UNIT TWO
THE MEANING, ORIGIN, FUNCTIONS, COMPOSITION AND
STRUCTURE OF THE ATMOSPHERE
Atmosphere: Meaning
 The atmosphere is a mixture of gases and
suspended particles of liquid and solid which
entirely envelop the earth.
 It constitutes the outer most layers of the
environmental spheres.
 It is a canopy over the hydrosphere and the
lithosphere, plays a crucial role in supporting the
biosphere.
 Its outer limit is not exactly known (10,000 kms
vs 1000 kms).
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 Atmosphere is colorless, odorless, tasteless and
cannot be felt except when it is in motion.
 Moreover it is mobile, elastic, compressible and
expandable.
Origin of the Atmosphere
 A variety of astronomical evidences and
insights suggest that the earth’s original
atmosphere probably developed around
the time the solar system formed; about
4.6 billion years ago.
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 Hence, the earth’s earliest atmosphere,
which is also called the primary
atmosphere, must have been comprised of
the same gases from which the earth and
other members of the solar system were
formed.
Nebula Theory
 According to this theory, the principal
gases that constituted the original
atmosphere are believed to have been
hydrogen, helium and hydrogen
compounds (e.g. NH4, NH3 etc) 3

 These gases, however, believed to have escaped
the full earth’s gravitational attraction to the
extraterrestrial space for they are extremely light
molecules.
 Hence, the earth is striped of its primitive
atmosphere.
 However, we do also have an atmosphere
today. Where did it come from?
 Scientists believe that billions of years ago
gases that might have trapped in the earth’s hot
interior during its formation expelled at the
surface latter by large number of volcanoes,
fissures, and fumaroles (steam vent)
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 Through the process of outgassing, the
outpouring of gases from the earth's interior,
many other gases were injected into the
atmosphere. These include:
• Water vapor (produced rain - rivers, lakes,
oceans)
• Carbon dioxide
• Nitrogen
 As outgassing occurred over a period of
millions of years, the atmosphere evolved to its
current state.
 The dominant gases in the present day
atmosphere are nitrogen (78%), oxygen (21%)
and Argon (0.93%). 5

Functions of the Atmosphere
Atmosphere performs a number of vital functions
 It supplies oxygen to animals and CO2 to plants
during respiration and photosynthesis,
respectively. Thus, it made the earth a livable
place.
 It protects our planet earth from the impact of
small falling meteors, which are usually
incinerated by friction before reaching the earth’s
surface.
 The blanketing influence of the atmosphere also
protects us from most lethal ultraviolet and
cosmic radiations
 Maintains the daily temperature balance by
regulating different proportions of both solar and
earth’s radiation.
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 This is because the atmosphere contains
greenhouse gases (heat trapping gases).
 It also regulates global temperature balance
through horizontal air circulation.
 The atmosphere makes the hydrological cycle
possible, a cycle that provides water to the
surface of the earth.
 It has made radio and TV communication
possible over long distances.
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PRESENT COMPOSITION OF THE ATMOSPHERE
Major Components of the Atmosphere
 The major components of the present day atmosphere
can be divided in to three major categories.
i) Non-variable (permanent) gases,
ii) Variable gases, and
iii) Particulates
The Non- Variable (Permanent) Gases
 These are gasses whose relative proportion remains
unchanged.
 These are gasses that are also sufficiently stable, both
physically and chemically.
 These gasses remain long enough in the atmosphere
with out taking part in certain chemical reactions.
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 They comprise 98 percent of the total atmosphere
by volume. These are gasses like Nitrogen (N2),
Oxygen (O2), Argon (Ar), Neon (Ne), Helium (He)
etc...
a. Nitrogen
 Nitrogen (78%)is a relatively inert gas produced
primarily by volcanic activity.
 Nitrogen, the most dominant gas in the
atmosphere that is chemically inactive and doesn’t
participate directly in the respiratory processes.
 Though it is removed by the nitrifying bacteria, it
returns to the atmosphere mainly through the
decaying of plants and animals matter.
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b. Oxygen
 Oxygen is removed from the atmosphere when
organic matter decays and when it combines with
other substances producing oxides. It is also taken
from the atmosphere during breathing.
 In contrary, the addition of oxygen to the
atmosphere occurs during photosynthesis.
c. Argon
 Argon is produced by radio active decay of K-40.
When the decay of k-40 occurs nearer to the
surface it diffuses to the atmosphere. It is one of
the inert gases.
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2. Variable Gases
 These are atmospheric gases that vary appreciably in
relative abundance over a short period of time.
 Although they collectively comprise only a very small
proportion of the atmosphere’s total mass and volume.
 Three of them are carbon dioxide, water vapor and ozone
and play a crucial role in the existence of life.
a. Carbon dioxide
 Carbon dioxide (CO2) makes up only 0.036% of the
atmosphere by volume.
 Carbon dioxide is essential to photosynthetic processes of
plants. Huge quantities of carbon is stored in plant tissue,
deposits of coal, peat, oil, and gas.
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 Carbon dioxide is taken in by plants and
during photosynthesis is combined with water
and energy to form oxygen and
carbohydrates.
 Because vegetation takes in so much carbon
dioxide, we often refer to plants as a "sink" for
it.
 Carbon dioxide in the atmosphere varies
throughout the year, decreasing slightly during
the summer as plants leaf out, and then
increases during the winter as plants go
dormant and photosynthesis decreases.
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b. Water Vapor
 Shows Spatio-temporal variation
 In warm tropical locations close to the surface, it
may account up to 4 percent of the atmospheric
gases while in cold polar regions, its
concentration may dwindle to a mere trace.
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 Spatio-temporal variation is ascribed to two major
factors
a) Air temperature: warm and moist air can hold
more water vapor than can cold and dry air.
b) The nature of the earth’s surface: air over land
surface is generally drier than air over water
surface.
 Water vapor is important from the following
perspectives.
 It is critically important to life on earth because it
is the source of all forms of the earth’s
precipitation.
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 Latent heat which is an important source of
atmospheric energy especially for storms, such
as thunder storms and hurricanes are powered
by the heat released during the condensation of
water vapor
 Water vapor strongly absorbs infrared radiation
given off by the earth, making it an important
green house gases in the earth’s heat balance
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c. Ozone
 Ozone is the third most abundant variable gas
which exists in triatomic form of oxygen.
 The majority (about 97%) of atmospheric ozone is
found in the upper atmosphere, that is, the
stratosphere where it is formed naturally as
oxygen atoms (O) combine with oxygen molecules
(O2).
 Though its concentration is less than 0.002% by
volume, ozone is important due to the fact that it
shields plants, animals and human being from the
damaging effects of ultraviolet rays.
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 Had the ultraviolet radiation reached the earth’s
surface with its full intensity, (1) the yields of
many agricultural crops would be greatly
decreased; (2) the incidence of skin cancer and
eye diseases such as cataract would be increased
Production of Ozone (03)
 When oxygen atoms (0) combine with oxygen
molecules (02) forms 03
- Not destructed by the ultra violet radiation
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 Although it mainly forms , above 25 kms in the
atmosphere, ozone gradually drifts downward by
mixing processes, producing a maximum
concentration near 25 km.
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Destruction of Ozone
 Research results have shown that the average
global concentration of stratospheric ozone
declined by about 3 percent between 1969 and
1985.
 The British Antarctic Survey also discovered a
hole in the ozone layer over Antarctica. The
Ozone layer depleted over Antarctica extends
form 12 to 20 km in altitude.
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 The stratospheric ozone (10-65 km) may be upset
by the CFC (Chlorofluorocarbons or
halocarbons) gasses injected in to the
atmosphere, i.e. in to the stratosphere.
What are the chemical processes (reactions)
involved in the destruction of ozone gas?
1. Individual chlorine atoms which are released from
CFC by photo dissociations are very important in
the destruction of the ozone layer (30 kms).
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 One Chlorine ion (Cl-) released combines with
ozone rapidly forming chlorine monoxide (ClO)
and molecular oxygen (O2).
 The chlorine monoxide reacts with another ozone
molecule to make chlorine dioxide, which
dissociates in to oxygen and a reactive chlorine
ion that initiates the cycle over again.
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The Major Sources of Chlorofluorocarbons Gas
 Propellants used in spray cans (aerosols) such as
hair spray, deodorants and fly killer
 Refrigerator coolant
 Air conditioner
 Foam producing industries, and
 Solvents
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III. Particulates
 Particulates consist of liquids and solids (with the
exception of water droplets or ice crystals)
particle that are found suspended in the
atmosphere because of their small size.
 Most particulates are solids rather than liquids,
and are collectively referred to as dust (aerosols)
 Aerosols are usually derived from two major
sources: natural and anthropogenic sources
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Natural Sources
Natural sources releasing atmospheric dust include:
 Dust generated when meteors incinerated in the
atmosphere
 Volcanic dust and ash released by volcanisms
 Wind blow surface material
 Salt crystals released from the evaporation of sea
spray
 Dust particles of biological origin including pollen,
spores, seeds and bacteria
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Antropogenic sources
Dust generating human activities include:
 Factory and automobile exhaust
 Smoke from home heating and cooling
 Farming areas
 Airborne pesticides
 Burning of grasses
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 Though most particulates (or atmospheric
impurities) are nuisance, as well as, health
hazards, (or pollutants), some air born
particulates play a vital role in atmospheric
processes and in the formation of clouds and
precipitation serving as condensation
(hygroscopic) nuclei.
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VERTICAL STRUCTURE (VERTICAL LAYERING) OF THE ATMOSPHERE
Compositional Layers
 The atmosphere is divided into two major
compositional layers: homosphere and heterosphere.
Homosphere
 It lies between the earth’s surface and 80 kilometers
above the surface
 It contains more than 99.9 percent of the total air
 In this part of the atmosphere, various gasses are kept
mixed by wind currents so that the proportion of the
non-variable gases remains constant
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 Because of its relatively homogeneous composition,
this portion of the atmosphere is some times termed
as homosphere
 The ability of the homosphere’s gases to remain
mixed actually attributed to the mixing action of
wind currents and the numerous collusions between
air molecules
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Heterosphere
 It is part of the atmosphere above 80 kms.
 Density of the atmosphere above 80 kilometer is
low so that the number of air molecules colliding
is greatly reduced
 Wind flows at these higher altitude is also
predominantly horizontal
 air currents are subjected to very little vertical
mixing.
 At higher altitudes (i.e. above 80k.m), various
gases exhibit an increasing tendency to be
layered on the basis of density (or atomic
weight).
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Compositional Layer
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Thermal Layers
These are layers on the basis of the vertical
distribution of air temperature
Troposphere
 It is lowest layer of the atmosphere.
 It extends on average to about 11 kilometers
from the surface.
 It extends upward from the surface to about
16 kilometers over the equator and only to
about 8 km from the surface over the Polar
Regions
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 75 percent of the total molecular or
gaseous mass of the atmosphere and
virtually, water vapor and aerosols, are
concentrated in this layer.
 This layer is responsible for all weather
condition (most weather phenomena
occur in this part of the atmosphere, it is
some times called the weather sphere).
 Temperature in this portion of the
atmosphere uniformly falls with increasing
altitude by 6.50C/ 1000 m ascent /descent
(normal environmental lapse rate)
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Vertical Thermal Layers of Atmosphere
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The Stratosphere
 It extends from the tropo-pause to an
altitude of 50 kilometers.
 In this layer, temperature inversion occurs
 It is the ozone gas that plays a major part
in heating the air at this altitude
(important because it absorbs energetic
ultraviolet solar radiation)
 At this part of the atmosphere, increased
motions of Ozone gas represent a higher
temperature.
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 The maximum concentration of ozone in
the stratosphere is observed near 25 km.
Yet, the stratospheric air temperature
reaches a maximum of near 50 kms.
 The reason for this is that, the air at 50 km
is less dense than that at 25 km, and so,
the absorption of intense solar energy, at
50 km rises the temperature of the fewer
molecules to much greater degree.
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Mesosphere
 It is also called middle sphere
 It represents part of the atmosphere that lies
between the stratosphere and thermosphere
(50-80km above the surface)
 Air at this level is extremely thin and the
atmospheric pressure is quite low (1013 mill
bars at sea level averages to1mb at 50 km
above sea level)
 temperature in the mesosphere decreases
with height there is little ozone in the air to
absorb solar radiation
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 Consequently, there would be more losing
of energy than absorbing.
 Thus, it is the coldest and the darkest part
of the atmosphere
 Because of air’s low density in this region,
our brain would soon become oxygen
starved, a condition known as hypoxia.
Thermosphere
 This is the outermost layer of the atmosphere
that extends outward from about 80 km to 1600
kms.
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 At this layer, temperature gradually rises to
eventually reach about 1500 C0
 This layer blocks a variety of harmful
cosmic radiation including x-rays, gamma
rays, and some ultraviolet radiation
 Ultraviolet solar radiation is absorbed
mainly by molecular nitrogen and oxygen
(O2)
 This radiation supplies enough energy to
break molecular oxygen in to two separate
oxygen atoms (O).
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 The energy "left over" after the separation
of the oxygen molecule increases the
speed of motion of the atoms.
 Because there are relatively few atoms and
molecules in the thermosphere, the
absorption of a small amount of solar
radiation can cause a large increase in
temperature, consequently there is an
inversion.
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The Ionosphere
 An electrified region with in the upper
atmosphere primarily the thermosphere
where fairly large concentration of ions
and free electrons exist.
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What are the primary causes of Ionization in
the ionosphere?
 The primary causes of ionization in the
ionosphere are:
- Ultraviolet radiation and x-ray from the sun
- High energy cosmic rays from the sun and from
super nova.
- Collusion between air molecules and energetic
particle.
 The electrical regions of the ionosphere play a
major role in radio communications. This is
because ions are reflectors of radio waves.
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Exosphere
 At very high altitudes above the earth the
atmosphere becomes extremely thin.
 At this level, atoms and molecules move
quite a distance before they collide with
one another
 At 250 km above the earth’s surface, an
atom can move an average distance called
mean free path of 1000 meter before
colliding with another atom.
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 Since the chance of molecular collusion is
reduced, many of the lighter, faster moving
molecules actually escape the earth’s
gravitational pull.
 Part of the atmosphere where atoms and
molecules shoot off in to space is called
exosphere, which represents the upper limit of
the earth’s atmosphere.
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