of the greenhouse cover, color of the surfaces inside
the greenhouse, and type of surfaces inside. Discuss in detail with examples.
Refer ref. 2 below.
3. What Factors Impact a greenhouse? Refer ref. 3 below.
References
1. http://oceanworld.tamu.edu/resources/oceanography-book/radiationbalance.htm
2. http://www.ucar.edu/learn/1_3_1.htm
3. http://www.ucar.edu/learn/1_3_2_13t.htm
57
11:
LECTURE ELEVEN
11.1: Components of the Atmosphere: Clouds and Aerosols
Introduction
Let us start defining these two terms. The word cloud according to American heritage dictionary is a
visible body of very fine water droplets or ice particles suspended in the atmosphere at attitude
ranging up to several miles above sea level. Therefore a cloud is a visible mass of droplets or
frozen crystals suspended in the atmosphere above the surface of the Earth. A cloud is also a visible
mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae.
Clouds are studied in the nephology or cloud physics branch of meteorology.
On Earth the condensing substance is normally water vapor, which forms small droplets or ice
crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals
they become visible as clouds. Dense deep clouds display a high reflectance (70% to 95%)
throughout the visible range of wavelengths. They thus appear white, at least from the top. Cloud
droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with
depth into the gases, hence the gray or even sometimes dark appearance at the base. Thin clouds
may appear to have acquired the color of their environment or background and clouds illuminated
by non-white light, such as during sunrise or sunset, may appear colored accordingly. In the nearinfrared range, clouds look darker because the water that constitutes the cloud droplets strongly
absorbs solar radiation at those wavelengths.
Fig. 9: Picture of the Clouds
58
On the other hand aerosol is different from the clouds. According to American heritage dictionary
aerosol is a gaseous suspension of fine solid or liquid particles. Technically, an aerosol is a
suspension of fine solid particles or liquid droplets in a gas. Examples are smoke, oceanic haze, air
pollution, smog and CS gas. In general discussion, "aerosol" usually refers to an aerosol spray can
or the output of such a can. The word aerosol derives from the fact that matter "floating" in air is a
suspension (a mixture in which solid or liquid or combined solid-liquid particles are suspended in a
fluid). To differentiate suspensions from true solutions, the term sol evolved—originally meant to
cover dispersions of tiny (sub-microscopic) particles in a liquid. With studies of dispersions in air,
the term aerosol evolved and now embraces both liquid droplets, solid particles, and combinations
of these. Anthropogenic aerosols, particularly sulfate aerosols from fossil fuel combustion, exert a
cooling influence on the climate which partly counteracts the warming induced by greenhouse gases
such as carbon dioxide. This effect is accounted for in many climate models. Recent research, as yet
unconfirmed, suggests that aerosol diffusion of light may have increased the carbon sink in the
earth's ecosystem. The question which environmentalists ask several times is what is the effect of
these natural things to our surroundings? We will go though on this so that we can get picture if
they can affect our environment.
11.2: Aerosols and Clouds
11.2.1:
Clouds
We have learned that cloud is a visible mass of droplets or frozen crystals floating in the atmosphere
above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by
gravity, such as masses of material in space called interstellar clouds and nebulae. On Earth the
condensing substance is typically water vapor, which forms small droplets or ice crystals, typically
0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible
as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range
of wavelengths. They thus appear white, at least from the top. Cloud droplets tend to scatter light
efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the
gray or even sometimes dark appearance at the base. Thin clouds may appear to have acquired the
colour of their environment or background and clouds illuminated by non-white light, such as
during sunrise or sunset, may appear coloured accordingly. In the near-infrared range, clouds look
59
darker because the water that constitutes the cloud droplets strongly absorbs solar radiation at those
wavelengths.
The colour of a cloud, as seen from the Earth, tells much about what is going on inside the cloud.
Clouds form when water vapor is light enough to rise due to becoming warmer than its surrounding.
As it rises it cools and the vapor condenses out of the air as micro-droplets. These tiny particles of
water are densely packed and sunlight cannot penetrate far into the cloud before it is reflected out,
giving a cloud its characteristic white colour. As a cloud matures, the droplets may combine to
produce larger droplets, which may combine to form droplets large enough to fall as rain. By this
process of accumulation, the space between droplets becomes increasingly larger, permitting light
to penetrate farther into the cloud. If the cloud is sufficiently large and the droplets within are
spaced far enough apart, it may be that a percentage of the light which enters the cloud is not
reflected back out before it is absorbed. A simple example of this is being able to see farther in
heavy rain than in heavy fog. This process of reflection/absorption is what causes the range of cloud
colour from white to black. For the same reason, the undersides of large clouds and heavy overcasts
can appear as various degrees of grey shades, depending on how much light is being reflected or
transmitted back to the observer.
Other colours occur naturally in clouds. Bluish-grey is the result of light scattering within the cloud.
In the visible spectrum, blue and green are at the short end of light's visible wavelengths, while red
and yellow are at the long end. The short rays are more easily scattered by water droplets, and the
long rays are more likely to be absorbed. The bluish colour is evidence that such scattering is being
produced by rain-sized droplets in the cloud.
A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud
emitting green is an imminent sign of heavy rain, hail, strong winds and possible tornadoes.
Yellowish clouds are rare but may occur in the late spring through early fall months during forest
fire season. The yellow colour is due to the presence of smoke. Red, orange and pink clouds occur
almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere.
The clouds do not become that colour; they are reflecting long and unscattered rays of sunlight,
which are predominant at those hours. The effect is much like if one were to shine a red spotlight on
a white sheet. In combination with large, mature thunderheads this can produce blood-red clouds.
60
11.2.2:
Aerosols
We can still define aerosols as particulates, alternatively referred to as particulate matter or fine
particles, and are tiny particles of solid or liquid suspended in a gas or liquid. That means from this
definition aerosol refers to particles and the gas together. Sources of particulate matter can be man
made or natural. In water pollution, particulates can either be in a solid or dissolved state. Solid
particulates can be removed by filters or settle from the water, and is referred to as insoluble
particulate matter. Whereas, dissolved particulate matter in water is collected by allowing the water
to evaporate, leaving behind the dissolved particulate matter. Salt is an example of dissolved
particulate matter. Some particulates occur naturally, originating from volcanoes, dust storms, forest
and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil
fuels in vehicles, power plants and various industrial processes also generate considerable amounts
of aerosols. The term anthropogenic aerosols refer to those made by human activities and currently
account for about 10 percent of the total amount of aerosols in our atmosphere.
11.3:
The importance of Clouds and Aerosols to Climate Change
Everything, from an individual person to Earth as a whole, emits energy. Scientists refer to this
energy as radiation. As Earth absorbs incoming sunlight, it warms up. The planet must emit some of
this warmth into space or increase in temperature. Two components make up the Earth's outgoing
energy: heat (or thermal radiation) that the Earth's surface and atmosphere emit; and sunlight (or
solar radiation) that the land, ocean, clouds and aerosols reflect back to space. The balance between
incoming sunlight and outgoing energy determines the planet's temperature and, ultimately, climate.
Both natural and human-induced changes affect this balance, called the Earth's radiation budget.
61
Fig. 10:
Earth's Radiation Balance Between Incoming and Outgoing Radiation
(Adopted from http://earthobservatory.nasa.gov/Features/CALIPSO/CALIPSO2.php)
Clouds affect the radiation budget directly by reflecting sunlight into space (cooling the Earth) or
absorbing sunlight and heat emitted by the Earth. When clouds absorb sunlight and heat, less energy
escapes to space and the planet warms. To understand how clouds impact the energy budget,
environmentalists need to know the composition of cloud particles, the altitude of clouds and the
extent to which clouds at different altitudes overlap each other.
Both natural processes and human activities produce aerosols. They either reflect or absorb energy,
depending on their size, chemical composition and altitude. The haze layer that is commonly seen in
the summertime is one example of an aerosol that primarily reflects sunlight. Soot emitted by diesel
engines is an example of an aerosol that absorbs sunlight. The reflection and absorption of energy
by aerosols act in a direct way to change the balance between incoming and outgoing energy. These
effects are called direct aerosol radiative forcing.
Aerosols also can affect the Earth's radiation budget indirectly by modifying the characteristics of
clouds. Cloud particles almost always form around aerosols such as natural sea salt particles or
human-made sulfate particles. The presence of additional aerosols can change the way clouds
62
radiate energy and the length of time they stay intact. A good example is the way that exhaust
particles emitted into the atmosphere by ships can increase the brightness of clouds along their
course. These effects are called indirect aerosol radiative forcing.
11.4:
Healthy Hazards of the Aerosols
The health effect of the particles is mainly depending on the size of the particle. The range is the
one determine the effect. The size of the particle is a main determinant of where in the respiratory
tract the particle will come to rest when inhaled. Common particles are airborne pollutants that fall
between 2.5 and 10 micrometers in diameter. The effects of inhaling particulate matter have been
widely studied in humans and animals and include asthma, lung cancer, cardiovascular issues, and
premature death. Because of the size of the particle, they can penetrate the deepest part of the lungs.
Larger particles are generally filtered in the nose and throat and do not cause problems, but
particulate matter smaller than about 10 micrometers, referred to as PM10, can settle in the bronchi
and lungs and cause health problems. The 10 micrometer size does not represent a strict boundary
between respirable and non-respirable particles, but has been agreed upon for monitoring of
airborne particulate matter by most regulatory agencies. Similarly, particles smaller than 2.5
micrometers, PM2.5, tend to penetrate into the gas-exchange regions of the lung, and very small
particles (< 100 nanometers) may pass through the lungs to affect other organs. In particular, a
study published in the Journal of the American Medical Association indicates that PM2.5 leads to
high plaque deposits in arteries, causing vascular inflammation and atherosclerosis a hardening of
the arteries that reduces elasticity, which can lead to heart attacks and other cardiovascular
problems. Researchers suggest that even short-term exposure at elevated concentrations could
significantly contribute to heart disease.
63
Study Question and Model Answer:
What is the origin and effect of fossil fuel in the environment
Answer: Fossil fuels are formed by the anaerobic decomposition of remains of organisms including phytoplankton
and zooplankton that settled to the sea (or lake) bottom in large quantities under anoxic conditions, millions of years
ago. Over geological time, this organic matter, mixed with mud, got buried under heavy layers of sediment. The
resulting high levels of heat and pressure caused the organic matter to chemically alter, first into a waxy material known
as kerogen which is found in oil shales, and then with more heat into liquid and gaseous hydrocarbons in a process
known as catagenesis. Terrestrial plants, on the other hand, tend to form coal. Many of the coal fields date to the
Carboniferous period of Earth's history.
According to data, the United States, more than 90% of greenhouse gas emissions come from the combustion of fossil
fuels. Combustion of fossil fuels also produces other air pollutants, such as nitrogen oxides, sulfur dioxide, volatile
organic compounds and heavy metals like Pb, Hg, Cd, Cu just to mention few .
Nitrogen oxides and sulphur dioxide emissions, which contribute to smog and acid rain and the formation of fine
particulate matter. It is the largest uncontrolled industrial source of mercury emissions in parts of developed countries
like Canada and USA. Fossil fuel-fired electric power plants also emit carbon dioxide, which may contribute to climate
change. In addition, the sector has significant impacts on water and habitat and species. In particular, hydro-dams and
transmission lines have significant effects on water and biodiversity.
Fossil fuels also contain radioactive materials, mainly uranium and thorium, which are released into the atmosphere. In
2000, about 12,000 metric tons of thorium and 5,000 metric tons of uranium were released worldwide from burning
coal.
Study Question
When the water evaporates into the sky the water particles form the cloud. There are 10 types of clouds in total
however, there are only three main types, and they are cirrus, stratus, and cumulus. Discuss.
References
1. Hinds, W. C., Aerosol Technology: Properties, Behavior, and Measurement of Airborne
Particles, Wiley-Interscience
2. Lave, L. B.; Eugene P. S. (1973), An Analysis of the Association Between U.S. Mortality
and Air Pollution, J. Amer. Statistical Association 68: 342.
3. Pope, C. A. (2002), Cancer, cardiopulmonary mortality, and long-term exposure to fine
particulate air pollution, J. Amer. Med. Assoc. 287: 1132–1141.
64
12:
LECTURE TWELVE
12.1: Air Pollution and Management
Introduction
We have just learned from the last lecture that particulate matter has a problem that researches
shows that about 1.9% increased risk of dying from cardiovascular disease. Taking example, what
will happen to those people living closer to the rough roads? In Tanzania what can you say about
those people live closer to cement industry like Twiga cement industry or Wazo hill? All these
contribute to Air pollution. Air pollution is the introduction of chemicals, particulate matter, or
biological materials that cause harm or discomfort to humans or other living organisms, or damages
the natural environment, into the atmosphere. Let us have a look on the picture of cement industry
in Texas USA producing dust storm. The same can be observed in our cement industries.
Fig. 11: Air Pollution (Adopted from http://en.wikipedia.org/wiki/File 28/04/2009)
Apart from industries, there are other sources of air pollution. We will discuss few of air pollutants
them and highlight the environmental effect to this pollutants.
12.2: Air Pollutants
An air pollutant is known as a substance in the air that can cause harm to humans and the
environment. Pollutants can be in the form of solid particles, liquid droplets, or gases. In addition,
they may be natural or man-made. Pollutants can be classified as either primary or secondary.
Usually, primary pollutants are substances directly emitted from a process, such as ash from a
65
volcanic eruption, the carbon monoxide gas from a motor vehicle exhaust or sulfur dioxide released
from factories.
Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants
react or interact. An important example of a secondary pollutant is ground level ozone - one of the
many secondary pollutants that make up photochemical smog. Note that some pollutants may be
both primary and secondary: that is, they are both emitted directly and formed from other primary
pollutants.
12.2.1:
Air Major Primary Pollutants
(a) Sulfur oxides (SOx) - Especially sulfur dioxide, a chemical compound with the formula SO2.
Sulfur dioxide is produced by volcanoes and in various industrial processes. Since coal and
petroleum often contain sulfur compounds, their combustion generates sulfur dioxide. Further
oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid
rain. This is one of the causes for concern over the environmental impact of the use of these
fuels as power sources. Sulfur dioxide is a colorless, non-flammable gas that is heavier than air.
It has a strong, suffocating odor that most people can readily identify as one of rotting eggs. It is
a liquid when under pressure, and dissolves in water very easily. Over 65% of this gas is
released in the air, more than 13 million tons per year, comes from electric utilities, especially
those that burn coal. On a global basis, fossil fuel combustion accounts for 75 to 85% of manmade of sulfur dioxide emissions, and industrial processes such as refining and smelting account
for the remainder.
As a gas, sulfur dioxide major route of toxicity is inhalation. However, it is also a strong irritant
to the eyes and moist skin where the gas combines with water to form both sulfuric and
sulfurous acids. Victims with chronic pulmonary disease, particularly asthma, have been shown
to be much more sensitive to the gas at lower concentrations. Children, too, may be at higher
risk because of their greater lung surface area to body weight ratio, increased minute volume to
weight ratio, and shorter structure.
(b) Nitrogen oxides (NOx) - Especially nitrogen dioxide are emitted from high temperature
combustion. Nitrogen dioxide is the chemical compound with the formula NO2. It is one of the
66
several nitrogen oxides. This reddish-brown toxic gas has a characteristic sharp, biting odor.
NO2 is one of the most prominent air pollutants. The toxicity of nitrous oxide (N2O) or
laughing gas, which is used as an anesthetic, is different from that of the other nitrogen oxides.
Children exposed to the same levels of nitrogen oxides as adults may receive larger doses
because they have greater lung surface area:body weight ratios and increased minute volumes:
weight ratios. In addition, they may be exposed to higher levels of nitrogen dioxide than adults
in the same location because of their short figure and the higher levels of nitrogen dioxide found
nearer to the ground. Exposure to relatively high air concentrations can produce eye irritation
and inflammation.
(c) Carbon monoxide - is a colourless, odourless, non-irritating but very poisonous gas. It is a
product by incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust
is a major source of carbon monoxide.
(d) Carbon dioxide (CO2) - a greenhouse gas emitted from combustion but is also a gas very
important to living organisms. It is a natural gas in the atmosphere.
(e) Volatile organic compounds - VOCs are an important outdoor air pollutant. In this field they are
often divided into the separate categories of methane (CH4) and non-methane (NMVOCs).
Methane is an extremely efficient greenhouse gas which contributes to enhance global warming.
Other hydrocarbon VOCs are also significant greenhouse gases via their role in creating ozone
and in prolonging the life of methane in the atmosphere, although the effect varies depending on
local air quality. Within the NMVOCs, the aromatic compounds benzene, toluene and xylene
are suspected carcinogens and may lead to leukemia through prolonged exposure. 1,3-butadiene
is another dangerous compound which is often associated with industrial uses.
(f) Particulate matter - Particulates, alternatively referred to as particulate matter (PM) or fine
particles, are tiny particles of solid or liquid suspended in a gas. In contrast, aerosol refers to
particles and the gas together. Sources of particulate matter can be man made or natural. Some
particulates occur naturally, originating from volcanoes, dust storms, forest and grassland fires,
living vegetation, and sea spray. Human activities, such as the burning of fossil fuels in vehicles,
power plants and various industrial processes also generate significant amounts of aerosols.
Averaged over the globe, anthropogenic aerosols—those made by human activities—currently
account for about 10 percent of the total amount of aerosols in our atmosphere. Increased levels
67
of fine particles in the air are linked to health hazards such as heart disease, altered lung
function and lung cancer.
(f) Toxic metals, such as lead, cadmium and copper.
(g) Chlorofluorocarbons (CFCs) - harmful to the ozone layer emitted from products currently
banned from use.
(h) Ammonia (NH3) - emitted from agricultural processes. Ammonia is a compound with the
formula NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia
contributes significantly to the nutritional needs of terrestrial organisms by serving as a
precursor to foodstuffs and fertilizers. Ammonia, either directly or indirectly, is also a building
block for the synthesis of many pharmaceuticals. Although in wide use, ammonia is both caustic
and hazardous.
(i) Odors - such as from garbage, sewage, and industrial processes
(j) Radioactive pollutants - produced by nuclear explosions, war explosives, and natural processes
such as the radioactive decay of radon.
12.2.2:
Air Major Secondary Pollutants
(a) Particulate matter formed from gaseous primary pollutants and compounds in photochemical
smog. Smog is a kind of air pollution; the word "smog" is a portmanteau of smoke and fog. Classic
smog results from large amounts of coal burning in an area caused by a mixture of smoke and sulfur
dioxide. Modern smog does not usually come from coal but from vehicular and industrial emissions
that are acted on in the atmosphere by sunlight to form secondary pollutants that also combine with
the primary emissions to form photochemical smog. A good example is fossil fuels or mineral fuels.
These are fuels formed by the anaerobic decomposition of buried dead organisms that lived up to
300 million years ago. These fuels contain high percentage of carbon and hydrocarbons.
Fossil fuels range from volatile materials with low carbon:hydrogen ratios like methane, to liquid
petroleum to nonvolatile materials composed of almost pure carbon, like anthracite coal. Methane
can be found in hydrocarbon fields, alone, associated with oil, or in the form of methane clathrates.
It is generally accepted that they formed from the fossilized remains of dead plants and animals by
exposure to heat and pressure in the Earth's crust over hundreds of millions of years.
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Ground level ozone (O3) formed from NOx and VOCs. Ozone (O3) is a key constituent of the
troposphere (it is also an important constituent of certain regions of the stratosphere commonly
known as the Ozone layer). Photochemical and chemical reactions involving it drive many of the
chemical processes that occur in the atmosphere by day and by night. At abnormally high
concentrations brought about by human activities (largely the combustion of fossil fuel), it is a
pollutant, and a constituent of smog.
Peroxyacetyl nitrate (PAN) - similarly formed from NOx and VOCs. Peroxyacyl nitrates, or PANs,
are powerful respiratory and eye irritants present in photochemical smog. They are formed from a
peroxyacyl radical and nitrogen dioxide, for example peroxyacetyl nitrate, CH3COOONO2:
Hydrocarbons + O2 + NO2 + light → CH3COOONO2
The general equation is;
CxHyO3 + NO2 → CxHyO3NO2
PANs are both toxic and irritating, as they dissolve more readily in water than ozone. They are
lachrymators, causing eye irritation at concentrations of only a few parts per billion. At higher
concentrations they cause extensive damage to vegetation. Both PANs and their chlorinated
derivates are said to be mutagenic, as they can be a factor causing skin cancer.
PANs are secondary pollutants, which mean they are not directly emitted as exhaust from power
plants or internal combustion engines, but they are formed from other pollutants by chemical
reactions in the atmosphere. Free radical reactions catalyzed by ultraviolet light from the sun
oxidize unburned hydrocarbons to aldehydes, ketones, and dicarbonyl compounds, whose secondary
reactions create peroxyacyl radicals, which combine with nitrogen dioxide to form peroxyacyl
nitrates. The most common peroxyacyl radical is peroxyacetyl, which can be formed from the free
radical oxidation of acetaldehyde, various ketones, or the photolysis of dicarbonyl compounds such
as methylglyoxal or diacetyl. Since they dissociate quite slowly in the atmosphere into radicals and
NO2, PANs are able to transport these unstable compounds far away from the urban and industrial
origin. This is important for tropospheric ozone production as PANs transport NOx to regions
where it can more efficiently produce ozone.
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12.2.3:
(a)
Air Minor pollutants
A large number of minor hazardous air pollutants. Some of these are regulated in USA under the
Clean Air Act and in Europe under the Air Framework Directive.
(b)
A variety of persistent organic pollutants, which can attach to particulate matter. Persistent
organic pollutants (POPs) are organic compounds that are resistant to environmental
degradation through chemical, biological, and photolytic processes. Because of this, they have
been observed to persist in the environment, to be capable of long-range transport,
bioaccumulate in human and animal tissue, biomagnify in food chains, and to have potential
significant impacts on human health and the environment. Example of compounds that make up
POPs are also classed as PBTs (Persistent, Bioaccumulative and Toxic) or TOMPs (Toxic
Organic Micro Pollutants). Many POPs are currently or were in the past used as pesticides.
Others are used in industrial processes and in the production of a range of goods such as
solvents, polyvinyl chloride, and pharmaceuticals. Though there are a few natural sources of
POPs, most POPs are created by humans in industrial processes, either intentionally or as
byproducts. Exposure to POPs can take place through diet, environmental exposure, or
accidents. POP exposure can cause death and illnesses including disruption of the endocrine,
reproductive, and immune systems; neurobehavioral disorders; and cancers possibly including
breast cancer. The lipid solubility of POPs allows them to bioaccumulate in fatty tissues of
animals. Many of the first generation organochlorine insecticides such as DDT were particularly
noted for this characteristic.
Study Question
(i)
Carbon monoxide (CO) is one of the known highly toxic gas. Discuss the sources and the
biological toxicity of this gas.
(ii)
Draw the general formula of peroxyacetyl nitrate.
References:
1
http://www.atsdr.cdc.gov/mhmi/mmg175.html
2
http://209.85.229.132/custom?q=cache:t5NPaOyWlMQJ:www.bioterrorism.slu.edu/bt/products/ahec_chem/scripts/
SulfurDioxide.pdf+toxicity+of+sulfur+dioxide&cd=1&hl=en&ct=clnk&client=pub
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13:
LECTURE THIRTEEN
13.1: Water Pollution and Management
Introduction
Water pollution is a major problem in the global context. It has been suggested that it is the leading
worldwide cause of deaths and diseases, and that it accounts for the deaths of more than 14,000
people daily. In addition to the acute problems of water pollution in developing countries like
Tanzania or industrialized countries continue to struggle with pollution problems as well. In the
most recent national report on water quality in the Tanzania shows that the country is blessed by
having big fresh water lakes and rivers although these water sources are not evenly distributed.
There are areas therefore where water is scarce while in other areas there is enough water.
Environmental degradation due to water pollution and poor land management affects water source
availability, suitability and sustainability. Water is typically referred to as polluted when it is
impaired by anthropogenic contaminants and either does not support a human use, like serving as
drinking water, and/or undergoes a marked shift in its ability to support its constituent biotic
communities, such as fish. Natural phenomena such as volcanoes, algae blooms, storms, and
earthquakes also cause major changes in water quality and the ecological status of water. Water
pollution has many causes and characteristics.
13.2: Water in the Atmosphere
Water vapour also called aqueous vapour is the gas phase of water. Water vapor is one state of the
water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling
of liquid water or from the sublimation of ice. Under normal atmospheric conditions, water vapor is
continuously generated by evaporation and removed by condensation.
Whenever a water molecule leaves a surface, it is said to have evaporated. Each individual water
molecule which transitions between a more associated (liquid) and a less associated (vapor/gas)
state does so through the absorption or release of kinetic energy. The aggregate measurement of this
kinetic energy transfer is defined as thermal energy and occurs only when there is differential in the
temperature of the water molecules. Liquid water that becomes water vapor takes a parcel of heat
71
with it, in a process called evaporative cooling. The amount of water vapor in the air determines
how fast each molecule will return back to the surface. When a net evaporation occurs, the body of
water will under go a net cooling directly related to the loss of water.
Evaporative cooling is restricted by atmospheric conditions. Humidity is the amount of water vapor
in the air. The vapor content of air is measured with devices known as hygrometers. The
measurements are usually expressed as specific humidity or percent relative humidity. The
temperatures of the atmosphere and the water surface determine the equilibrium vapor pressure;
100% relative humidity occurs when the partial pressure of water vapor is equal to the equilibrium
vapor pressure. This condition is often referred to as complete saturation. Humidity ranges from 0
gram per cubic metre in dry air to 30 grams per cubic metre when the vapor is saturated at 30 °C.
Another form of evaporation is sublimation, by which water molecules become gaseous directly
from ice without first becoming liquid water. Sublimation accounts for the slow mid-winter
disappearance of ice and snow at temperatures too low to cause melting.
Water vapor will only condense onto another surface when that surface is cooler than the
temperature of the water vapor, or when the water vapor equilibrium in air has been exceeded.
When water vapor condenses onto a surface, a net warming occurs on that surface. The water
molecule brings a parcel of heat with it. In turn, the temperature of the atmosphere drops slightly. In
the atmosphere, condensation produces clouds, fog and precipitation (usually only when facilitated
by cloud condensation nuclei). The dew point of an air parcel is the temperature to which it must
cool before water vapor in the air begins to condense.
Also, a net condensation of water vapor occurs on surfaces when the temperature of the surface is at
or below the dew point temperature of the atmosphere. Deposition, the direct formation of ice from
water vapor, is a type of condensation. Frost and snow are examples of deposition.
13.2: Physical Properties of Water in the Atmosphere
The molecular mass or weight of water is 18.02g/mol, as calculated from the sum of the atomic
masses of its constituent atoms. The average molecular mass of air (Approx. 79% nitrogen, N2; 21%
Oxygen, 02) is 28.57g/mol at standard temperature and pressure (STP). Using Avogadro's Law and
the ideal gas law, water vapor and air will have a molar volume of 22.414 litre/mol at STP. A molar
72
mass of air and water vapor occupy the same volume of 22.414 litres. The density (mass/volume) of
water vapor is 0.804g/litre, which is significantly less than that of dry air at 1.27g/litre at STP.
Note that STP conditions include a temperature of 0°C, at which the ability of water to become
vapor is very restricted. Its concentration in air is very low at 0°C. The red line on the chart to the
right is the maximum concentration of water vapor expected for a given temperature. The water
vapor concentration increases significantly as the temperature rises, approaching 100% (steam, pure
water vapor) at 100°C. However the difference in densities between air and water vapor would still
exist.
13.3: Chemical Properties of Water in the Atmosphere
A chemical property is any of a material's properties that becomes evident during a chemical
reaction; that is, any quality that can be established only by changing a substance's chemical
identity. Simply speaking, chemical properties cannot be determined just by viewing or touching the
substance; the substance's internal structure must be affected for its chemical properties to be
investigated.
Water is a single chemical compound whose molecules consist of two hydrogen atoms attached to
one oxygen atom. The chemical formula of this compound is H2O. Considering that a hydrogen
atom weighs only about one-sixteenth as much as an oxygen atom, most of the weight in water is
due to oxygen: 88.8 percent of the weight is oxygen and 11.2 percent is hydrogen. This percentage
remains the same from a single water molecule to a lake full of water molecules.
Water can be made (synthesized) from hydrogen and oxygen, both of which are gases. When these
two gases are mixed, however, they do not react unless the reaction is started with a flame or spark.
Then they react with explosive violence. The tremendous energy that is released is a signal that
water is an extremely stable compound. It is hard to break a water molecule apart into its
components.
The normal boiling point of water is 100°C and its freezing point is 0°C. As water is cooled to make
ice, it becomes slightly denser, like all liquids. But at 4°C, it reaches its maximum density. When
cooled below that temperature, it becomes less dense. At 0°C, water freezes and expands. Since ice
is less dense than water, ice floats on it.
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In pure water, 1 out of every 555 million molecules is broken down into a hydrogen ion and a
hydroxide ion (an ion is an electrically charged atom or group of atoms). These ions are enough to
make water a slight conductor of electricity. That is why water is dangerous when there is electricity
around.
Because water dissolves so many substances (it is called the universal solvent), all of the water on
Earth is in the form of solutions. Water can react with many things in the atmosphere to produce
different compounds. Some of the compounds formed and there effect in the environment is acid
rain. In the following lecture we will discuss the formation and effect of acid rain to our
environment.
Study Question
Discuss in detail the difference between smog and water vapor
References:
1. Chang, R. (1991), Chemistry, 4th ed McGraw-Hill, Inc
2. Manahan S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher
3. Wright, J. (2005), Environmental Chemistry, Taylor & Francis e-Library. NY.
4. Petrucci, H. and Harwood, W. S. (1997), General Chemistry, Principles and Modern
Application, 7th ed, Prentice hall Upper Saddle River, New Jersey.
74
14:
LECTURE FOURTEEN
14.1: Acid rain and the Effect to the Environment
Introduction
An acid a word from the Latin word acidus meaning sour is traditionally considered as any
chemical compound that, when dissolved in water, gives a solution with a hydrogen ion with a pH
less than 7.0. other scientists like Johannes Nicolaus Brønsted and Martin Lowry, defined an acid as
a compound which donates a hydrogen ion (H+) to another compound (called a base). The following
is the reaction to describe what we just say:
O
O
C
R
H
O
+
H
O
H
C
R
O
+
OH3
O
R
C
O
H
O
+
NH3
R
C
+
O
NH4
You can just see how acid donate the Hydrogen ion (H+).
Another scientist is A Arrhenius who defines an acid as a molecule which increases the
concentration of the hydronium (H3O+) ion when dissolved in water. On the other hand an
Arrhenius base is a molecule which increases the concentration of the hydroxide ion when dissolved
in water. A third concept was proposed by Gilbert N. Lewis which includes reactions with acid-base
characteristics that do not involve a proton transfer. A Lewis acid is a species that accepts a pair of
electrons from another species; in other words, it is an electron pair acceptor. Brønsted acid-base
reactions are proton transfer reactions while Lewis acid-base reactions are electron pair transfers.
Example of reaction showing Lewis acid is showing in the reaction below:
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In the first reaction a fluoride ion, F-, donate an electron pair to boron trifluoride to form the
product tetrafluoroborate. Fluoride "loses" a pair of valence electrons because the electrons shared
in the B—F bond are located in the region of space between the two atomic nuclei and are therefore
more distant from the fluoride nucleus than they are in the lone fluoride ion. BF3 is a Lewis acid
because it accepts the electron pair from fluoride. Common examples of an acid include acetic acid
(in vinegar) and sulfuric acid (used in car batteries). We have learned earlier that water moisture is
present in the atmosphere. Therefore, it is wise to say acid is possible to be formed in the
atmosphere. That’s where the concept of acid rain comes. Let us discuss in detail about this acid
rain and the consequences it has on our environment.
14.2:
Acid Rain and its Source
"Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog and cloudwater, dew) and dry (acidifying particles and gases) acidic components. A more accurate term is
“acid deposition”. Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids
with a pH less than 7 are acidic, and those with a pH greater than 7 are bases. “Clean” or unpolluted
rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together
to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains
other chemicals.
H2O (l) + CO2 (g) → H2CO3 (aq)
Carbonic acid then can ionize in water forming low concentrations of hydronium ions:
2H2O (l) + H2CO3 (aq)
CO32- (aq) + 2H3O+(aq)
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The most important gas which leads to acidification is sulfur dioxide. Emissions of nitrogen oxides
which are oxidized to form nitric acid are of increasing importance due to stricter controls on
emissions of sulfur containing compounds. The form of SO2 comes from fossil fuel combustion and
industry, or from wildfires and also from volcanoes. There are several sources of the acidity rain.
These are:
14.2.1:
Human Activity
The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as
electricity generation, factories, and motor vehicles. Coal power plants are one of the most
polluting. The gases can be carried hundreds of km in the atmosphere before they are converted to
acids and deposited. In the past, factories had short funnels to let out smoke, but this caused many
problems locally; thus, factories now have taller smoke funnels. However, dispersal from these
taller stacks causes pollutants to be carried farther, causing widespread ecological damage.
14.2.2:
Chemical Processes
In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an
intermolecular reaction:
SO2 + OH· → HOSO2· which is followed by:
HOSO2· + O2 → HO2· + SO3
In the presence of water, sulfur trioxide (SO3) is converted rapidly to sulfuric acid:
SO3(g) + H2O(l) → H2SO4(l)
Nitric acid is formed by the reaction of OH with nitrogen dioxide:
NO2 + OH· → HNO3
14.3: The Effect of Acid Rain in the Environment
Acid rain has been shown to have adverse impacts on forests, freshwaters and soils, killing insect
and aquatic life-forms as well as causing damage to buildings and having impacts on human health.
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14.3.1:
Effect on Surface Waters and Aquatic Animals
Both the lower pH and higher aluminum concentrations in surface water that occur as a result of
acid rain can cause damage to fish and other aquatic animals. At pHs lower than 5 most fish eggs
will not hatch and lower pHs can kill adult fish. As lakes and rivers become more acidic
biodiversity is reduced. Acid rain has eliminated insect life and some fish species, including the
brook trout in some lakes, streams, and creeks in geographically sensitive areas, such as the
Adirondack Mountains of the United States. However, the extent to which acid rain contributes
directly or indirectly via runoff from the catchments to lake and river acidity (i.e., depending on
characteristics of the surrounding watershed) is variable.
14.3.2:
Effect on Soils
Soil biology and chemistry can be seriously damaged by acid rain. Some microbes are unable to
tolerate changes to low pHs and are killed. The enzymes of these microbes are denatured (changed
in shape so they no longer function) by the acid. The hydronium ions of acid rain also mobilize
toxins, e.g. aluminium, and leach away essential nutrients and minerals.
2H+ (aq)+ Mg2+ (clay)
2H+ (clay)+ Mg2+(aq)
Soil chemistry can be dramatically changed when base cations, such as calcium and magnesium, are
leached by acid rain thereby affecting sensitive species, such as sugar maple (Acer saccharum).
14.3.3: Effect on Forests and other Vegetation
Fig. 12:
Effect of acid rain on a forest, Jizera Mountains, Czech Republic
78
Adverse effects may be indirectly related to acid rain, like the acid's effects on soil (see above) or
high concentration of gaseous precursors to acid rain. High altitude forests are especially vulnerable
as they are often surrounded by clouds and fog which are more acidic than rain.
Other plants can also be damaged by acid rain but the effect on food crops is minimized by the
application of lime and fertilizers to replace lost nutrients. In cultivated areas, limestone may also be
added to increase the ability of the soil to keep the pH stable, but this tactic is largely unusable in
the case of wilderness lands. When calcium is leached from the needles of red spruce, these trees
become less cold tolerant and exhibit winter injury and even death.
14.3.4:
Effect on Human Health
Fine particles, a large fraction of which are formed from the same gases as acid rain (sulfur dioxide
and nitrogen dioxide), have been shown to cause illness and premature deaths such as cancer and
other diseases. For more information on the health effects of aerosols see particulate health effects.
14.3.5:
Other Adverse Effects
Acid rain can also cause damage to certain building materials and historical monuments. This
results when the sulfuric acid in the rain chemically reacts with the calcium compounds in the
stones (limestone, sandstone, marble and granite) to create gypsum, which then flakes off.
CaCO3 (s) + H2SO4 (aq)
CaSO4 (aq) + CO2 (g) + H2O (l)
This result is also commonly seen on old gravestones where the acid rain can cause the inscription
to become completely illegible. Acid rain also causes an increased rate of oxidation for iron.
Visibility is also reduced by sulfate and nitrate aerosols and particles in the atmosphere.
79
Study Questions:
1. With vivid equations differentiate between Arrhenius acid, Lewis acid and
Brønsted acid.
2.
In terms of electron losing and gaining define the terms oxidation and reduction
process. Using equations of definition of acid, show which specie is oxidizing
agent and which one is reducing agent.
References:
1. Chang, R. (1991), Chemistry, 4th ed McGraw-Hill, Inc
2. Manahan S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher
3. Wright, J. (2005), Environmental Chemistry, Taylor & Francis e-Library. NY.
4. Petrucci, H. Harwood, W. S. (1997), General Chemistry, Principles and Modern Application, 7th
ed, Prentice hall Upper Saddle River, New jersey.
5. http://en.wikipedia.org/wiki/File:Acid-base.png
6. Ebbing, D. D., & Gammon, S. D. (2005). General chemistry, 8th ed, Boston, MA: Houghton
Mifflin
80
15:
15.1:
LECTURE FIFTEEN
Soil and Ground Water Pollution Management
Introduction
Until the late 1960s, soil chemistry focused primarily on chemical reactions in the soil that
contribute to affect of plant growth. Since then concerns have grown about environmental pollution,
organic and inorganic soil contamination and potential ecological health and environmental health
risks. Consequently, the emphasis in soil chemistry has shifted from pedology and agricultural soil
science to an emphasis on environmental soil science.
Knowledge of environmental soil chemistry is paramount to predicting the fate, mobility and
potential toxicity of contaminants in the environment. The vast majority of environmental
contaminants are initially released to the soil. Once a chemical is exposed to the soil environment a
myriad of chemical reactions can occur that may increase/decrease contaminant toxicity. These
reactions include adsorption/desorption, precipitation, polymerization, dissolution, complexation,
and oxidation/reduction. These reactions are often disregarded by scientists and engineers involved
with environmental remediation. Understanding these processes enable us to better predict the fate
and toxicity of contaminants and provide the knowledge to develop scientifically correct and costeffective remediation strategies.
Contamination of water and soil is one of the critical issue in our country. There are several laws
and bylaws to protect these systems. Despite the questions that have ensued concerning the
strictness and perhaps the inappropriateness of some of the regulations contained in environmental
laws, the fact remains that the public is very concerned about the quality of the environment. They
have expressed an overwhelming willingness to spend substantial tax shillings to ensure a clean and
safe environment.
15.2: Soil and Groundwater
This is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic
formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a
usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become
81
completely saturated with water is called the water table. Groundwater is recharged from, and
eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps, and
can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal and
industrial use by constructing and operating extraction wells. The study of the distribution and
movement of groundwater is hydrogeology, also called groundwater hydrology.
Typically, groundwater is thought of as liquid water flowing through shallow aquifers, but
technically it can also include soil moisture, permafrost (frozen soil), immobile water in very low
permeability bedrock, and deep geothermal or oil formation water. Groundwater is hypothesized to
provide lubrication that can possibly influence the movement of faults. It is likely that much of the
Earth's subsurface contain some water, which may be mixed with other fluids in some instances.
Groundwater may not be confined only to the Earth. The formation of some of the landforms
observed on Mars may have been influenced by groundwater.
An aquifer is a layer of relatively porous substrate that contains and transmits groundwater. When
water can flow directly between the surface and the saturated zone of an aquifer, the aquifer is
unconfined. The deeper parts of unconfined aquifers are usually more saturated since gravity causes
water to flow downward.
The upper level of this saturated layer of an unconfined aquifer is called the water table or phreatic
surface. Below the water table, where generally all pore spaces are saturated with water is the
phreatic zone. Substrate with relatively low porosity that permits limited transmission of
groundwater is known as an aquitard. An aquiclude is a substrate with porosity that is so low it is
virtually impermeable to groundwater.
A confined aquifer is an aquifer that is overlain by a relatively impermeable layer of rock or
substrate such as an aquiclude or aquitard. If a confined aquifer follows a downward grade from its
recharge zone, groundwater can become pressurized as it flows. This can create artesian wells that
flow freely without the need of a pump and rise to a higher elevation than the static water table at
the above, unconfined, aquifer.
The characteristics of aquifers vary with the geology and structure of the substrate and topography
in which they occur. Generally, the more productive aquifers occur in sedimentary geologic
formations. By comparison, weathered and fractured crystalline rocks yield relatively smaller
quantities of groundwater in many environments. Unconsolidated to poorly cemented alluvial
82
materials that have accumulated as valley-filling sediments in major river valleys and geologically
subsiding structural basins are included among the most productive sources of groundwater.
The high specific heat capacity of water and the insulating effect of soil and rock can mitigate the
effects of climate and maintain groundwater at a relatively steady temperature. In some places
where groundwater temperatures are maintained by this effect at about 10°C, groundwater can be
used for controlling the temperature inside structures at the surface. For example, during hot
weather relatively cool groundwater can be pumped through radiators in a home and then returned
to the ground in another well. During cold seasons, because it is relatively warm, the water can be
used in the same way as a source of heat for heat pumps that is much more efficient than using air.
The relatively constant temperature of groundwater can also be used for heat pumps. Groundwater
makes up about twenty percent of the world's fresh water supply, which is about 0.61% of the entire
world's water, including oceans and permanent ice.
Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when
this recharge reaches the water table. It is estimated that the volume of groundwater comprises
30.1% of all freshwater resource on earth compared to 0.3% in surface freshwater; the icecaps and
glaciers are the only larger sources of fresh water on earth at 68.7%.
15.3: Contamination of Water and Soil
There are a number of inorganic and organic contaminants that are important in water and soil.
These include plant nutrients such as nitrate and phosphate; heavy metals such as cadmium,
chromium, and lead; oxyanions such as arsenite, arsenate, and selenite; organic chemicals;
inorganic acids; and radionuclides. The sources of these contaminants include fertilizers, pesticides,
acidic deposition, agricultural and industrial waste materials, and radioactive fallout. We can just
find out human activity play key role in the contamination of water and soil. Let us now start one by
one looking on how different sources are contaminated.
15.3.1: Groundwater Pollution
Interactions between groundwater and surface water are complex. Consequently, groundwater
pollution, sometimes referred to as groundwater contamination, is not as easily classified as surface
water pollution. By its very nature, groundwater aquifers are susceptible to contamination from
83
sources that may not directly affect surface water bodies, and the distinction of point vs. non-point
source may be irrelevant. Ground water contamination is the result of polluted water infiltrating
through the soil and rock and eventually reaching the ground water. This process might take many
years and might take place at a distance from the well where the contamination is found. Once the
ground water is contaminated, it is very difficult to remediate. No doubt that the new technologies
will always reduce the pollution level. But the underground water quality in this basin based on
various factors, influx of industrial effluent, influx of water through rainfall, soil, agriculture pattern
etc., so we can say that by these factors, the underground water quality can be varied qualitatively
and quantitatively.
A spill of a chemical contaminant on soil, located away from a surface water body, may not
necessarily create point source or non-point source pollution, but nonetheless may contaminate the
aquifer below. Analysis of groundwater contamination may focus on soil characteristics and
hydrology, as well as the nature of the contaminant itself.
15.3.2: Surface and Ground Water Quality Situation: Tanzania Case
Natural mineralization as in the case of occurrence of high concentrations of fluoride in water in
some parts of Arusha, Kilimanjaro, Singida, Shinyanga and other areas is an example of natural
factors. An interpretation of the data obtained from the sampling of boreholes, protected springs
open springs, streams, ponds around the country indicates that boreholes are the best water sources
except for natural mineralization as in the case with many borehole with high fluoride concentration
or salinity, or if the beneficiaries pollute during water drawing in the case of open wells. The quality
of drinking water in the investigated sources in the country reveals how the sources are as indicated
in the following table.
Table 1: Types of Water Sources in Tanzania
Water sources
Description
Boreholes
Suitable for use without treatment provided pump base and
sealants are firmly intact.
Rain water and protected
Suitable as well
Springs and impoundments
Moderate quality depending on the location
Streams, rivers, lakes, open Worst in quality, thus need treatment
wells
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15.3.3: Pollution of Water Resources
There are two basic types of pollution—point and nonpoint. Point pollution is contamination that
can be traced to a particular source such as an industrial site, septic tank, or wastewater treatment
plant. Nonpoint pollution results from large areas and not from any single source and includes both
natural and human activities. Sources of nonpoint pollution include agricultural, human, forestry,
urban, construction, and mining activities and atmospheric deposition. There are also naturally
occurring nonpoint source pollutants that are important. These include geologic erosion, saline
seeps, and breakdown of minerals and soils that may contain large quantities of nutrients.
The specific contaminants leading to pollution in water include a wide spectrum of chemicals,
pathogens, and physical or sensory changes such as elevated temperature and discoloration. While
many of the chemicals and substances that are regulated may be naturally occurring (calcium,
sodium, iron, manganese, etc.) the concentration is often the key in determining what is a natural
component of water, and what is a contaminant.
Oxygen-depleting substances may be natural materials, such as plant matter (e.g. leaves and grass)
as well as man-made chemicals. Other natural and anthropogenic substances may cause turbidity
(cloudiness) which blocks light and disrupts plant growth, and clogs the gills of some fish species.
Many of the chemical substances are toxic. Pathogens can produce waterborne diseases in either
human or animal hosts. Alteration of water’s physical chemistry includes acidity (change in pH),
electrical conductivity, temperature, and eutrophication. Eutrophication is the fertilization of surface
water by nutrients that were previously scarce.
High levels of pathogens may result from inadequately treated sewage discharges. This can be
caused by a sewage plant designed with less than secondary treatment (more typical in lessdeveloped countries). In developed countries, older cities with aging infrastructure may have leaky
sewage collection systems (pipes, pumps, valves), which can cause sanitary sewer overflows. Some
cities also have combined sewers, which may discharge untreated sewage during rain storms.
Contaminants may include organic and inorganic substances.
85
15.3.4: Organic Water Pollutants:
•
Detergents
•
Disinfection by-products found in chemically disinfected drinking water, such as chloroform
•
Food processing waste, which can include oxygen-demanding substances, fats and grease
•
Insecticides and herbicides, a huge range of organohalides and other chemical compounds
•
Petroleum hydrocarbons, including fuels (gasoline, diesel fuel, jet fuels, and fuel oil) and
lubricants (motor oil), and fuel combustion byproducts, from storm water runoff
•
Tree and brush debris from logging operations
•
Volatile organic compounds (VOCs), such as industrial solvents, from improper storage.
Chlorinated solvents, which are dense non-aqueous phase liquids (DNAPLs), may fall to the
bottom of reservoirs, since they don't mix well with water and are denser.
•
Various chemical compounds found in personal hygiene and cosmetic products
15.3.5: Inorganic Water Pollutants:
•
Acidity caused by industrial discharges (especially sulfur dioxide from power plants)
•
Ammonia from food processing waste
•
Chemical waste as industrial by-products
•
Fertilizers containing nutrients--nitrates and phosphates--which are found in storm-water
runoff from agriculture, as well as commercial and residential use[12]
•
Heavy metals from motor vehicles (via urban storm-water runoff)[12]
[13]
and acid mine
drainage
•
Silt (sediment) in runoff from construction sites, logging, slash and burn practices or land
clearing sites
15.3.6: Macroscopic Pollution
These are visible items polluting the water may be termed "floatables" in an urban storm-water
context, or marine debris when found on the open seas, and can include such items as:
Trash (e.g. paper, plastic, or food waste) discarded by people on the ground, and that are washed by
rainfall into storm drains and eventually discharged into surface waters
Nurdles, small ubiquitous waterborne plastic pellets, shipwrecks, large derelict ships
86
15.3.7: Thermal Pollution
Thermal pollution is the rise or fall in the temperature of a natural body of water caused by human
influence. A common cause of thermal pollution is the use of water as a coolant by power plants
and industrial manufacturers. Elevated water temperatures decreases oxygen levels (which can kill
fish) and affects ecosystem composition, such as invasion by new thermophilic species. Urban
runoff may also elevate temperature in surface waters. Thermal pollution can also be caused by the
release of very cold water from the base of reservoirs into cooler rivers.
15.7: Waste Management as Source of water Pollution Tanzania Case
Waste management is the collection, transport, processing, recycling or disposal, and monitoring of
waste materials. The term usually relates to materials produced by human activity, and is generally
undertaken to reduce their effect on health, the environment or aesthetics. Waste management is
also carried out to recover resources from it. Waste management can involve solid, liquid, gaseous
or radioactive substances, with different methods and fields of expertise for each.
Waste management practices differ for developed and developing nations like Tanzania, for urban
and rural areas, and for residential and industrial producers. Management for non-hazardous
residential and institutional waste in metropolitan areas is usually the responsibility of local
government authorities, while management for non-hazardous commercial and industrial waste is
usually the responsibility of the generator.
Waste collection methods vary widely between different countries and regions. Domestic waste
collection services are often provided by local government authorities, or by private industry. Some
areas, especially those in less developed countries, do not have a formal waste-collection system. In
Dar es salaam city we have management structure of the Dar es Salaam Health department under
City council as shown here:
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Fig. 13:
Waste Management Structure of the Dar es Salaam Health Department
Each of Dar es Salaam’s three districts (Kinondoni, Temeke, and Ilala) has its own day-to-day setup
for solid-waste management, headed by the health officer, after who come the foreperson, the
heads, and the cleaners. All daily operations are based at site offices in the three districts. The
activities of city cleansing are supervised by the Health Standing Committee, which plans,
evaluates, and advises on all matters concerning health, including waste removal and disposal. To
see waste removal as a health issue is perhaps to take a narrow focus, as it subsumes other aspects
of waste generation and management. The concentration on cleansing does not bring into the
forefront important aspects, such as waste generation and recycling. Some times the house hold
waste products are just thrown closer to the premises. Let us see the picture taken at to illustrate the
environmental impact.
88
Fig. 14:
Trash Closer to Living Premises with Colored Discharge Find its Way to
Water/Soil System
15.4: Soil Pollution
Soil pollution is caused by the presence of man-made chemicals or other alteration in the natural soil
environment. This type of contamination typically arises from the rupture of underground storage
tanks, application of pesticides, percolation of contaminated surface water to subsurface strata, oil
and fuel dumping, leaching of wastes from landfills or direct discharge of industrial wastes to the
soil. The most common chemicals involved are petroleum hydrocarbons, solvents, pesticides, lead
and other heavy metals. This occurrence of this phenomenon is correlated with the degree of
industrializations and intensities of chemical usage.
Soil contaminants can have significant toxic consequences for ecosystems. These changes can be
evident in the alteration of metabolism of common microorganisms and arthropods local in a given
soil environment. The result can be virtual abolition of some of the primary food chain, which in
turn have major consequences for predator or consumer species. Even if the chemical effect on
lower life forms is small, the lower pyramid levels of the food chain may ingest alien chemicals,
which normally become more concentrated for each consuming rung of the food chain. Many of
89
these effects are now well known, such as the concentration of persistent DDT materials for avian
consumers, leading to weakening of egg shells, increased chick mortality and potentially species
extinction.
Effects occur to agricultural lands which have certain types of soil contamination. Contaminants
typically alter plant metabolism, most commonly to reduce crop yields. This has a secondary effect
upon soil conservation, since the languishing crops cannot shield the Earth's soil mantle from
erosion phenomena. Some of these chemical contaminants have long half-lives and in other cases
derivative chemicals are formed from decay of primary soil contaminants.
Study Questions:
1. Consider Table 1 above. Discuss each category shown by identifying how they
can be contaminated.
2. What can you tell about contamination of ground water using figure 1 above?
Class Activity:
Discuss in detail the role of the private formal sector in waste management in your city. What do
you think the Government can do the rescue the life of the citizens due to waste around our
houses?
References:
1. http://ipsnews.net/news.asp?idnews=47932
2. Mwanthi, M. A., Nyabola, L. O., (1997), Solid Waste Management in Nairobi City: Knowledge
and Attitudes, Environmental Health Dec. 1997(Dec. 1997): 23-29.
3. Teerlink, H. and Frank, E. D., (1993), The Brown Environment, Employment and Local Agenda
21, Third World Planning Review 15(2): 195-207.
4. Yhdego, M., (1995), Urban solid waste management in Tanzania Issues, concepts and
challenges, Resource, Conservation and Research 14: 1-10.
90
16:
Lecture 16
16.1: Health Problems of Water Pollution
Introduction
People have known for a long time that drinking water can spread disease; however, until it was
understood that microbes in the water were responsible for disease, not much could be done to
prevent waterborne illness. It is not true that the outbreak of diseases due to unsafe water is only in
Africa. Example is in 1854, one of the first incidents occurred that indicated drinking water could
be treated to prevent disease. An outbreak of cholera was raging through several London
neighborhoods and no one understood the cause (cholera is caused by bacteria and one of the
sources for the bacteria is feces, or stool, from infected people). In one of the worst infected
neighborhoods, an individual named John Snow made an interesting observation. We will discuss
these problems in detail in this lecture.
16.2: Unsafe Water a Source of Waterborne Diseases
Several reasons are responsible for the possible contamination of the big water systems, the main
ones being aging infrastructure and most likely poor water treatment practices, which have led to a
dramatic emergence of cholera epidemics and high rate of diarrhea diseases in the country. These
are normally spread when infected human faeces gain access to water. The disease causing
organisms, which may be present in the excreta, are often passively carried into the water. Out
breaks occur when the contaminated water is drunk untreated. To prevent the water borne diseases
water for domestic use must always be free from faecal contamination. The situation calls for
improvement of water quality and this can be achieved by disinfection. But generally the water
being supplied should adhere to drinking water quality standards.
Waterborne diseases are pathogenic microorganisms which are directly transmitted when
contaminated fresh water is consumed. Contaminated fresh water, used in the preparation of food,
can be the source of food-borne disease through consumption of the same microorganisms.
According to the World Health Organization, diarrheal disease accounts for an estimated 4.1% of
the total daily global burden of disease and is responsible for the deaths of 1.8 million people every
year. It was estimated that 88% of that burden is attributable to unsafe water supply, sanitation and
91
hygiene, and is mostly concentrated in children in developing countries. Waterborne disease can be
caused by protozoa, viruses, or bacteria, many of which are intestinal parasites.
Table 2: Protozoal Infections
Disease and
Transmission
Microbial Agent
Sources of Agent in Water Supply
General Symptoms
Protozoan (Entamoeba
Abdominal discomfort, fatigue,
Amoebiasis (handSewage, non-treated drinking water, flies
histolytica) (Cyst-like
weight loss, diarrhea, bloating,
to-mouth)
in water supply
appearance)
fever, abdominal pain
Cryptosporidiosis
(oral)
Protozoan
(Cryptosporidium
parvum)
Flu-like symptoms, watery
Collects on water filters and membranes
diarrhea, loss of appetite,
that cannot be disinfected, animal
substantial loss of weight,
manure, seasonal runoff of water.
bloating, increased gas, nausea
Cyclosporiasis
Protozoan parasite
(Cyclospora
cayetanensis)
Sewage, non-treated drinking water
Giardiasis (oralfecal) (hand-tomouth)
Protozoan (Giardia
lamblia) Most
common intestinal
parasite
Untreated water, poor disinfection, pipe
breaks, leaks, groundwater
Diarrhea, abdominal
contamination, campgrounds where
discomfort, bloating, and
humans and wildlife use same source of
flatulence
water. Beavers and muskrats create
ponds that act as reservoirs for Giardia.
Microsporidiosis
The genera of Encephalitozoon
Protozoan phylum
intestinalis has been detected in
(Microsporidia), but
groundwater, the origin of drinking
closely related to fungi
water
cramps, nausea, vomiting,
muscle aches, fever, and fatigue
Diarrhea and wasting in
immunocompromised
individuals
Table 3: Parasitic Infections (Kingdom Animalia)
Disease and
Transmission
Microbial Agent Sources of Agent in
Water Supply
General Symptoms
Schistosomiasis
(immersion)
Members of the
genus
Schistosoma
Fresh water
contaminated with
certain types of snails
that carry schistosomes
Rash or itchy skin. Fever, chills, cough, and muscle
aches
Dracunculiasis
(Guinea Worm
Disease)
Dracunculus
medinensis
Stagnant water
containing larvae
Allergic reaction, urticaria rash, nausea, vomiting,
diarrhea, asthmatic attack.
Taeniasis
Tapeworms of
Drinking water
the genus Taenia contaminated with eggs
Intestinal disturbances, neurologic manifestations,
loss of weight, cysticercosis
Fasciolopsiasis
Fasciolopsis
buski
Drinking water
contaminated with
encysted metacercaria
GIT disturbance, diarrhea, liver enlargement,
cholangitis, cholecystitis, obstructive jaundice.
Hymenolepiasis
(Dwarf Tapeworm
Infection)
Hymenolepis
nana
Drinking water
contaminated with eggs
Abdominal pain, anorexia, itching around the anus,
nervous manifestation
Echinococcosis
Echinococcus
Drinking water
Liver enlargement, hydatid cysts press on bile duct
92
(Hydatid disease)
granulosus
contaminated with feces and blood vessels; if cysts rupture they can cause
(usually canid)
anaphylactic shock
containing eggs
coenurosis
multiceps
multiceps
contaminated drinking
water with eggs
increases intacranial tension
Ascariasis
Ascaris
lumbricoides
Drinking water
contaminated with feces
(usually canid)
containing eggs
Mostly, disease is asymptomatic or accompanied
by inflammation, fever, and diarrhea. Severe cases
involve Löffler's syndrome in lungs, nausea,
vomiting, malnutrition, and underdevelopment.
Enterobiasis
Enterobius
vermicularis
Drinking water
contaminated with eggs
Peri-anal itch, nervous irritability, hyperactivity and
insomnia
Table 4: Bacterial Infections
Disease and
Transmission
Microbial Agent
Sources of Agent in Water
Supply
Bacteria can enter a wound
from contaminated water
sources. Can enter the
gastrointestinal tract by
consuming contaminated
drinking water or (more
commonly) food
General Symptoms
Dry mouth, blurred and/or double
vision, difficulty swallowing, muscle
weakness, difficulty breathing, slurred
speech, vomiting and sometimes
diarrhea. Death is usually caused by
respiratory failure.
Botulism
Clostridium botulinum
Campylobacteriosis
Most commonly caused
Drinking water contaminated
by Campylobacter
with feces
jejuni
Produces dysentery like symptoms
along with a high fever. Usually lasts
2-10 days.
Cholera
Spread by the
bacterium Vibrio
cholerae
Drinking water contaminated
with the bacterium
In severe forms it is known to be one
of the most rapidly fatal illnesses
known. Symptoms include very
watery diarrhea, nausea, cramps,
nosebleed, rapid pulse, vomiting, and
hypovolemic shock (in severe cases),
at which point death can occur in 1218 hours.
E. coli Infection
Certain strains of
Escherichia coli
(commonly E. coli)
Water contaminated with the
bacteria
Mostly diarrhea. Can cause death in
immunocompromised individuals, the
very young, and the elderly due to
dehydration from prolonged illness.
M. marinum infection
Mycobacterium
marinum
Naturally occurs in water,
most cases from exposure in
swimming pools or more
frequently aquariums; rare
infection since it mostly
infects immunocompromised
individuals
Symptoms include lesions typically
located on the elbows, knees, and feet
(from swimming pools) or lesions on
the hands (aquariums). Lesions may
be painless or painful.
Dysentery
Caused by a number of
species in the genera
Water contaminated with the
Shigella and
bacterium
Salmonella with the
most common being
Frequent passage of feces with blood
and/or mucus and in some cases
vomiting of blood.
93
Shigella dysenteriae
Pontiac fever produces milder
symptoms resembling acute influenza
without pneumonia. Legionnaires’
disease has severe symptoms such as
fever, chills, pneumonia (with cough
that sometimes produces sputum),
ataxia, anorexia, muscle aches,
malaise and occasionally diarrhea and
vomiting
Legionellosis (two
distinct forms:
Legionnaires’ disease
and Pontiac fever)
Caused by bacteria
belonging to genus
Legionella (90% of
cases caused by
Legionella
pneumophila)
Leptospirosis
Water contaminated by the
Caused by bacterium of
animal urine carrying the
genus Leptospira
bacteria
Begins with flu-like symptoms then
resolves. The second phase then
occurs involving meningitis, liver
damage (causes jaundice), and renal
failure
Otitis Externa
(swimmer’s ear)
Caused by a number of Swimming in water
bacterial and fungal
contaminated by the
species.
responsible pathogens
Ear canal swells causing pain and
tenderness to the touch
Salmonellosis
Caused by many
bacteria of genus
Salmonella
Drinking water contaminated
with the bacteria. More
common as a food borne
illness.
Typhoid fever
Salmonella typhi
Characterized by sustained fever up to
40ºC (104ºF), profuse sweating,
diarrhea, less commonly a rash may
Ingestion of water
contaminated with feces of an occur. Symptoms progress to
infected person
delerium and the spleen and liver
enlarge if untreated. In this case it can
last up to four weeks and cause death.
Vibrio Illness
Vibrio vulnificus,
Vibrio alginolyticus,
and Vibrio
parahaemolyticus
Can enter wounds from
contaminated water. Also got
by drinking contaminated
water or eating undercooked
oysters.
Contaminated water: the
organism thrives in warm
aquatic environments.
Symptoms include diarrhea, fever,
vomiting, and abdominal cramps
Symptoms include explosive, watery
diarrhea, nausea, vomiting, abdominal
cramps, and occasionally fever.
Table 5: Viral Infections
Disease and
Transmission
Microbial Agent
Sources of Agent in
Water Supply
Manifests itself in
improperly treated
water
Adenovirus
infection
Adenovirus
Gastroenteritis
Astrovirus,
Manifests itself in
Calicivirus, Enteric
improperly treated
Adenovirus, and
water
Parvovirus
General Symptoms
Symptoms include common cold symptoms,
pneumonia, croup, and bronchitis
Symptoms include diarrhea, nausea, vomiting, fever,
malaise, and abdominal pain
SARS (Severe
Acute Respiratory Coronavirus
Syndrome)
Manifests itself in
improperly treated
water
Symptoms include fever, myalgia, lethargy,
gastrointestinal symptoms, cough, and sore throat
Hepatitis A
Can manifest itself in
Symptoms are only acute (no chronic stage to the
Hepatitis A virus
94
(HAV)
water (and food)
virus) and include Fatigue, fever, abdominal pain,
nausea, diarrhea, weight loss, itching, jaundice and
depression.
Enters water through
the feces of infected
individuals
Poliomyelitis
(Polio)
Poliovirus
Polyomavirus
infection
Very widespread, can
manifest itself in water,
Two of
Polyomavirus: JC ~80% of the population
virus and BK virus has antibodies to
Poltomavirus
90-95% of patients show no symptoms, 4-8% have
minor symptoms (comparatively) with delirium,
headache, fever, and occasional seizures, and spastic
paralysis, 1% have symptoms of non-paralytic aseptic
meningitis. The rest have serious symptoms resulting
in paralysis or death
BK virus produces a mild respiratory infection and
can infect the kidneys of immunosuppressed
transplant patients. JC virus infects the respiratory
system, kidneys or can cause progressive multifocal
leukoencephalopathy in the brain (which is fatal).
Class Activity
Discuss in detail common epidemic diseases caused by unhygienic water at your place.
References:
1. Berman, E. (1969), Toxic Metals and their analysis 6th Ed. Heyden and Sons Ltd
2. Jocobs, A. and Worwood, M. (1974); In Blood and its Disorder, Hardesty, R. M. and
Weatherall, D. J. , Oxford, Blackwell
3. Jones, A.; Johnston, D. O.; Netterville, J. T. and Wood, J. L. (1983), Chemistry, Man and
Society, Saunder.
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17:
LECTURE SEVENTEEN
17.1: Water Treatment
Introduction
Water treatment describes those processes used to make water more acceptable for a desired enduse. These can include use as drinking water, industrial processes, medical and many other uses.
The goal of all water treatment process is to remove existing contaminants in the water, or reduce
the concentration of such contaminants so the water becomes fit for its desired end-use. One such
use is returning water that has been used back into the natural environment without adverse
ecological impact. The processes involved in treating water for drinking purpose may be solids
separation using physical such as settling and filtration, chemical such as disinfection and
coagulation. Biological processes are also employed in the treatment of wastewater and these
processes may include, for example, aerated lagoons, activated sludge or slow sand filters.
On the other hand water purification is the removal of contaminants from untreated water to
produce drinking water that is pure enough for its intended use, most commonly human
consumption. Substances that are removed during the process of drinking water treatment include
bacteria, algae, viruses, fungi, minerals such as iron, manganese and sulphur, and man-made
chemical pollutants including fertilizers.
It is important to take measures to make available water of desirable quality at the consumer end.
That leads to protection of the treated water during conveyance and distribution after treatment. It is
common practice to have residual disinfectants in the treated water in order to kill any
bacteriological contamination after water treatment.
World Health Organization (WHO) guidelines are generally followed throughout the world for
drinking water quality requirements. In addition of the WHO guidelines, each country or territory or
water supply body can have their own guidelines in order for consumers to have access to safe
drinking water.
17.2: Various Processes Used in Drinking Water Treatment
The combination of following processes is used for municipal drinking water treatment worldwide
Pre-chlorination - For algae control and arresting any biological growth Aeration - along with prechlorination for removal of dissolved iron and manganese.
96
Coagulation –This process is used for flocculation. Coagulant aids also known as polyelectrolytes to improve coagulation and for thicker floc formation.
Sedimentation - For solids separation, that is, removal of suspended solids entrapped in the floc.
Filtration - for removal of carried over floc.
Disinfection - for killing the bacteria
Various technologies & chemicals are developed and are being developed continuously by various
organizations to deliver the above mentioned processes. There is no unique solution (selection of
processes) for any type of water. Also, it is very difficult to standardise the solution in the form of
processes for water from different sources. Treatability studies for each source of water in different
seasons need to be carried out to arrive at most appropriate processes. Let us discuss one type:
17.3: Sewage Treatment
Sewage treatment is the process that removes the majority of the contaminants from wastewater or
sewage and produces both a liquid effluent suitable for disposal to the natural environment and a
sludge. To be effective, sewage must be conveyed to a treatment plant by appropriate pipes and
infrastructure and the process itself must be subject to regulation and controls. Some wastewaters
require different and sometimes specialized treatment methods. At the simplest level, treatment of
sewage and most wastewaters is carried out through separation of solids from liquids, usually by
settlement. By progressively converting dissolved material into solids, usually a biological floc
which is then settled out, an effluent stream of increasing purity is produced.
Sewage is created by residences, institutions, hospitals and commercial and industrial
establishments. Raw influent (sewage) includes household waste liquid from toilets, baths, showers,
kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes
liquid waste from industry and commerce.
The separation and draining of household waste into grey-water and black-water is becoming more
common in the developed world, with grey-water being permitted to be used for watering plants or
recycled for flushing toilets. A lot of sewage also includes some surface water from roofs or hardstanding areas. Municipal wastewater therefore includes residential, commercial, and industrial
liquid waste discharges, and may include storm-water runoff. Sewage systems capable of handling
storm-water are known as combined systems or combined sewers. Such systems are usually avoided
since they complicate and thereby reduce the efficiency of sewage treatment plants owing to their
seasonality. The variability in flow also leads to often larger than necessary, and subsequently more
97
expensive, treatment facilities. In addition, heavy storms that contribute more flows than the
treatment plant can handle may overwhelm the sewage treatment system, causing a spill or overflow
(called a combined sewer overflow, or CSO, in the United States). It is preferable to have a separate
storm drain system for storm-water in areas that are developed with sewer systems.
As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants
including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil
and grease. Some jurisdictions require storm-water to receive some level of treatment before being
discharged directly into waterways. Examples of treatment processes used for storm-water include
sedimentation basins, wetlands, buried concrete vaults with various kinds of filters, and vortex
separators (to remove coarse solids).
Sewage can be treated close to where it is created (in septic tanks, biofilters or aerobic treatment
systems), or collected and transported via a network of pipes and pump stations to a municipal
treatment plant (see sewerage and pipes and infrastructure). Sewage collection and treatment is
typically subject to local, state and federal regulations and standards. Industrial sources of
wastewater often require specialized treatment processes.
Conventional sewage treatment involves three stages, called primary, secondary and tertiary
treatment. First, the solids are separated from the wastewater stream. Then dissolved biological
matter is progressively converted into a solid mass by using indigenous, water-borne microorganisms. Finally, the biological solids are neutralized then disposed of or re-used, and the treated
water may be disinfected chemically or physically (for example by lagoons and microfiltration).
The final effluent can be discharged into a stream, river, bay, lagoon or wetland, or it can be used
for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for
groundwater recharge or agricultural purposes. We can summarize the whole process in the fig. 13.
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Fig. 15: Process Flow Diagram for a Typical Large-Scale Treatment Plant
17.3.1:
Pre-treatment
The typical materials that are removed during pre treatment include fats, oils, and greases (also
referred to as FOG), sand, gravels and rocks (also referred to as grit), larger settleable solids and
floating materials (such as rags and flushed feminine hygiene products). In developed countries,
sophisticated equipment with remote operation and control are employed. The developing countries
still rely on low cost equipment like manually cleaned screen etc.
17.3.2:
Screening
The influent sewage water is worried to remove all large objects carried in the sewage stream, such
as rags, sticks, tampons, cans, fruit, etc. This is most commonly done with a manual or automatic
mechanically raked bar screen. The raking action of a mechanical bar screen is typically paced
according to the accumulation on the bar screens and/or flow rate. The bar screen is used because
large solids can damage or clog the equipment used later in the sewage treatment plant. The large
solids can also hinder the biological process. The solids are collected and later disposed in a landfill
or incineration.
Pre treatment also typically includes a sand or grit channel or chamber where the velocity of the
incoming wastewater is carefully controlled to allow sand grit and stones to settle, while keeping
the majority of the suspended organic material in the water column. This equipment is called a de-
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gritter or sand catcher. Sand, grit, and stones need to be removed early in the process to avoid
damage to pumps and other equipment in the remaining treatment stages. Sometimes there is a sand
washer (grit classifier) followed by a conveyor that transports the sand to a container for disposal.
The contents from the sand catcher may be fed into the incinerator in a sludge processing plant, but
in many cases, the sand and grit is sent to a landfill.
17.3.3: `
Primary Treatment - Sedimentation
In the primary sedimentation stage, sewage flows through large tanks, commonly called "primary
clarifiers" or "primary sedimentation tanks". The tanks are large enough that sludge can settle and
floating material such as grease and oils can rise to the surface and be skimmed off. The main
purpose of the primary sedimentation stage is to produce both a generally homogeneous liquid
capable of being treated biologically and a sludge that can be separately treated or processed.
Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive
the collected sludge towards a hopper in the base of the tank from where it can be pumped to further
sludge treatment stages.
17.3.4:
Secondary Treatment
Secondary treatment is designed to substantially degrade the biological content of the sewage such
as are derived from human waste, food waste, soaps and detergent. The majority of municipal plants
treat the settled sewage liquor using aerobic biological processes. For this to be effective, the biota
require both oxygen and a substrate on which to live. There are a number of ways in which this is
done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic
contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the
less soluble fractions into floc. Secondary treatment systems are classified as fixed film or
suspended growth. Fixed-film treatment process including trickling filter and rotating biological
contactors where the biomass grows on media and the sewage passes over its surface. In suspended
growth systems, such as activated sludge, the biomass is well mixed with the sewage and can be
operated in a smaller space than fixed-film systems that treat the same amount of water. However,
fixed-film systems are more able to cope with drastic changes in the amount of biological material
and can provide higher removal rates for organic material and suspended solids than suspended
growth systems.
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Roughing filters are intended to treat particularly strong or variable organic loads, typically
industrial, to allow them to then be treated by conventional secondary treatment processes.
Characteristics include typically tall, circular filters filled with open synthetic filter media to which
wastewater is applied at a relatively high rate. They are designed to allow high hydraulic loading
and a high flow-through of air. On larger installations, air is forced through the media using
blowers. The resultant wastewater is usually within the normal range for conventional treatment
processes.
17.3.5:
Activated Sludge
In general, activated sludge plants encompass a variety of mechanisms and processes that use
dissolved oxygen to promote the growth of biological floc that substantially removes organic
material. The process traps particulate material and can, under ideal conditions, convert ammonia to
nitrite and nitrate and ultimately to nitrogen gas. Others are calling this denitrification.
17.3.6:
Surface Aerated Basins
Most biological oxidation processes for treating industrial wastewaters have in common the use of
oxygen (or air) and microbial action. Surface-aerated basins achieve 80 to 90% removal of
Biochemical Oxygen Demand with retention times of 1 to 10 days. The basins may range in depth
from 1.5 to 5.0 metres and use motor-driven aerators floating on the surface of the wastewater.
In an aerated basin system, the aerators provide two functions: they transfer air into the basins
required by the biological oxidation reactions, and they provide the mixing required for dispersing
the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the
floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kW·h.
However, they do not provide as good mixing as is normally achieved in activated sludge systems
and therefore aerated basins do not achieve the same performance level as activated sludge units.[1]
Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of
biological reactions increase with temperature. Most surface aerated vessels operate at between 4 °C
and 32 °C.
17.3.7:
Filter Beds (Oxidizing Beds)
In older plants and plants receiving more variable loads, trickling filter beds are used where the
settled sewage liquor is spread onto the surface of a deep bed made up of coke (carbonized coal),
limestone chips or specially fabricated plastic media. Such media must have high surface areas to
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support the biofilms that form. The liquor is distributed through perforated rotating arms radiating
from a central pivot. The distributed liquor trickles through this bed and is collected in drains at the
base. These drains also provide a source of air which percolates up through the bed, keeping it
aerobic. Biological films of bacteria, protozoa and fungi form on the media’s surfaces and eat or
otherwise reduce the organic content. This biofilm is grazed by insect larvae and worms which help
maintain an optimal thickness. Overloading of beds increases the thickness of the film leading to
clogging of the filter media and ponding on the surface.
17.3.8:
Biological Aerated Filters
Biological Aerated (or Anoxic) Filter (BAF) or Bio-filters combine filtration with biological carbon
reduction, nitrification or de-nitrification. BAF usually includes a reactor filled with a filter media.
The media is either in suspension or supported by a gravel layer at the foot of the filter. The dual
purpose of this media is to support highly active biomass that is attached to it and to filter
suspended solids. Carbon reduction and ammonia conversion occurs in aerobic mode and sometime
achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF is operated either
in up flow or down flow configuration depending on design specified by manufacturer.
17.3.9:
Membrane Bioreactors
Membrane bioreactors (MBR) combines activated sludge treatment with a membrane liquid-solid
separation process. The membrane component uses low pressure microfiltration or ultra filtration
membranes and eliminates the need for clarification and tertiary filtration. The membranes are
typically immersed in the aeration tank (however, some applications utilize a separate membrane
tank). One of the key benefits of a membrane bioreactor system is that it effectively overcomes the
limitations associated with poor settling of sludge in conventional activated sludge (CAS)
processes. The technology permits bioreactor operation with considerably higher mixed liquor
suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling.
The process is typically operated at MLSS in the range of 8,000–12,000 mg/L, while CAS is
operated in the range of 2,000–3,000 mg/L. The elevated biomass concentration in the membrane
bioreactor process allows for very effective removal of both soluble and particulate biodegradable
materials at higher loading rates. Thus increased Sludge Retention Times (SRTs)—usually
exceeding 15 days—ensure complete nitrification even in extremely cold weather.
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The cost of building and operating a MBR is usually higher than conventional wastewater
treatment, however, as the technology has become increasingly popular and has gained wider
acceptance throughout the industry, the life-cycle costs have been steadily decreasing. The small
footprint of MBR systems, and the high quality effluent produced, makes them particularly useful
for water reuse applications.
17.4: Secondary Treatments
17.4:1:
Secondary Sedimentation
The final step in the secondary treatment stage is to settle out the biological floc or filter material
and produce sewage water containing very low levels of organic material and suspended matter.
17.4.2:
Rotating Biological Contactors
Rotating biological contactors (RBCs) are mechanical secondary treatment systems, which are
robust and capable of withstanding surges in organic load. RBCs were first installed in Germany in
1960 and have since been developed and refined into a reliable operating unit. The rotating disks
support the growth of bacteria and micro-organisms present in the sewage, which breakdown and
stabilise organic pollutants. To be successful, micro-organisms need both oxygen to live and food to
grow. Oxygen is obtained from the atmosphere as the disks rotate. As the micro-organisms grow,
they build up on the media until they are sloughed off due to shear forces provided by the rotating
discs in the sewage. Effluent from the RBC is then passed through final clarifiers where the microorganisms in suspension settle as sludge. The sludge is withdrawn from the clarifier for further
treatment.
A functionally similar biological filtering system has become popular as part of home aquarium
filtration and purification. The aquarium water is drawn up out of the tank and then cascaded over a
freely spinning corrugated fiber-mesh wheel before passing through a media filter and back into the
aquarium. The spinning mesh wheel develops a biofilm coating of microorganisms that feed on the
suspended wastes in the aquarium water and are also exposed to the atmosphere as the wheel
rotates. This is especially good at removing waste urea and ammonia urinated into the aquarium
water by the fish and other animals.
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17.4.3:
Tertiary Treatment
The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality
before it is discharged to the receiving environment (sea, river, lake, ground, etc.). More than one
tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is
always the final process. It is also called "effluent polishing".
17.4.4:
Filtration
Sand filtration removes much of the residual suspended matter. Filtration over activated carbon
removes residual toxins. Lagooning provides settlement and further biological improvement
through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and
colonization by native macrophytes, especially reeds, is often encouraged. Small filter feeding
invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine
particulates.
17.4.5:
Constructed Wetlands
Constructed wetlands include engineered reedbeds and a range of similar methodologies, all of
which provide a high degree of aerobic biological improvement and can often be used instead of
secondary treatment for small communities. In Tanzania this is area of researching where most of
Engineers has this project in Shinyanga.
17.4.6:
Nutrient Removal
Wastewater may contain high levels of the nutrients nitrogen and phosphorus. Excessive release to
the environment can lead to a build up of nutrients, called eutrophication, which can in turn
encourage the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an
algal bloom, a rapid growth in the population of algae. The algae numbers are unsustainable and
eventually most of them die. The decomposition of the algae by bacteria uses up so much of oxygen
in the water that most or all of the animals die, which creates more organic matter for the bacteria to
decompose. In addition to causing deoxygenation, some algal species produce toxins that
contaminate drinking water supplies. Different treatment processes are required to remove nitrogen
and phosphorus.
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17.4.7:
Nitrogen Removal
The removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia
(nitrification) to nitrate, followed by denitrification, the reduction of nitrate to nitrogen gas.
Nitrogen gas is released to the atmosphere and thus removed from the water. Nitrification itself is a
two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of
ammonia (NH3) to nitrite (NO2−) is most often facilitated by Nitrosomonas spp. (nitroso referring to
the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO3−), though traditionally
believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional
group), is now known to be facilitated in the environment almost exclusively by Nitrospira spp.
Denitrification requires anoxic conditions to encourage the appropriate biological communities to
form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be
used to reduce nitrogen, but the activated sludge process (if designed well) can do the job the most
easily. Since denitrification is the reduction of nitrate to dinitrogen gas, an electron donor is needed.
This can be, depending on the wastewater, organic matter (from faeces), sulfide, or an added donor
like methanol. Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary
treatment.
17.4.8:
Phosphorus Removal
Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water
systems (for negative effects of algae see Nutrient removal). It is also particularly important for
water reuse systems where high phosphorus concentrations may lead to fouling of downstream
equipment such as reverse osmosis.
Phosphorus can be removed biologically in a process called enhanced biological phosphorus
removal. In this process, specific bacteria, called polyphosphate accumulating organisms (PAOs),
are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20%
of their mass). When the biomass enriched in these bacteria is separated from the treated water,
these biosolids have a high fertilizer value.
Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g.
ferric chloride), aluminum (e.g. alum), or lime. This may lead to excessive sludge productions as
hydroxides precipitates and the added chemicals can be expensive. Despite this, chemical
phosphorus removal requires significantly smaller equipment footprint than biological removal, is
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easier to operate and is often more reliable than biological phosphorus removal. Once removed,
phosphorus, in the form of a phosphate rich sludge, may be land filled or, if in suitable condition,
resold for use in fertilizer.
17.4.8:
Disinfection
The purpose of disinfection in the treatment of wastewater is to substantially reduce the number of
microorganisms in the water to be discharged back into the environment. The effectiveness of
disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of
disinfection being used, the disinfectant dosage (concentration and time), and other environmental
variables. Cloudy water will be treated less successfully since solid matter can shield organisms,
especially from ultraviolet light or if contact times are low. Generally, short contact times, low
doses and high flows all militate against effective disinfection. Common methods of disinfection
include ozone, chlorine, or ultraviolet light. Chloramine, which is used for drinking water, is not
used in wastewater treatment because of its persistence.
Chlorination remains the most common form of wastewater disinfection in North America due to its
low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual
organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful
to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic
material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic
species, the treated effluent must also be chemically dechlorinated, adding to the complexity and
cost of treatment.
Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no
chemicals are used, the treated water has no adverse effect on organisms that later consume it, as
may be the case with other methods. UV radiation causes damage to the genetic structure of
bacteria, viruses, and other pathogens, making them incapable of reproduction. The key
disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and
the need for a highly treated effluent to ensure that the target microorganisms are not shielded from
the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from
the UV light). In the developed coutries, light is becoming the most common means of disinfection
because of the concerns about the impacts of chlorine in chlorinating residual organics in the
wastewater and in chlorinating organics in the receiving water. Ozone O3 is generated by passing
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oxygen O2 through a high voltage potential resulting in a third oxygen atom becoming attached and
forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in
contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer
than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event
of an accidental release), ozone is generated onsite as needed. Ozonation also produces fewer
disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of
the ozone generation equipment and the requirements for special operators.
17.4.9:
Package Plants and Batch Reactors
In order to use less space, treat difficult waste, deal with intermittent flow or achieve higher
environmental standards, a number of designs of hybrid treatment plants have been produced. Such
plants often combine all or at least two stages of the three main treatment stages into one combined
stage. When a large number of sewage treatment plants serve small populations, package plants are
a viable alternative to building discrete structures for each process stage.
One type of system that combines secondary treatment and settlement is the sequencing batch
reactor (SBR). Typically, activated sludge is mixed with raw incoming sewage and mixed and
aerated. The resultant mixture is then allowed to settle producing a high quality effluent. The settled
sludge is run off and re-aerated before a proportion is returned to the head of the works.
The disadvantage of such processes is that precise control of timing, mixing and aeration is
required. This precision is usually achieved by computer controls linked to many sensors in the
plant. Such a complex, fragile system is unsuited to places where such controls may be unreliable,
or poorly maintained, or where the power supply may be intermittent.
Package plants may be referred to as high charged or low charged. This refers to the way the
biological load is processed. In high charged systems, the biological stage is presented with a high
organic load and the combined floc and organic material is then oxygenated for a few hours before
being charged again with a new load. In the low charged system the biological stage contains a low
organic load and is combined with flocculate for a relatively long time.
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17.4.10:
Electricity Use
Wastewater treatment plants are, along with water treatment plants, often the largest users of energy
in a community. In developed countries these processes account for 3% of electricity use. As an
example, Nazareth Wastewater Treatment Plant in Nazareth, PA uses 1.6 million kWh per year to
process 365 million gallons of wastewater. For each gallon of water treated, the electric power used
is equivalent to leaving a 100-W equivalent (23W) Compact Fluorescent Light bulb on for 11.4
minutes, or a 60-W equivalent (14W) bulb for 18.8 minutes.
17.4.11:
Sludge Treatment and Disposal
The sludge’s accumulated in a wastewater treatment process must be treated and disposed of in a
safe and effective manner. The purpose of digestion is to reduce the amount of organic matter and
the number of disease-causing microorganisms present in the solids. The most common treatment
options include anaerobic digestion, aerobic digestion, and composting.
Choice of a wastewater solid treatment method depends on the amount of solids generated and other
site-specific conditions. However, in general, composting is most often applied to smaller-scale
applications followed by aerobic digestion and then lastly anaerobic digestion for the larger-scale
municipal applications.
17.4.12:
Anaerobic Digestion
Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process
can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C,
or mesophilic, at a temperature of around 36°C. Though allowing shorter retention time (and thus
smaller tanks), thermophilic digestion is more expensive in terms of energy consumption for
heating the sludge. One major feature of anaerobic digestion is the production of biogas, which can
be used in generators for electricity production and/or in boilers for heating purposes.
17.4.13:
Aerobic Digestion
Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic
conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. The
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operating costs are characteristically much greater for aerobic digestion because of the energy costs
needed to add oxygen to the process.
17.4.14:
Composting
Composting is also an aerobic process that involves mixing the sludge with sources of carbon such
as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater
solids and the added carbon source and, in doing so, produce a large amount of heat.
17.4.15:
Sludge Disposal
When a liquid sludge is produced, further treatment may be required to make it suitable for final
disposal. Typically, sludges are thickened (dewatered) to reduce the volumes transported off-site for
disposal. There is no process which completely eliminates the need to dispose of biosolids. There is,
however, an additional step some cities are taking to superheat the wastewater sludge and convert it
into small pelletized granules that are high in nitrogen and other organic materials. In New York
City, for example, several sewage treatment plants have dewatering facilities that use large
centrifuges along with the addition of chemicals such as polymer to further remove liquid from the
sludge. The removed fluid, called centrate, is typically reintroduced into the wastewater process.
The product which is left is called "cake" and that is picked up by companies which turn it into
fertilizer pellets. This product is then sold to local farmers and turf farms as a soil amendment or
fertilizer, reducing the amount of space required to dispose of sludge in landfills.
Many processes in a wastewater treatment plant are designed to mimic the natural treatment
processes that occur in the environment, whether that environment is a natural water body or the
ground. If not overloaded, bacteria in the environment will consume organic contaminants, although
this will reduce the levels of oxygen in the water and may significantly change the overall ecology
of the receiving water. Native bacterial populations feed on the organic contaminants, and the
numbers of disease-causing microorganisms are reduced by natural environmental conditions such
as predation or exposure to ultraviolet radiation. Consequently, in cases where the receiving
environment provides a high level of dilution, a high degree of wastewater treatment may not be
required. However, recent evidence has demonstrated that very low levels of certain contaminants
in wastewater, including hormones (from animal husbandry and residue from human hormonal
contraception methods) and synthetic materials such as phthalates that mimic hormones in their
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action, can have an unpredictable adverse impact on the natural biota and potentially on humans if
the water is re-used for drinking water. In the US and EU, uncontrolled discharges of wastewater to
the environment are not permitted under law, and strict water quality requirements are to be met. A
significant threat in the coming decades will be the increasing uncontrolled discharges of
wastewater within rapidly developing countries.
Class activity:
Anaerobic digestion is one of famous technique used these days to treat sewage. Refer to
website http://en.wikipedia.org/wiki/Anaerobic_digestion to discuss in detail the
importance of this method over others.
References
1. Seinfeld J. H. and Pandis, S.N. (1998), Atmospheric Chemistry and Physics, John Wiley &
Sons, Inc. New York.
2. Hites, R. A. (2007), Elements of Environmental Chemistry, John Wiley & Sons, Inc.
3. Khopkar, S. M. (2004). Environmental Pollution Monitoring and Control. New Age
International.
4. Beychok, M.R. (1971), Performance of surface-aerated basins. Chemical Engineering Progress
Symposium Series 67 (107): 322–339.
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18:
Additional Practice Questions and Model Answers
Questions on Primary Air Pollution
Mention different known primary pollutants and their sources and also what are the health effect
of each?
(a) CO (carbon monoxide): The source of CO is natural-forest fires: Causes basically by human
(anthropogenic) -transportation industry (automobile exhaust), incomplete combustion of fossil
fuels: eg. 2CH4 + 3O2 → 2CO + 4H2O
The health effect is acute exposure- headaches, dizziness, and decreased physical performance:
chronic expo-stress on cardiovascular system, heart attack
(b) NOx (nitrogen oxides): The source is natural-forest fires: Causes basically by human
(anthropogenic) stationary combustion sources (factories and power plants), transportation:
N2(g) + O2 (g) → 2NO (g)
2NO (g) + O2 (g) → 2NO2 (g)
The health effect is acute-lung irritation: chronic-bronchitis (persistent inflammation of bronchial
tubes)
(c) SOx (sulfur oxides): The source is natural volcanoes Causes basically by human
(anthropogenic)stationary combustion sources, industry, found in metal ores, coal:
S + O2 → SO2
2S + O2 → 2SO3
The health effect is acute-inflammation of respiratory tract, asthma: chronic-emphysema
(breakdown of alveoli in lungs), bronchitis
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(d) Particulates: The source is natural forest fires, volcanoes, windstorms, pollen, sea salts: Causes
basically by human (anthropogenic) industry (smoke, cement, ash), domestic-heating.
The health effect is irritation of respiratory system and cancer.
(e) HC (hydrocarbons): The source is from living and decaying plants: Causes basically by human
(anthropogenic) transportation.
What are the Possible Control and Prevention Measures of Primary air Pollution
Catalytic converters- devices attached to a vehicle's exhaust system to convert CO and HC into
water and carbon dioxide. (CO, HC) Scrubbers- pollutant-laden air is passed through a mixture of
water and lime, trapping particulates and sulfur oxide gases Filters- cloth bags through which smoke
is passed to stop particulates from flowing into the air.
Question on Ozone Depletion
What is the Evidence for Ozone Depletion?
(a) A thin layer of ozone gas (O3) encircles the earth and prevents about 99% of the ultraviolet light
(UV) from the sun from reaching the earth.
(b) Measurements made by British scientists at Halley Bay, Antarctica demonstrated the thinning of
the ozone layer.
(c) Satellite measurements of the ozone layer over Antarctica revealed that there was a hole in the
layer, the size of the United States. The ozone layer over Antarctica was depleted by more than
50% in 20 years (1965-1985) while other data from satellites showed that the ozone layer was
depleted even over non-polar regions (about 3%)
(d) A similar hole was discovered above the Arctic, but smaller in size.
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How the ozone formed and the depletion of ozone: (the natural process)
The UV light strikes O3 molecules, which split apart. The products reunite, giving off heat and
reforming ozone. The ozone layer continues renewing itself while converting UV light into heat
(infrared radiation).
Balanced equations:
O3(ozone) + UV light → O + O2
O + O2 → O3 + Infrared radiation (heat)
What are the Pollutants of Ozone layer and their sources?
(a) CFC's: Propellants-spray cans: insecticides, paint, aerosols: Coolants-refrigerators, air
conditioners, freezers: Styrofoam
(b) NOx: High-altitude jets: supersonic transport (SST), commercial jets: Detonation of nuclear
weapons, fertilizers, volcanoes (minor)
What are the Catalysts for ozone depletion?
(a) CFC's: These react with stratospheric ozone. When CFC's are broken down, chlorine free
radicals are produced. These can react with more than 100,000 molecules of ozone.
(b) NOx: These nitric oxides react with ozone to form oxygen gas (O2), thus interfering with the
natural process of formation of ozone.
What is the Environmental Effects of Ozone Depletion?
(a) Ozone depletion increases the amount of UV light that strikes the earth's surface. An excess of
this type of light causes damage to living organisms, including:
Eyes cataracts- blurred vision or blindness. Skin: severe burns/cancer. Immune system: weaker
immune response therefore more susceptible to diseases. Crops: interference with photosynthesis
and hence lower crop yields. On the other hand marine ecosystem: plankton near surface die
therefore disrupts food chain
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What is the evidence for global warming?
The history shows the level of carbon dioxide in the Earth's atmosphere had remained relatively
stable until about 100 years ago. This was the time that the human race began burning fossil fuels at
a high level. In just over a century, industrial civilization has added 360 billion tons of carbon
dioxide to our atmosphere. Carbon dioxide is very difficult to get rid of- the natural processes are
very slow and we aren't intelligent enough to have invented our own way of doing it.
It is well now documented that there is a correlation between the rise in CO2 levels and the rise in
temperature over the years.
What are the main gases involved in a global warming?
The gases that trap the heat near the Earth's surface and radiate it back:
CH4, H2O (water vapor), CO2 and chlorofluorocarbons (CFC's) and nitrous oxide.
Their Sources:
CH4: Methane is produced by the bacterial decomposition of vegetation under water that occurs in
flooded rice fields. Cattle- "ruminants" (because they "ruminate") i.e. cows, etc. have four
stomachs. In the complicated process of digesting the the food is fermented, producing a large
quantity of methane gas as a by-product which the cow expels.
H2O: Water vapor is always present in the atmosphere the water cycle.
CO2 It is produced naturally by the decomposition of organic materials, growing plants, and
weathering of rocks. (While they are burning millions of trees in the tropical rain forest, or using
fire woods not only they suffocating us, and killing many species of animals, but they are also
contributing to global warming. It is produced by the burning of fossil fuels- coal, oil and natural
gas. i.e. use of furnaces and coal fires, aircrafts and cars, industrial plants, etc.
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Influence of Gases on Global warming
The greenhouse theory proposes that the greenhouse gases are increasing in the atmosphere to such
an extent that less reradiated energy is able to escape from the atmosphere. The greater quantities of
greenhouse gases absorb this long-wave radiation, effectively trapping it. The energy is then
reradiated into the lower atmosphere. The gases may be said to be acting like a blanket, absorbing
more and more radiation in the atmosphere. In theory, this could lead to a progressive increase in
temperatures and eventually to climatic change.
Methane: This gas is present in the atmosphere only in very small quantities, but it absorbs radiation
more effectively than CO2.
Carbon dioxide: Naturally occurring CO2 allows sunlight to pass through the atmosphere and heat
the Earth, but also absorbs infrared radiation escaping from the Earth's surface and radiates it back
to earth. This process helps maintain Earth's temperature. The increase of CO2 concentration in the
air slows down the escape of heat global warming. Like almost anything in the world, little carbon
dioxide is good; too much may be devastating.
Effects of Global Warming
Oceans will absorb more heat energy, making hurricanes and typhoons more common.
There will be a change in ocean current patterns. The world's weather patterns will be altered
significantly and flooding in some areas/droughts in others.
Some areas like USA and Canada will become warmer and drier and the agricultural plains
might become too dry to support dry-land farming.
Melting of polar ice caps and glaciers: This would raise the sea level, flooding up to 20% of the
world's land mass. (Sinking of islands like Zanzibar, Florida and especially Holland, which is
already half-inside the sea-they just retrieved it by building canals.
The construction of climate models is difficult because of feedback processes: The Earth warms
then more water evaporates, more clouds form, clouds reduce the amount of heat reaching Earth
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and the warming trend will slow down or polar ice caps melt, ice reflects back sunlight instead
of absorbing, when ice decreases, warming speeds up.
What is the Influence of Particulates on Temperature?
Particulates consist of smoke, ash, soot, dust, lead, and other particles from burning fuel. They come
from industrial processes and motor vehicles that burn fossil fuel, burning wood, and dust from
construction and agriculture. They can form clouds that reduce visibility and cause a variety of
respiratory problems. Particulates have also been linked with cancer. They also corrode metals,
erode buildings and sculptures, and soil fabrics, and they can lower the temperature by reflecting
sunlight.
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Additional Reading List
1. Thomas, G. S. and William, M. S. (2003). Chemistry of the Environment, Prentice Hall.
2. Stanley, E. M. (2000). Environmental Chemistry, Lewis Publishers.
3. Miller, T. G . (1999). Living in the Environment. Brooks/Cole Publishing.
4. Baird, C. and Cann, M. Environmental Chemistry, (2004), 3rd edition, W.H. Freeman and Co,
New York.
5. Jacob, D. J. (1999), Introduction to Atmospheric Chemistry, Princeton University Press.
6. Seinfeld J. H. and Pandis, S.N. (1998), Atmospheric Chemistry and Physics, John Wiley &
Sons, Inc. New York.
7. Hites, R. A. (2007), Elements of Environmental Chemistry, John Wiley & Sons, Inc.
8. http://en.wikipedia.org/wiki/Ozone_depletion (15 April, 2009)
9. Chang, R. (1991), Chemistry, 4th Edn McGraw – Hill, Inc.
10. UNEP (1989) Environmental Effects Panel Report, UNEP, Nairobi, 64 pp.
11. http://en.wikipedia.org/wiki/Global_warming
12. Royal Society (2005). "Joint science academies' statement: Global response to climate change"
Retrieved on 19 April 2009
13. http://en.wikipedia.org/wiki/Greenhouse_gas . Retrieved on 27/04/2009
14. Hites, R. A. (2007); Elements of Environmental Chemistry, John Wiley and Sons Inc.
Publication.
15. McNeill, V. F.; et al (2006). "The effect of varying levels of surfactant on the reactive uptake of
N2O5 to aqueous aerosol". Atmospheric Chemistry and Physics (Strasbourg, France: European
Geosciences Union)
16. http://www.atmos-chem-phys-discuss.net/6/17/2006/acpd-6-17-2006-print.pdf.
Retrieved
on
2008-10-17.
17. Ritter L; Solomon KR, Forget J, Stemeroff M, O'Leary C.. "Persistent organic pollutants".
United Nations Environment Programme. http://www.chem.unep.ch/pops/ritter/en/ritteren.pdf.
Retrieved on 2007-09-16.
18. Berg, U. T.; Heilmann, S.; Hemmingsen, V.; Dahi, E. and Thogersen, J. water use Pattern and
bacterial Growth During Water Storage by Consumers in Dar es Salaam,(1992) Workshop
Research on Hygenic Water in Dare s Salaam.
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19. Mihale, M. (2002); Chemometrics of Obsolete Pesticides at Vikuge Farm, Kibaha District,
Tanzania , M. Sc. Thesis, University of Dar es Salaam.
20. United Republic of Tanzania, Ministry of Water and Irrigation, MAJI WEEK 16 – 22 March
2008.
21. Appropriate Technology for Sewage Pollution Control in the Wider Caribbean Region,
Caribbean Environment Programme Technical Report #40 1998
22. Massoud, T. and Ahmad, A. (2005), Integrated Approach to Water and Wastewater
Management for Tehran,Iran, Water Conservation, Reuse, and Recycling: Proceedings of the
Iranian-American Workshop, National Academies Press .
23. Joshi, P.K., Tyagi, N.K. & Svendsen, M. 1995. Measuring crop damage due to soil salinity. In
M.S.A. Gulati, ed. Strategic change in Indian irrigation. New Delhi, Macmillan India Limited.
24. Kahnert, F. & Levine, G. 1989. Key findings, recommendations, and summary. Groundwater
irrigation and the rural poor: options for development in the Gangetic basin. Washington, DC,
the World Bank.
25. Alexander, S. (1977), Soil Microbiology, 2nd Ed., Wiley Interscience
26. Bohn, M., and O'Connor, N. (1985), Soil Chemistry, 2nd Ed, Wiley Interscience
27. Seinfeld, J. H.; Pandis, S. N. (1998). Atmospheric Chemistry and Physics - From Air Pollution
to Climate Change. John Wiley and Sons, Inc.
28. Schroeder, D. (2000), Thermal Physics, Addison Wesley Longman.
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