Climate in cities
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Unit 3:
Acid rain
Acid rain, acid fogs and acid snow directly and indirectly affect our health.
Breathing acid fogs can damage our respiratory system and acidic waters
contain higher than average levels of heavy metals. Drinking these waters can
cause many serious diseases.
The main pollutants responsible for acid rain are the nitrogen oxides and
sulphur dioxide. It's easier to reduce emissions of these gases by saving energy
and reducing car use than it is to prevent damage by acid rain. Once the acid
rain is formed all we can do is partially neutralise its effects with liming
being the most widely used method.
Part 1: Origin
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Origin of acid rain
Acidic species are deposited to the ground both in precipitation (rain,
snow, hail) and as dry particles. Sunlight reacts with sulphur dioxide
and nitrogen oxide gases in the air to form sulphuric acid and nitric
acid. The presence of other substances, including volatile organic
compounds, also contribute to acid rain.
A closer look at pH
In the basics level chapter "What is acid rain?" we introduced the idea of pH as
a measure of how acid a solution is. The pH scale was developed by Soren
Sorenson in 1909 and the pH value of a solution is given by:
pH = -log[H+]
where [H+] is the hydrogen ion activity. The activity of a substance is its active
concentration, i.e. the amount that can actually react with another chemical
species. In most dilute solutions, the activity of a substance is directly related
to its concentration.
The construction of the pH scale is based on the fact that even pure water
dissociates (splits) into its constituent ions, the hydrogen ion (H+) and the
hydroxide ion (OH-). This process is known as auto-ionisation. In an aqueous
solution, hydrogen ions are associated with water molecules to form the
hydronium ion (H3O+).
The reaction is reversible so the products can recombine to reform the initial
reactants and then dissociate again. The ratio of the products (H3O+ and OH-)
to the reactants (two H2O molecules) is always constant. This constant is called
the ion product constant of water, Kw, and always has a value of 10-14.
So Kw = [H3O+] [OH-] = 10-14
If we now take logs of this equation:
pKw = pH + pOH = 14
In pure water [H3O+] = [OH-] = 10-7 mol L-1 and pKw = 7 + 7 = 14
Because Kw has a constant value, we can define acidic and basic solutions
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based on the relative amounts of H3O+ and OH- in solution:
Acid solution - [H3O+] > [OH-]
Neutral solution - [H3O+] = [OH-]
Basic (alkaline) solution - [H3O+] < [OH-]
Discovery of the acid rain problem
Acid rain was first recognised as a problem as far back as the end of the 17th
Century. In 1692, Robert Boyle published his book "A general history of the
air", where he described it as "nitrous or salino-sulfurous spirits". The term
"acid rain" was first used in 1872 by the Manchester scientist, Robert Angus
Smith (1817-1884), in his book "Air and Rain: The Beginnings of Chemical
Climatology". It wasn't, however, until the 1960's that the large scale problem
of acid rain was discovered. Studies showed that the acidity of lakes in
Scandinavia and North America were increasing and fish populations were
decreasing and that forests were dying. As a result of these scientific studies,
political action was taken to try to reduce the problem of acid rain.
Anthropogenic emissions as a source of acid rain
Apart from emissions of sulphur dioxide and nitrogen oxides from the
combustion of fossil fuels, there are also other substances which contribute to
acid rain formation. These include hydrochloric acid and organic carboxylic
acids. These organic acids include acetic acid and formic acid and are formed
when volatile organic compounds (VOC's) are oxidised in the air. The VOC's
have both natural and human sources. Human sources are primarily
from vehicle exhausts and plants are the main natural source. Natural sources
dominant the global emissions of VOC's, they are about ten times higher than
human sources.
The double role of ammonia
Ammonia (NH3) is also a very important gas in the atmosphere. It is a basic
gas and reacts with acids in the air to form ammonium (NH4+)
aerosols. Ammonia gas reacts with sulphuric acid to form ammonium sulphate
(NH4)2SO4 and with nitric acid to form NH4NO3. These reactions neutralise the
acids and so reduce the acidity of the atmosphere. However, when ammonium
aerosols are deposited to the ground they react with oxygen and a process
called nitrification can occur:
NH4+ + 2O2 -> 2H+ + NO3- + H2O
In this process, hydrogen ions are released. So ammonium deposition and its
subsequent nitrification leads directly to soil acidification.
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1. Sources and effects of acid rain and other forms of acid deposition. Author: Sebastian Wypych.
Deposition
Deposition is the process by which chemical constituents are removed from
the atmosphere to the surface of the Earth.
Wet deposition is the removal of substances in water (rain, snow, hail, fog).
Dry deposition is the removal of substances as particles or gases. When acid
species are removed from the atmosphere to the ground the process is called
acid deposition.
Wet deposition
The process by which chemicals are removed from the atmosphere and
deposited on the Earth's surface via rain, sleet, snow, hail, cloud water, and fog
is called wet deposition.
The dominant species responsible for acid rain are sulphuric acid and nitric
acid. These are formed in the air from the precursor gases, sulphur dioxide and
the nitrogen oxides (NOx = NO + NO2). Combustion of coal and oil in
powerstations to produce energy is an important source of both these gases and
car exhausts are an additional important source of nitrogen oxides. The gases
react with the hydroxyl radical (OH) and oxygen atoms (O) to form the acids.
Acid particles are very hygroscopic (i.e. they absorb water easily) so they can
act as cloud condensation nuclei and enhance cloud formation. When these
clouds rain, acid rain reaches the ground. On the way to the ground, the rain
drops can scavenge more acid particles so increasing the rain acidity further.
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2. a) Trends in sulphate wet deposition (kg ha1
) in the Eastern United States from NADP/NTN
Monitoring Data. The image on the left shows data
for the period 1989-1991, while the image on the
right shows data for 1995-1998. In the early
1990's, sulphate wet deposition was highest in a
broad region of the Mid-western and Eastern
U.S.A. including the Ohio River Valley, Western
Pennsylvania, and the Mid-Appalachians (see
1989-1991).
2. b) As a result of significant reductions in
sulphur dioxide emissions beginning in 1995
(Phase I of the Acid Rain Program), total sulphur
deposition in rain decreased by up to 25% over
a large area of the Eastern U.S., this reduction
was unprecedented in magnitude and spatial
extent (see 1995-1998). Source: U.S.
Environmental Protection Agency.
Dry deposition
Acid deposition occurs all the time, not just on rainy days. The settling of acidic
gases and particles out of the atmosphere is called acid dry deposition. If the
gases and particles fall into water bodies such as lakes they can cause
acidification, if they fall on plants and buildings they can directly damage them.
About half of the acidity in the atmosphere falls back to the Earth as dry
deposition.
Dry deposited gases and particles can be washed from trees and other surfaces
by rain. When this happens, the acidity of the rain increases further. Dry
deposition usually occurs close to the emission source, whereas wet deposition
can fall up to around a thousand kilometers away from the emission source.
Use of high factory chimneys to disperse pollutants have greatly improved air
quality in industrialised areas. The improvement of air quality has come at a
cost and air pollutants are now blown large distances by the wind and affect
much larger areas. Because of this long range transport, acid rain generally
creates greater problems than acid dry deposition.
Part 2: Impact 2
The impact of acid rain on human health and the economy
Acid rain has direct and indirect impact on both the natural
environment and on people and our infrastructure (buildings, roads,
etc.). Deterioration of our historical monuments by acid rain is a
particular problem.
Human health
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The pollutants that cause acid rain (mainly sulphur dioxide (SO2) and the
nitrogen oxides (NOx)) affect human health, particularly the respiratory
system. Have a look in the bioclimate section for more information.
Polluted air
Sulphur dioxide, nitrogen oxides and acid rain can cause respiratory problems
including asthma, bronchitis, headaches and eye, nose, and throat irritations.
The fine sulphate and nitrate particles which form from the gases are less than
2.5 µm in diameter (and known as PM 2.5). They can be transported long
distances by the wind and then inhaled by people far away from their sources.
Because the particles are so small, they get deep into the lungs and can cause
cancer.
Reaction of these acid gases with fog can cause acid smog. As a result of
pollution control, these acid smogs are now thankfully rare. In 1952, the pH of
an acid smog in London was as low as 1.5 and 4000 people died as a result.
Polluted water
1. Ranges of lead levels in blood,
among children, in micrograms per
deciliter, in chosen countries.
Explanations: C - China (data from
1988), M - Mexico (1995), P - Poland
(1992-1994), R - Romania (1995). The
red line marks the level of 10
micrograms lead per deciliter of blood.
See the text on the left for more
information. Author: Anita Bokwa.
Source of data: http://www.wri.org/wr98-99/metals.htm
Acidic waters corrode water systems and
dissolve the heavy metals in pipes so polluting
the water. In addition, heavy metals are
released into solution when acid rain falls on
soils and this water may also enters the
drinking water supply. The heavy metals can
be absorbed both by plants (through soil and
direct contact) and by animals (from food or by
direct contact). These metals can then enter
the human body where they accumulate
causing cancer. Brain damage, kidney
problems and Alzheimers have all been linked
to people eating "toxic" animals and plants.
Lead levels above ten milligrams per decilitre
of blood are known to have substantial
negative health impacts. Figure 1.
shows concentrations of lead in blood
in children from developing countries. The
majority of young children in cities in
developing countries have average blood lead
levels greater than 10 micrograms per deciliter
and suffer health problems as a consequence.
Buildings and monuments
Acid rain damages stone work in two main ways: dissolution and alterations. It
is particularly damaging to buildings made up of limestone and marble. These
are primarily composed of the mineral calcite (calcium carbonate, CaCO3)
which dissolves readily in weak acid solutions. The reaction which occurs:
CaCO3(s) + 2H+(aq)
Ca2+(aq) +CO2(g) + H2O
converts the solid calcium carbonate into aqueous phase calcium ions and gas
phase carbon dioxide and neutralises the acid. As a result, exposed areas of
buildings and statues dissolve and carved details are lost. The same process
occurs in calcium carbonate rich soils and when soil-dust particles that contain
CaCO3 collide with acidified raindrops in the air.
When sulphuric acid covers calcite, the hydrogen ion H+ from the acid dissolves
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the calcite, as described above. The sulphate ion also released then reacts with
the calcium ions and forms a clear-to-white gypsum crust over the marble or
limestone:
Ca2+ +SO42- + 2H2O
CaSO4 * 2H2O
rain removes some of the gypsum crust over time, creating tiny crevices and
causing erosion.
Even though the acidity of rain has been reduced in many places in recent
years, buildings are still showing signs of damage. This is because the acid rain
has permanently changed the stone from which the building is made. Other
materials vulnerable to acid rain damage include carbon-steel, nickel, zinc,
copper, paint, some plastics, leather and textiles. Stainless steel and aluminium
are more resistant materials.
2. A limestone sculpture of one of the twelve
apostles at the front of the St. Peter and St. Paul
church in the old town of Cracow, Poland. Acid
rain has caused incredible damage and copies of
this and the other eleven apostles have been
made to replace these originals. Photo: Sebastian
Wypych.
3. The copy made to replace the damaged
original apostle sculpture. Photo: Sebastian
Wypych.
The restoration of monuments and buildings is costly. For example, acid rain
caused damage to Westminister Abbey in London, England which cost 10 million
pounds to repair in the early 1990's. Although economic losses can be
calculated, the aesthetic appeal of the world's cultural treasures cannot be
price-tagged. The Taj Mahal in India, the Acropolis in Greece and even newer
buildings including Canada's Parliament Building and the U.S. Capitol Building
are seeing the effects of acid rain.
Cars
Automotive coatings are damaged by all forms of acid
rain, including dry deposition, and particularly when
acid dry deposition is mixed with dew or rain.
However, it is difficult to quantify the specific
contribution of acid rain to paint finish damage relative
to damage caused by other forms of environmental
fallout, by the improper application of paint or by
deficient paint formulations. Usually the damage is
permanent; once it has occurred, the only solution is
to respray the car.
4. Acid rain causes
rapid corrosion and rust
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formation on metal objects
used outdoors such as chains,
roofs and cars. This,,in turn,
generates higher costs of
maintenance and renovation.
Source of photo:
www.freefoto.com
Part 3: What can we do?
What can we do to counteract acid rain?
All of us can do something to limit the formation of acid rain or help
reduce its effects. These actions vary from international agreements,
through soil and water liming, to energy saving.
Two directions
Efforts undertaken to counteract acid rain can be divided into two groups:


limiting the causes of acid rain.
neutralising the effects of acid rain.
Limiting the causes
1. Causes of acid rain and what
we can do to counteract them.
Author: Anita Bokwa.
The image on the left shows the chain of processes
that cause acid rain to form. At each step there is
something we can do to counteract it. The paragraphs
below explain these activities.
Each of us can help!
Each of us can change our lifestyle and, as a
result, reduce emissions of SO2 and NOx. First of
all, energy saving leads to reductions in the use
of fossil fuels and thus lower emissions.
Use of clean renewable energy resources also reduces
SO2 and NOx emissions.
Better technology
Many new technologies have been
developed for industry which
reduce the emissions of pollutants
into the air. These can be applied
before, during or after combustion.
Examples of pre-combustion
sulphur control technology
(removing sulphur before burning)
include coal scrubbing and oil
desulphurisation. One of the postcombustion sulphur controls
(removing sulphur after burning) is
Flue Gas Desulfurisation (FGD).
This process is, however, rather
expensive. The cost of
removing one kilogram of sulphur
is between three and six US dollars
(between three and
five Euros). Figure 2. shows
2. The Dry Flue Gas Desulphurisation process uses high
reactivity lime as the primary reagent material and
requires particulate collection equipment downstream
of the spray dryer absorbers. Source: Hamon
Research-Cottrell, Inc. http://www.hamonresearchcottrell.com/Prod_FlueGasDry.asp
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how a dry scrubbing system works.
Nitrogen oxide emissions from cars can
be reduced by fitting catalytic
converters. These convert the
nitrogen oxides into harmless nitrogen
gas. Figures 3. and 4. explain how a
catalytic converter works.
3. Location of a catalytic converter in car. The
catalytic converter cleans up the exhaust before it
leaves the car removing a lot of the pollution.
Most modern cars are equipped with three-way
catalytic converters. See the next figure to learn
more about them! Source: HowStuffWorks.
http://www.howstuffworks.com/catalyticconverter.htm
Nitrogen oxides can also be
removed from the flue gases of
power stations. One method
involves mixing the flue gases with
ammonia which converts
the nitrogen oxides to nitrogen gas
and water. This process can be
used on both existing and new
power stations and can reduce
emissions by up to 90%.
Unlike sulphur, it's impossible to
reduce the nitrogen content of coal
and oil before combustion. The
nitrogen is bound to the organic
matter that makes up the fuel and so
can't be removed by physical
cleaning. Instead, nitrogen oxides can
be removed during combustion.
Advanced low nitrogen oxides burners
can reduce emissions by up to 30%.
Such burners can be installed either
on new or existing power plants.
4. A three-way catalytic converter. "Three-way" refers
to the three gases it helps to reduce -- carbon
monoxide, VOC's and NOx. The converter uses two
different types of catalysts, a reduction catalyst and an
oxidation catalyst. Both types consist of a ceramic (or
metal) structure coated with a metal catalyst, usually
platinum, rhodium and/or palladium. The idea is to
create a structure that exposes the maximum surface
area of catalyst to the exhaust stream, while also
minimizing the amount of catalyst required (they are
very expensive). There are two main types of
structures used in catalytic converters -- honeycomb
and ceramic beads. Most cars today use a honeycomb
structure. Explanations: A - reduction catalyst; B oxidization catalyst; C - honeycomb.
Source: HowStuffWorks.
International co-operation
Concern in the late 1970's led to international efforts to identify the causes and
effects of long-range (transboundary) transport of air pollutants. During
the 1980's much research was conducted in Europe and North America.
International legislation during the 1980's and 1990's has led to huge
reductions in SO2 emissions in many countries but reductions in NOx emissions
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have, unfortunately, been much less.
In 1979, the United Nations Economic
Commission for Europe (UNECE)
implemented the Convention on Long-Range
Transboundary Pollution (the socalled Geneva Convention). In 1985, in
Helsinki, most UNECE members adopted the
Protocol on the Reduction of Sulphur
Emissions, agreeing to reduce SO2
emissions by 30% (from 1980 levels) by
1993. This was called the 30% club. All of
the countries that signed the Protocol
achieved this reduction. Many other
countries have now also met these targets.
5. Legislation on long-range and acidifying
emissions. See text for explanations.
Author: Anita Bokwa
The Sofia Protocol for reducing NOx emissions was adopted in 1988.
The protocol is based on 1987 levels. Many countries are unlikely to meet these
targets, due to increases in road traffic and despite European Union legislation
requiring cars built after 1993 to be fitted with a catalytic converter.
In June 1994, a number of European countries signed the Second Protocol for
sulphur. Its main objective was to reduce acidifying emissions to a level at
which critical loads are not exceeded, i.e. to reduce sulphur emissions by 7080% by the year 2000 (against 1980 levels). Eastern European countries
generally have a lower target of 40-50% (against 1980 levels).
The most recent UNECE Convention on Long Range Transboundary Air
Pollution was signed by 27 countries in December 1999. The Gothenburg
Protocol, designed to Abate Acidification, Eutrophication and Ground-level
Ozone aims to cut emissions of four pollutants: SO2, NOx, VOC's and NH3, by
setting country-by-country emission ceilings to be achieved by the year 2010.
Neutralising the effects
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Once acid rain forms, all we can do
is neutralise its effects. Action has
to be taken quickly to have any
effect at all and the most
commonly used method is liming.
This is described below.
6. Effects of acid rain on natural environment and what
we can do to counteract them.
Author: Anita Bokwa
Natural and artificial buffers
The acidity of a water body is greatly influenced by the amount of limestone in
the surrounding rock formations. River beds and lake beds formed from
limestone rocks are partially dissolved by the acidic water and this neutralises
the acid. At present the main way of artificially reversing acidification in
freshwaters is liming the water body or its surrounding catchment by
adding sodium hydroxide (NaOH) or slaked lime (calcium hydroxide Ca(OH)2).
Liming increases the pH of the water, causing aluminum and other metals to
come out of solution and fall to the bottom of the lake as metal rich sediments.
This improves the conditions for surface living organisms but causes toxicity
problems for organisms living on the lake bed. Acidified lakes in Sweden, the
USA and Canada have all been restored in the short term by liming. Liming on a
large scale, however, is expensive.
Liming provides only a temporary solution, it's far better to attack the source of
the problem by reducing emissions of the acidifying pollutants, sulphur dioxide
and the nitrogen oxides.
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