Environmental Monitoring
& Technology Series
Collect & evaluate
meteorological data
For Technicians
Study module 3
Atmospheric composition & structure
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Collect & evaluate meteorological data
Study Module 3
Assessment details
Purpose
This subject covers the ability to site and set up basic ‘ground level’ meteorological
equipment and collect and record reliable data. It also includes the ability to assess data
quality, interpret significant data features and use the data to ensure the validity of air and
noise monitoring measurements.
Instructions
◗ Read the theory section to understand the topic.
◗ Complete the Student Declaration below prior to starting.
◗ Attempt to answer the questions and perform any associated tasks.
◗ Email, phone, book appointment or otherwise ask your teacher for help if required.
◗ When completed, submit task by email using rules found on last page.
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Details
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Class code
CMD
Assessment name
SM3
Due Date
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Total Marks Available
42
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Weighting
This is one of seven formative assessments that make up 20% of
the overall mark for this unit
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Introduction
This chapter will hopefully answer any question you have ever had about the atmosphere,
such as ‘what is air made up from?’, and ‘where does the atmosphere end?’, and many
more. The idea that the atmosphere is just a thick ‘pillow’ of air is wrong, it is remarkably
more complex that. Here we describe how the atmosphere is assembled, and from what
gaseous materials it is composed of.
For example, most people are not aware that the atmosphere consists of both "permanent"
and "variable" atmospheric gases, and that the atmosphere is layered like a cake! By the
end of this chapter, you will have a very good understanding of what the atmosphere is
made from, and how the atmosphere is assembled.
Exercise 3.1
Complete the Important Terms found at the end of this Chapter before continuing to aid
your comprehension
Composition of the atmosphere
So, let’s ask the question, what gases is air made up of? Most people are aware that the air
is mainly nitrogen and oxygen. Some would be aware of carbon dioxide (due to the concern
about global warming), and a few would be aware of methane gas, but what about
everything else? What about water? We need to be able to discuss the composition of the
atmosphere as being either dry or moist. Dry atmosphere values are used purely for
calculations, as the atmosphere is never truly dry.
Permanent Atmospheric gases
By permanent gas, we obviously mean that the gas in question is always present, and that
their proportions are nearly constant near the earth's surface, as not all gases are! Gases fall
into two categories, abundant (common) and trace (rare). The two abundant gases are
nitrogen and oxygen and are the only gases that exist in the dry atmosphere that have
concentrations above one percent near the earth's surface. The third gas in the dry
atmosphere is argon. Argon is also considered a permanent gas. The "permanent"
concentration of Argon is just less than one percent, and is considered trace (about 0.93% to
be more exact).
Variable atmospheric gases
Unlike the few "permanent" gases, the concentrations of the numerous other substances
found in the earth's atmosphere are variable. With the exception of water vapour, each one
of these variable gases (substances) exists in the atmosphere in concentrations far less than
one percent by volume. Because these constituents exist in such small amounts, their
proportions are often recorded in parts per million (ppm) and parts per billion (ppb) by
volume.
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Water Vapour in the Atmosphere
Of the variable substances in the atmosphere, water vapor (H2O) is the most variable (and
important) with concentrations ranging from 0-4% by volume. Most water vapour enters
the atmosphere via evaporation and transpiration. Evaporation occurs when a single water
molecule on a liquid water surface gains enough kinetic energy (often by solar radiation) to
break the bond which holds the molecules together. Transpiration is better explained in
terms of vapour pressure. The process of water escaping from plants is referred to as
transpiration. Evaporation and transpiration are collectively known as evapo-transpiration.
Constituent
Symbol
% by volume
Molecular weight (dry air)
Nitrogen
N2
78.1
28.02
Oxygen
O2
20.9
32.00
Argon
Ar
0.9
39.88
Carbon Dioxide*
CO2
0.036 And rising!
44.00
Neon
Ne
0.0001818
20.18
Helium
He
0.000524
4.00
Hydrogen
H
0.00005
2.02
Nitrogen Dioxide*
NO2
<0.00002
46.00
Sulfur Dioxide*
SO2
<0.0001
64.00
Water Vapour*
H2O
>0<5
18.02
Table 3.1 - Percentage composition of the atmosphere. * indicates variable gases.
Physical Properties of the Atmosphere
Physical properties refer to the basic ‘laws of nature’ that we find all around us, all of the
time. Having a basic understanding of these properties is essential in understanding even
the most fundamental of concepts in meteorology.
Density
In our atmosphere, density decreases rapidly with height (i.e., number of molecules which
make up the air decreases with height). This is due to the Earth's gravitational pull.
Molecules which are in the atmosphere are pulled towards the center of the earth.
Therefore, there are higher concentrations (numbers per unit volume) of molecules near
the surface of the earth than there are 16 km up.
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In fact, over 90 percent of all molecules in the atmosphere are within the first 16 km. So
why doesn’t the atmosphere collapse into one thin layer on the surface? Well, there is a lot
of mixing and vertical motion and other forces that keeps all those molecules moving
around. The dramatic decrease in density as you go up affects the air pressure, causing it to
decrease at a similar rate. This effect is visible in figure 3.1
Pressure
The molecules in the atmosphere are in constant motion. While moving, they bump into
each other and other objects on the order of 10 billion times a second and each collision
exerts a small force on the other object. These collisions, in conjunction with gravity and
some other forces mentioned later, all come together to create atmospheric pressure. As
the density of molecules decreases with height, so do the number of collisions and
therefore, the associated pressure. Because the density of the atmosphere decreases as you
go up, air pressure decreases in the same proportion.
Figure 3.1 – Graph showing how atmospheric density and pressure change with height. Adapted from
wikipedia
The same concept applies the other way, and this force can be thought of as the weight of
all the air above any point on the earth's surface, and it turns out that air is quite heavy,
‘weighing in’ at an impressive 1013.25 hectopascals (hPa). Ultimately, air pressure is a
measure of all the air above any point, influenced by the molecules' forces of motion and
gravity.
It is also important to understand that the pressure at any given point changes with time
because the air molecules do not stay in the same location. This changing pressure is the key
to our concept of weather. Generally, as pressure decreases, the weather becomes stormier
(more unstable).
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Temperature
When we measure the temperature of something we are actually measuring the average
kinetic energy of molecules. Therefore, when we measure the air temperature, we are
measuring the average kinetic energy of the molecules that make up the composition of the
air. When air molecules bombard a thermometer, the kinetic energy of the air molecules is
transferred to the liquid in the thermometer.
This transfer of energy causes the liquid to heat up and expand and the thermometer to
"rise." In the upper layer of the atmosphere, the thermosphere, the temperature is very hot,
at 115 km it can be as hot as 65 C, but you would freeze because there are so few molecules
that high up to bombard your body.
Air temperature also changes with height. Since the number of molecules decreases with
height, it is sometimes assumed that temperature also decreases with height. This,
however, is not always the case. Each layer in the atmosphere has its own temperature
profile. For our purposes, we will talk about temperature within the descriptions of each
layer.
Volume
Density, pressure and temperature will change with altitude, but what about volume.
Obviously, the volume is what we describe it to be, it is not a physical attribute, but because
the concept of volume is incorporated into so many other aspects of meteorological study,
it is critical to have a good understanding of the concept.
Making sense of Units and Standards
Nothing about size, scale or amount makes any sense unless you have something to
compare it to for comparison. In order for there to be some kind of standardization for
meteorological (and other) data collection, all data is related back to standard values.
The first standard deals with height, and is called sea level. The reason for this is that the
Earth is not even, so some measurements are taken at different heights, and so we can
makes sense of the results, we must correct them to make them relative to the standard of
sea level.
The standard air density at sea level is about 1.2 kilograms (kg) per cubic meter.
The standard air pressure at sea level (known as mean sea level pressure, MSLP) is 1013.25
mb (101.325 kPa). Meteorologists normally refer to pressure in units of either millibars (mb)
or kilo Pascals (kPa), which is equivalent to 14.7 pounds per square inch (1013.25 millibars
or 101.325 kilo Pascals) at sea level. One millibar is equal to 100 Pascals (Pa) or 0.1 kilo
Pascals (kPa). The Pascal is the international unit of pressure.
The standard temperature for sea level is 15 °C.
By using these values as standard for the atmosphere, we can compare all other measured
values at various locations around the world and make a valued comparison. It means
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nothing to see on the news that the pressure outside is 940 hPa (94.0 kPa) if you do not
know the standards.
If you do, you will realize that you are probably in the middle of a ferocious storm. However,
if you saw a pressure of 1020 hPa (102.0 kPa), you would know that you were probably
having clear skies.
Structure of the Atmosphere
Heterosphere
On the largest scale of the atmosphere, there are two distinct layers; a lower layer, where
the composition is uniform, and an upper layer, where there is no uniformity. The upper
layer (above about 100 km) is called the heterosphere, in which the atmosphere varies with
altitude.
Figure 3.2 – Structure of the atmosphere with relative examples
This is essentially because, in the absence of mixing, the density of a gas decreases with
increasing altitude, but at a rate which depends on the atomic weight of the particular
gases. Thus, the higher mass constituents, such as oxygen and nitrogen, fall off more quickly
than lighter constituents such as helium, molecular hydrogen, and atomic hydrogen. As the
altitude increases, the atmosphere is dominated successively by helium, molecular
hydrogen, and atomic hydrogen.
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The precise altitude of the heterosphere and the layers it contains varies significantly with
temperature. Below the heteorsphere, at an altitude of about 100 km (at a level called the
turbopause), the Earth's atmosphere has a somewhat uniform composition which creates
another atmospheric layer the homosphere.
Homosphere
There are 4 main layers within the atmosphere, which we will discuss in turn. They are the
troposphere, the stratosphere, the mesosphere and the thermosphere.
Troposphere
The troposphere is the lowest layer of the atmosphere. This is the layer where most
weather takes place. Most thunderstorms don't go much above the top of the troposphere
(about 10-16 km). In this layer, pressure and density rapidly decrease with height, and
temperature generally decreases with height at a constant rate.
The other main characteristic of the troposphere is that it is well-mixed. The name
troposphere is derived from the Greek tropein, which means to turn or change. Air
molecules can travel to the top of the troposphere (about 10 km up) and back down again in
a just a few days. This mixing encourages changing weather.
The troposphere is bounded above by the tropopause; a boundary marked as the point
where the temperature stops decreasing with height and becomes constant with height.
Stratosphere
The stratosphere is the layer above the troposphere, characterized primarily as a stable,
stratified layer with a large temperature inversion throughout which prevents large storms
from extending much beyond the tropopause. The stratosphere is also the location of the
ozone layer. The ozone layer is that part of the atmosphere which absorbs the strong ultraviolet light, the stuff which can cause us serious harm (UVc) and prevents it from reaching
the earth's surface.
The maximum concentrations of ozone are at about 25 km above the surface, or near the
middle of the stratosphere. The interaction between UV light, ozone and the atmosphere at
that level releases heat, which warms the atmosphere and helping to create the
temperature inversion in this layer. The stratosphere is capped above by the stratopause,
where there is another pause in temperature increase as we go higher.
Mesosphere
The mesosphere is the middle layer in the atmosphere (hence, mesosphere). The
mesosphere is similar to the troposphere in terms that it is in lapse (decreasing temperature
with height). At the top of the mesosphere, air temperature reaches its coldest value,
around -90 degrees Celsius. The mesosphere is bounded above by the mesopause. The
average height of the mesopause is about 85 km.
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Thermosphere
The thermosphere is a warm layer above the mesosphere. In this layer, there is a significant
temperature inversion. The few molecules that are present in the thermosphere receive
extraordinary amounts of energy from the sun, causing the layer to warm.
Atmospheric Boundary Layer (ABL)
Friction is generated by the earth's surface, but that aloft friction is negligible in comparison.
At some point in the atmosphere, there is a zone where friction goes from significant to
insignificant.
Atmospheric Boundary Layer
The lower layer of air which is subjected to frictional processes is known as the atmospheric
boundary layer (ABL). The remaining air in the troposphere is known as the free atmosphere
(because it is free of frictional influences).
Figure 3.3 - Diagram showing relative position of the PBL and the greater troposphere.
But what is this boundary layer? It turns out that there is a well defined, but quite variable
height, where the influence of friction stops. Below this point, the earth's surface has direct
influences on motion in the atmosphere. These influences include evapo-transpiration, heat
and energy transfer, and pollution emission. The ABL is that layer which is influenced
directly by the earth's surface.
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Processes in the Atmospheric Boundary Layer
The planetary boundary layer is the area of the atmosphere in which we live, and nearly all
of our activities take place there. So, what are some of the processes that occur in the ABL?
Weather Processes
The ABL is where nearly all of our weather is produced. Temperature and pressure gradients
caused by differential heating force the winds that drive air masses together producing
warm and cold fronts. The lifting mechanisms produce the upward motion which causes the
cooling necessary for cloud development to occur and precipitation to form. Though, each
of these processes are important in the role they play in the production of various weather
events, these processes in the ABL are also important in the role they play in the transport,
dispersion, and removal of pollution.
Pollutant Removal Mechanisms
Pollutants are released in various forms and from various sources (sulphur dioxide from
factory stacks, carbon monoxide from car exhausts, etc). These pollutants are mixed into the
air and advected down wind. Many of these pollutants do not remain in the atmosphere but
are removed by natural processes that occur within the planetary boundary layer.
These processes are called removal mechanisms, and the duration a pollutant resides or is
suspended in the atmosphere is referred to as its residence time. There are basically three
removal mechanisms that act on airborne pollutants;
Wet deposition involves the absorption of pollutants, both particles and gases, into liquid
droplets or ice crystals. These pollutants undergo sedimentation to the surface of the Earth
in one of the many forms of precipitation where they generally cause negative health effects
upon both animals and plants.
Dry deposition, as the name suggests, refers to the removal of pollutants that are not
absorbed into liquid or ice, but rather, are removed as either dry particles or gases.
Chemical reactions that transform different substances are also considered a removal
mechanism.
Evapo-transpiration
Many of the processes that occur in the boundary layer are dependent on the presence of
moisture in the air. Water vapour in the air varies from 0 to 4 percent, and this water
vapour enters the atmosphere through evapo-transpiration.
Evapo-transpiration is the combined process of evaporation and transpiration. It is used to
describe the exchange of water vapour from the surface to the air via water reservoirs, soils,
and plant life. Evapo-transpiration is, therefore, an important process within the
atmospheric boundary layer.
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Assessment Task
After reading the theory above, answer the questions below. Note that;

Marks are allocated to each question.

Keep answers to short paragraphs only, no essays.

Make sure you have access to the references (last page)

If a question is not referenced, use the supplied notes for answers
Complete the following table of important terms. 0 mk
Term
Definition
Permanent gas
Type your answer here.
Variable gas
Type your answer here.
Water vapor
Type your answer here.
Evapo-transpiration
Type your answer here.
Density
Type your answer here.
Pressure
Type your answer here.
Concentration
Type your answer here.
Temperature
Type your answer here.
Heat
Type your answer here.
Kinetic energy
Type your answer here.
Volume
Type your answer here.
Sea level
Type your answer here.
Mean sea level pressure Type your answer here.
Heterosphere
Type your answer here.
Homosphere
Type your answer here.
Troposphere
Type your answer here.
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Stratosphere
Type your answer here.
Mesosphere
Type your answer here.
Thermosphere
Type your answer here.
Atmospheric
boundary layer
Type your answer here.
Study Module 3
Removal mechanisms Type your answer here.
Residence time
Type your answer here.
Answer the following questions
a) What is the most abundant gas in the atmosphere? 1 mk
Type your answer here
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b) What is meant by a permanent gas? 1 mk
Type your answer here
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c) List five common permanent gases found in the atmosphere. 5 mk
Type your answer here
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d) What is meant by a variable gas? 1 mk
Type your answer here
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e) List 3 common variable gases in the atmosphere. 3 mk
Type your answer here
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f) Why is the amount of water in the atmosphere so variable? 2 mk
Type your answer here
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g) Describe how pressure and density vary with altitude. 4 mk
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h) How does temperature vary with altitude? 5 mk
Type your answer here
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i) Why does temperature vary with altitude? 4 mk
Type your answer here
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j) Why must we have standard values for atmospheric variables such as pressure and
density? 3 mk
Type your answer here
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k) What is the difference between the homosphere and the heterosphere? 2 mk
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l) List and describe the four main layers of the homosphere? 4 mk
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m) What is meant by the term ‘pause’ with regards to atmospheric layers? 2 mk
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n) What is the atmospheric boundary layer? 1 mk
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o) Why is it important to us on the ground? 2 mk
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Type your answer here
Leave blank for assessor feedback
p) List and describe some of the important processes that are unique to the atmospheric
boundary layer? 2 mk
Type your answer here
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Assessment & submission rules
Answers
◗ Attempt all questions and tasks
◗ Write answers in the text-fields provided
Submission
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Penalties
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on the cover page), it may not be considered for marking without justification.
Results
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graded with feedback.
◗ You have the right to appeal your results within 3 weeks of receipt of the marked work.
Problems?
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contact your assessor at the earliest convenience to get the problem resolved. The name of
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Resources & references
References
Turco, R.P., (1997). Earth under Siege: from Air Pollution to Global Change. Oxford
University press. New York. USA.
Sturman, A.P, Tapper, N.J., (2000). The weather and climate of Australia and New Zealand.
Oxford University Press. Melbourne. Australia.
The Shodor Education Foundation Inc. Air Quality Meteorology. A Developmental Course of
the US Environmental Protection Agency in conjunction with the US National Oceanic and
Atmospheric Administration. http://www.shodor.org/metweb/index.html
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Further reading and online aids
www.bom.gov.au
http://bpesoft.com/s/wleizero/xhac/?h=5360 for info at all altitudes
http://www.chemistry.ohio-state.edu/betha/nealGasLaw/frb2.2.html
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html#c4, for the mathematically inclined
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