Chapter 2 Composition of the Atmosphere
1. Introduction
The trails of meteors when enters the earth's atmosphere give clues as to its
composition. For last four decades, our knowledge of atmospheric chemistry, especially the
upper atmosphere, has been enormously enlarged, thanks to satellite and instrument
technology. Space programs have yielded just as much valuable information about the
atmosphere of our own planet as about the Moon and other planets .
The atmosphere refers to a protective blanket of gases which is able to sustain life on
the earth and protects it from the hostile environment of outer space. It constitutes the
source of carbon dioxide for plant photosynthesis and of oxygen for respiration. It provides
the nitrogen which nitrogen-fixing bacteria and man's ammonia manufacturing plants
employ to produce chemically bound nitrogen essential for life. As a basic component of the
hydrologic cycle the atmosphere transports water from the oceans to land, thereby acting as
the condenser in a vast solar-powered skill. It is unfortunate that the man has also made the
atmosphere a dumping ground for many pollutant materials, such as sulphur dioxide,
aerosol etc. These cause bad effects to vegetation and materials, shortens human life, and
possibly alters the characteristics of the atmosphere itself.
The atmosphere forms an insulating blanket around the earth. Without it the
temperature at the equator would rise to 180° during the day and drop as low as -220 °F at
night. It burns up meteors that would bombard the surface of the earth from space . Without
the atmosphere there would be no sound. There would be no conventional long-distance
radio communication, which is dependent on the electrons in the upper atmosphere.
Without air there would be no lightning, no clouds, no wind, no rain, no snow and no fire.
The surface of the earth would be as bleak and sterile as of moon.
The atmosphere absorbs most of the cosmic rays from outer space and protects living
things from their effects. It absorbs most of the electromagnetic radiation from the sun. Only
radiation in the wavelength regions of 300 to 2500 nm and 0.01 to 40 m gets transmitted to
any appreciable extent by the atmosphere. The first region consists of near-ultraviolet,
visible and near-infrared radiation; the second consists of radio-waves. One segment is the
optical window, consisting essentially of the visible spectrum of light, from near-ultraviolet
to near infrared . The other is the radio window, consisting of radio waves from about 1
centimeter to 40 meters in length. It is particularly fortunate for life on earth that the
atmosphere filters out tissue-damaging ultraviolet radiation below about 300 nm.
The atmosphere also plays an important role in maintaining the heat balance of the
earth. The atmosphere absorbs infrared radiation emitted by the sun. It also absorbs energy
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Chapter 2 Composition of the Atmosphere
re-emitted from the earth in the form of infra-red radiation. Therefore, it serves an
important heat stabilizing function and prevents the tremendous temperature extremes
which occur on planets and moons lacking substantial atmosphere.
2. Composition of the Atmosphere
The composition of the atmosphere are gases present in large amounts, water vapor
and solid particles in considerably less amounts. Gases that in the atmosphere are divided
into two kinds, based on their concentration, viz., constant gases and variable gases.
Constant gases are the ones, whose concentrations do not change over time, and their
concentrations almost remain same. But, variable gases are present in different
concentrations at different places and times.
Nitrogen and oxygen are the two major constant gases that make up 99 percent of
the air. Both are important to sustain life on earth. Nitrogen constitutes 78.09 % and oxygen
20.94 percent by volume thus, making the bulk of the atmosphere. The remaining 0.97 % is
constituted by nitrous oxide and inert gases such as, argon, helium, krypton, xenon and also
by variable gases – carbon dioxide, water vapor, methane, ozone and particulates among
others.
Nitrogen in its gaseous form in the atmosphere is not that important. It is nonpoisonous. But, indirectly, it is very important, as it is converted into useful forms for life by
certain micro-organisms. Nitrogen is very important in the formation of amino acids, which
are building blocks of proteins, and also in the formation of nucleotides, which are part of
the genetic materials, viz., DNA and RNA. This is one of the best examples that the natural
processes are in perfect harmony with each other.
It is needless to stress the importance of oxygen. Every living organism, including
humans, need oxygen for respiration. Respiration is the process through which the chemical
energy (food) is converted into usable form of energy by the living cells. Inert gases, as such
do not play any role in the environment. However, they are used for commercial purposes
such as in neon lights.
2.1. Carbon Dioxide in the Atmosphere
There are many natural sources of carbon dioxide including animal and plant
respiration and decay, combustion through forest and grassland fires and volcanic activity.
Carbon dioxide is emitted by all living organisms as an end product of respiration.
This carbon dioxide, in turn is used by producer organisms (green plants and certain micro19
Chapter 2 Composition of the Atmosphere
organisms) for the synthesis of food. This process is called, photosynthesis. This is another
example how natural processes are interrelated and interdependent. As a large amount of
carbon dioxide is utilized by plants, on average, it comprises only 0.04 percent of dry air.
Nevertheless, it plays significant role in keeping the atmosphere at temperatures that permit
life. The concentration of carbon dioxide, nowadays, is increasing due to human activities.
Today, man burns large quantities of fossil fuels for various purposes. As a result, huge
amounts of carbon dioxide are emitted into the atmosphere. Its ever increasing
concentration has already resulted in global warming and in some places, melting of ice.
2.2. Water Vapor in the Atmosphere
Water vapor, though present in small quantities, plays a crucial role. It i s responsible
for cloud formation in the atmosphere and precipitation. Its concentration varies over time
at a given place and at different places. It is an important component of the atmosphere in
determining the weather of a place at a given time. It is responsible for fog formation during
early morning hours in winter, for feeling sultry near coastal regions, during summer.
Water vapor also absorbs outgoing radiation from earth as CO2 does and it has a
crucial role in greenhouse effect. Thus, both CO2 and H2O along with ozone (in
troposphere), methane and N2O are called, greenhouse gases.
The main source of water in the stratosphere is the photochemical dissociation of
methane which involves many steps: CH4 + 2O2  CO2 + 2H2O
2.3. Ozone in the Atmosphere
Ozone (O3) is both beneficial and harmful to life on Earth. Much of the ozone in the
atmosphere is found in the stratosphere. Here, ozone absorbs UV light from the sun
preventing it from reaching the surface. Without this blanket, humans would be exposed to
serious sun burn and potential risk of skin cancer. Ozone is also found in the lowest layer of
the atmosphere, the troposphere. Here ozone can act as an eye and respiratory irritant.
Ozone also causes cellular damage inside the leaves of plants causing brown splotches,
impairing carbon dioxide uptake and disrupting the photosynthetic apparatus. Such damage
can cause economic losses through reduced crop yields. It also damages the carbon sink role
of vegetation leaving more carbon dioxide in the atmosphere to enhance the greenhouse
effect and potential global warming.
Human-produced compounds such as chlorofluorocarbons (CFC) and halides
containing chlorine and bromine destroy ozone, and have disrupted the fragile stratospheric
20
Chapter 2 Composition of the Atmosphere
ozone layer over Antarctica and the Arctic. Ozone depletion over Antarctica occurs during
the spring when sunlight returns to the South Pole and the temperatures are still very cold.
2.4. Methane in the Atmosphere
Methane (CH4) is a greenhouse gas contributing to about 18% of global warming
and has been on the rise over the last several decades. Though methane contributes a
smaller proportion of the atmosphere by volume (0.0002 %) than carbon dioxide, it is 20
times more potent than CO2 as a greenhouse gas. Methane is a product of the decomposition
of organic matter, with major natural sources being that which occurs from wetlands,
termites, the oceans, and hydrates.
A major source of methane is from termites. Termites eat wood and produce methane
as a result of the breakdown of cellulose in their digestive tracts. They are thought to be
responsible for 11% of the methane in the atmosphere (some estimates are as high as 20 % 40 %).
Human activities have contributed to the rise of methane in our atmosphere.
Landfills, rice paddy agriculture, natural gas systems, and livestock production appear to be
significant contributors of anthropogenic sources of methane.
In presence of NO, under conditions of temperature inversion, low humidity and
sunlight these hydrocarbons produce undesirable photochemical smog, manifested by the
obscured visibility due to particles, oxidants such as ozone, and noxious organic species
such as aldehydes.
2.5. Other Particles in the Atmosphere
Particulate matter refers to the solid and liquid particles that are dispersed into
ambient air. These particles can be classified in several ways. Firstly, they can be classified
into primary and secondary particles based on the mechanism of their formation. Primary
particles are emitted directly as particles, whereas secondary particles are formed from
precursor gases in the atmosphere via gas-to-particle conversion. Both types of particles are
subject to growth and transformations since there can be formation of secondary material
on the surface of existing particles.
Secondly, particles can be classified by their physical size, the size is from a few
nanometers (nm) to tens of micrometers (µm) in diameter. Size is the single most important
determinant of the properties of particles and it has implications on formation, physical and
chemical properties, transformation, transport, and removal of particles from the
atmosphere.
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Chapter 2 Composition of the Atmosphere
2.5.1. Inorganic Particles
Among the constituents of inorganic particulate matter found in polluted
atmospheres are salts, oxides, nitrogen compounds, sulfur compounds, various metals, and
radionuclides.
The major trace elements that typically occur at levels above 1 µg/m3 in
particulate matter are aluminum, calcium, carbon, iron, potassium, sodium, and silicon.
Most of these tend to originate from terrestrial sources. Lesser quantities of copper, lead,
titanium, and zinc, and even lower levels of antimony, beryllium, bismuth, cadmium, cobalt,
chromium, cesium, lithium, manganese, nickel, rubidium, selenium, strontium, and
vanadium are commonly observed. The likely sources of some of these elements are given
below:
- Al, Fe, Ca, Si: Soil erosion, rock dust, coal combustion;
- C: Incomplete combustion of carbonaceous fuels;
- Na, Cl: Marine aerosols, chloride from incineration of organohalide polymer wastes;
- Sb, Se: Very volatile elements, possibly from the combustion of oil, coal, or refuse;
- V: Combustion of residual petroleum;
- Zn: Tends to occur in small particles, probably from combustion;
- Pb: Combustion of leaded fuels and wastes containing lead.
2.5.2. Organic Particles
Organic atmospheric particles occur in a wide variety of compounds. For example,
Polycyclic Aromatic Hydrocarbons (PAH) in atmospheric particles have received a great deal
of attention because of the known carcinogenic effects of some of these compounds.
Prominent among these compounds are benzo(a)pyrene, benz(a)anthracene, chrysene,
benzo-(e)pyrene, benz(e)acephenanthrylene, benzo(j)fluoranthene, and indenol.
Atmospheric polycyclic aromatic hydrocarbons are found almost exclusively in the
solid phase, largely sorbed to soot particles. Soot itself is a highly condensed product of
PAHs. Soot contains 1–3 % hydrogen and 5–10 % oxygen, the latter due to partial surface
oxidation. Benzo(a)pyrene adsorbed on soot disappears very rapidly in the presence of light
yielding oxygenated products, the large surface area of the particle contributes to the high
rate of reaction. Oxidation products of benzo(a)-pyrene include epoxides, quinones,
phenols, aldehydes, and carboxylic acids.
2.5.3. Toxic Metals
Some of the metals found predominantly as particulate matter in polluted
atmospheres are known to be hazardous to human health. All of these except beryllium are
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Chapter 2 Composition of the Atmosphere
so-called “heavy metals.” Lead is the toxic metal of greatest concern in the urban
atmosphere because it comes closest to being present at a toxic level, mercury ranks second.
Others include beryllium, cadmium, chromium, vanadium, nickel, and arsenic.
2.5.3.1. Atmospheric Mercury
Atmospheric mercury is of concern because of its toxicity, volatility, and mobility.
Some atmospheric mercury is associated with particulate matter. Much of the mercury
entering the atmosphere does so as volatile elemental mercury from coal combustion and
volcanoes. Volatile organomercury compounds such as dimethylmercury, (CH3)2Hg, and
monomethylmercury salts, such as CH3HgBr, are also encountered in the atmosphere.
2.5.3.2. Atmospheric Lead
With the reduction of leaded fuels, atmospheric lead is of less concern than it used to
be. However, during the decades that leaded gasoline containing tetraethyllead was the
predominant automotive fuel, particulate lead halides were emitted in large quantities. This
occurs through the action of dichloroethane and dibromoethane added as halogenated
scavengers to prevent the accumulation of lead oxides inside engines.
2.5.4. Radioactive Particles
Some of the radioactivity detected in atmospheric particles is of natural origin. This
activity includes that produced when cosmic rays act on nuclei in the atmosphere to
produce radionuclides, including 7Be,
10
Be,
14
C,
39
Cl, 3H,
22
Na,
32
P, and
33
P. A significant
natural source of radionuclides in the atmosphere is radon, a noble gas product of radium
decay. Radon may enter the atmosphere as either of two isotopes,
and
220
222
Rn (half-life 3.8 days)
Rn (half-life 54.5 seconds). Both are alpha emitters in decay chains that terminate
with stable isotopes of lead. The initial decay products,
218
Po and 216Po, are nongaseous and
adhere readily to atmospheric particulate matter. The rate of travel of radioactive particles
through the atmosphere is a function of particle size.
3. Structure of the Atmosphere
The earth is surrounded by a thin layer of air, called atmosphere. The atmosphere,
from the surface of earth extends up to 60,000 km. Where, most of the mass of the
atmosphere is found near the planetary surface. It is near the earth's surface from surface to
about 80 to 100 km. This is due to the earth's gravity, which pulls the atmospheric
constituents, towards its center. As a result, the most of the atmosphere are within this
thickness.
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Chapter 2 Composition of the Atmosphere
3.1. Layers of the Atmosphere based on Composition of Constituents
It is the thin layer, in which most of the atmospheric processes take place. According
to the concentration of the gases, atmosphere is divided into:
Homo-sphere: the lower region, extending from the surface of the earth to a height of 80 to
100 km above the earth. In this layer, gases are more or less uniform in their chemical
composition.
Hetero-sphere: it starts from the upper portion of homo-sphere and extends to height of
60,000 km above the earth. In this layer, chemical composition changes with height.
Concentration of gases keep decreasing as one goes up. Inter molecular distance increases
with height and hence, concentration decreases.
3.2. Layers of the Atmosphere based on Temperature Variation
The atmosphere again can be divided into four distinct layers according to their
temperature characteristics:
Troposphere: it is the bottom layer of the atmosphere. It contains 70 % mass of the
atmosphere. It extends to an average height of 12 km. In this region the composition of the
atmosphere is more less uniform in the absence of air pollution mainly due to the constant
circulation of air masses in this region. A very important feature of this layer is that
temperature decreases with height in this layer. The rate of decrease of temperature with
altitude is called, lapse rate. Average lapse rate in troposphere is -6.4 °C / km. Troposphere
ends at tropopause. Tropopause is just like a lid over the troposphere, where temperature
stops decreasing with height.
Stratosphere: it lies just above the tropopause. It extends to a height of 50 km from earth’s
surface. Ozonosphere, a very important layer is found within this stratosphere. Ozone
present in the ozonosphere prevents the harmful ultra-violet rays from reaching the earth,
thereby protecting the life. Thus, ozonosphere acts as a protective umbrella.
Stratosphere is a calm layer consisting of relatively clean air. Water vapor in this layer is
almost absent, and hence clouds will not form in this layer. In this layer, temperature
increases with height just opposite to that in troposphere. The flying of jet air planes was
partly responsible for the destruction of ozone. Above the stratosphere, temperature neither
decreases nor increases with height up to some level. This small layer is called, stratopause.
The residence times of molecules or particles in the stratosphere are quite long because of
slow mixing. If the pollutants can somehow reach the stratosphere, they pose long-term
global hazards compared to their impact in the much denser troposphere.
Mesosphere: it starts from the edge of the stratopause, just at an approximate height of 52
km from earth’s surface. It extends to height of 80 km from the ground. In this layer,
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Chapter 2 Composition of the Atmosphere
temperature decreases with height as in troposphere. This layer, as such does not have any
impact on life. But, it gains importance as it plays crucial role in radio communication.
Sunlight passing through this layer, converts the individual molecules to individual charged
ions i.e. ionization. Ionized particles are concentrated as a zone called the D-layer. This Dlayer reflects radio waves sent from earth. But this D-layer blocks the communications
between earth and astronauts.
In this layer, during summer, at night times, a spectacular display of wispy clouds can be
seen sometimes over high latitudes. It is presumed that meteoric dust particles coated with
ice crystals reflect the sunlight resulting in wispy clouds. Just above the atmosphere lies,
mesopause, in which temperature neither decreases nor increases.
The principal chemical species in this region are O2+ and NO+.
Thermosphere: it is found approximately above 80 km from earth's surface. It extends to the
edge of space at about 60,000 km from earth’s surface. Temperature keeps rising with
altitude in this layer. It is likely to reach 900 °C at an altitude of 350 km. However, these
high temperatures are not felt as in lower layers. The atmospheric gases, particularly
oxygen and nitric oxide, split into atoms and also undergo ionization by the absorption of
very high energy radiation. It results in individual charged ions. This process produces two
ionized belts, viz., E- and F-layers. These layers also reflect radio waves and have influence
over radio communications. In the upper thermosphere, further concentration of ions are
seen that comprise the Van Allen radiation belts. This layer is sometimes called,
magnetosphere. It is thus called as earth’s magnetic field has more influence over the
movement of particles rather than earth’s gravitational field. Thermosphere as such has no
definable upper boundary and gradually blends with space.
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