Introduction

The atmosphere is a layer of air (mixture of gases) that expands up to 700 kilometers above the Earth's surface. The atmosphere is a compound and speedily changing part of 12 the earth. The   fluid nature of the atmosphere causes it to be the most changeable part of the Earth. The atmosphere is constantly under the influence of gravity, earth's rotation and differential heating by solar radiation. The atmosphere drives the weather (atmospheric variables that change   rapidly-air temperature, humidity, percentage of cloudiness, kind and amount of precipitation, air pressure and wind velocity) and the climate (the average weather situation in a region over a long period of time).

The gases in the air directly or indirectly support life on the Earth; during respiration,  living things consume oxygen and release carbon dioxide; plants absorb carbon dioxide during photosynthesis and release oxygen; nitrogen is converted to useful nutrients to support plant growth and hence, that of the animals that depend on plants for survival.

General Composition of the Atmosphere

A superior percentage of the air in the atmosphere is made up of clear and odourless gases-nitrogen and oxygen. Such 2 gases are said the primary or 'permanent' gases. According to their relative volumes, average composition of the dry unpolluted atmosphere is specified in Table

1. Fluctuating amounts of most of such gases might be originated in each of the layers of the atmosphere. Several of the concentrations are uncertain because (i) analytical procedures  for some components have only recently reached the stage where good data can be obtained;  (ii) some components such as CH₄, N₂O and  CO₂  are known  to  be  increasing  in  concentration  at  an appreciable rate; and (iii) it is doubtful whether any parts of the atmosphere can be considered completely free of pollutants or not.

The primary gases don't influence the weather unlike the trace gases. The lower atmosphere encloses varying amounts of water vapour which determines its humidity. Water vapour (and to a lesser degree, CO₂) is of great  significance  in  radiative  transfer  since  it  absorbs  and  emits strongly the infra-red (IR), the region of the spectrum at which earth radiates energy back to space. Carbon dioxide plays a major role in the greenhouse result and the attendant temperature attenuation of the earth. Various gaseous and solid impurities such as Chlorofluorocarbons (CFCs) in the atmosphere resulting from human activities contribute to the ozone layer depletion. The air also carries many kinds of dust of meteorite and terrestrial origin alongside micro-organisms, pollen and other particulate matters of anthropogenic origin.

The earth's atmosphere is a little thicker at the Equator and a little thinner at the poles because, the earth's spinning motion reasons it to bulge slightly at the Equator. Since of the pull of gravity, the density of the atmosphere and the pressure exerted by air molecules are greatest near the earth's surface (approximately 1g/103m³ and 106 dynes/cm², correspondingly). Air pressure decreases speedily through rising altitude, reaching one-half of its sea-level value at about 5,500 m (5.5 kilometres). At standard temperature and pressure (s.t.p.), 22.4 L of air weighs 28.97g.

Table: Average Composition of the Dry Unpolluted Atmosphere

Composition and Characteristics of the Earth's

Atmospheric Layers

The Earth's atmosphere is composed of distinct layers, namely; the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere and the Exosphere.

The troposphere

The troposphere holds most of the air and is the place where storms occur. It extends from the Earth surface upward to a height of about 10 kilometres  at  the  poles,  11.3  kilometres  in  mid-latitudes  and  16.1 kilometres  at the Equator. The air in the troposphere is in constant horizontal and vertical motions. Throughout the troposphere, temperature reduces through altitude at an average lapse rate of about 2^oC per 305 m reaching about -57^oC at the tropopause (the peak of the troposphere).

The troposphere might be considered in 2 smaller components: the part in contact through the earth surface is said the boundary layer; above it is the free troposphere. The boundary layer is usually bounded at its upper extreme by a temperature inversion (a horizontal band in that temperature enhances through height) through which little exchange of air can take place by the free troposphere above. The depth of the boundary layer is classically around 100 metres at night and 1000 metres during the day, even though such figures do fluctuate greatly. Pollutant emissions are generally emitted into the boundary layer and are constrained inside it to a huge extent. Free tropospheric air encloses the longer-lived atmospheric components together with contributions from pollutants, that have escaped the boundary layer, and from several downward mixing stratospheric airs.

The troposphere is the layer of greatest interest through respect to pollution problems since; it is the layer in that most living things survive.

Fig: The Atmospheric Layers

Some important tropospheric chemical Transformations

1. Formation of Hydroxyl Radical (OH)

(a) Photolysis of ozone: When ozone undergoes photolysis in the presence of light of short wavelength (λ < 315mm) singlet atomic oxygen O (ID) is formed. This might relax to the triplet state O (3P) or might react through water vapour to form OH.

O₃ + hv    O (lD) + O₂ (λ<315nm)

O(lD) + M O(3P) + M

or O(²D) + H₂O    2OH

This is the primary source of OH in the atmosphere. (M is an inert energy absorbing molecule for example N₂).

(b)  Reactions of O(lD) with CH₄ and H₂. This is one of the minor sources of OH in the atmosphere.

 CH₄ + O(^lD)  CH₃ + OH

H₂  + O(^lD)   H + OH

(c) Photolysis of HONO and H2O2. This produces OH directly.

HONO + hv OH + NO (λ<400nm)

H₂O₂ + hv 2OH (λ<360nm)

(d) From HO₂ radical. In polluted atmospheres, HO₂ is able to give rise to OH formation.

HO₂ + NO NO₂ + OH

The hydroxyl radicals play a central role in the formation of ozone, peroxyacetyl nitrate (PAN), sulphuric acid and nitric acid.

2. Formation of ozone: Ozone could be shaped in the atmosphere (troposphere particularly) from the oxidation of CH₄ and CO in processes including the hydroxyl radicals.

CH₄ + OH             CH₃ +H₂O

CH₃ + O₂+M        CH₃O₂ + M

CH₃O₂ + NO       CH₃O + NO₂

CH₃O + O₂  CH₂O + HO₂

As well, CO + OH   CO₂ + H  

H + O₂+M HO₂ +M

Now, HO₂ + NO  OH + NO₂

NO₂ + hv NO + O(³P) (λ < 435nm)

O(³P) + O₂+M O₃ + M  

Formation of peroxyacetyl nitrate (PAN): PAN is of interest as a trait product of atmospheric photochemistry, as a probable reservoir of reactive nitrogen in remote atmospheres and since of its unfavorable health consequences on plants. Its formation is acetyl radicals, CH₃CO, formed from acetaldehyde oxidation.

CH₃CHO + OH CH₃CO + H₂O

CH₃CO + O₂  CH₃C(O)OO

CH₃C(O)OO + NO₂   CH₃C - OONO₂

                                                 O   (PAN)

4. Formation of sulphuric acid:  Atmospheric oxidation of SO₂ could proceed via a range of mechanisms but formation through the hydroxyl radical reaction in the gas phase is of overwhelming importance.

SO₂ + OH  HOSO₂

HOSO₂ + O₂  HO₂ + SO₃

SO₃ + H₂O H₂SO₄

5. Nitric acid formation:  The main daytime route of HNO₃ formation is from the reaction:

NO₂ + OH HNO₃

At night, reaction of NO₃ radical becomes significant that wasn't operative during the day due to photolytic breakdown of NO₃. The radical is shaped as follows:

NO₂ + O₃ NO₃ + O₂ and NO₃+ RH

M NO₃ + NO₂        N₂O₅ + H₂O

HNO₃ + R    N₂O₅ 2HNO3

One of the atmospheric changes that have attracted a widespread attention over the last few decades is the phenomenon of acid rain or acid deposition. Acid rain results when  gaseous emissions such as sulphur oxides (SOx) and nitrogen oxides (NOx) interact with water vapour and sunlight  and  are  converted  to  strong acidic compounds as specified (Nos. 4 and 5) above.

The stratosphere

The stratosphere is the second layer of the atmosphere as one move upward from the earth's surface. It lies above the troposphere and below his mesosphere. The stratosphere starts at approximately 10 kilometres and extends to approximately 50 kilometres elevated.  The elevation of the bottom of the stratosphere varies by latitude and seasons. The bottom of the stratosphere is around 16 kilometres near the equator, around 10 kilometres at mid-latitudes and around 8 kilometres near the poles. It is slightly lower in winter at mid and high-latitudes and slightly higher in the summer.

Inside the stratosphere, temperature raise as altitude increases, reaching about - 3^oC at its top (the stratopause). Above the stratopause, temperature again reduces through height. In the stratosphere, air doesn't flow up and down, but flows parallel to the earth in an extremely fast moving air streams. This dynamic stability of the stratosphere is due to the warmer layers above and cooler layer below the stratosphere. The heating of the upper layer is reasoned via an ozone layer that absorbs solar ultra violet (UV) radiation.

The stratosphere is extremely dry; it encloses air by little water vapour. As a consequence of this, few clouds are found in this layer through most clouds occurring in the lower (more humid) troposphere. Although, polar stratospheric clouds (PSCs) nacreous clouds as well termed show in the lower stratosphere near the poles in winter. They are originating at altitudes of 15 to 25 kilometres and form only when temperatures at those heights dip below - 78^oC. They are suspected to play some roles in the formation of 'holes in the ozone layer' through catalyzing certain chemical reactions that destroy ozone. Since bacterial life survives in the stratosphere, it is considered part of the biosphere.

The stratospheric ozone layer chemistry

The ozone layer is a diffused concentration of ozone (O₃) originates predominantly in the stratosphere. About 90 percent of the atmospheric ozone is placed within the stratosphere. Ozone actually occurs in trace amounts throughout the atmosphere through a peak concentration of about 10 ppm by volume in the lower stratosphere between about 20 to 25 kilometres altitude.  The concentration of ozone in the stratosphere illustrates daily, annual and seasonal variations of numerous per cent. Consequently, it is hard to recognize and quantify changes in concentration caused via anthropogenic effects unless such are huge.

The ozone layer shields life from the harmful UV rays of the sun. Whereas acid rain and photochemical oxidant pollution are somewhat localised environmental problems, modification of stratospheric ozone is a global phenomenon. The electromagnetic UV radiation from the sun that falls on the upper layers of the atmosphere is subdivided into three regions of different energy or wavelength (λ). These regions are: highest energy (λ < 290 nm), transitional energy (λ = 290 - 320 nm) and lowest energy (λ = 320 - 400 nm).

At altitudes above 400 kilometres, the majority of the oxygen is atomic since the bonds in the dioxygen molecule might be broken through UV (λ < 290 nm) to form free U ox V ygen atoms.

 O₂ 2(O).

At lower altitudes, as the no. of dioxygen and dinitrogen molecules enhances, ozone (O₃) is shaped.

O + O₂ + M     O₃+ M.

The molecular species, M, (generally N₂ or O₂) eliminates the excess energy generated when ozone is shaped and so prevents the ozone from decomposing immediately. Ozone is destroyed by the absorption of UV and via several other naturally occurring chemical species, X, (X = NO and HO generally) in the stratosphere through a  reaction  sequence  that regenerates such species so that they can react once more .

O₃  (UV) O + O₂

or O₃ + X     O₂  + O

 XO + O   O₂ + X.

Currently, there have been significant increases in the stratospheric concentrations of Chlorine (Cl) and Bromide (Br). It has been estimated that one atom of Cl can demolish 100 000 molecules of ozone before the cycle is interrupted. The bromine cycle isn't easily broken; once shaped, the ozone shifts towards the poles and as well to lower altitudes.

There is generally a higher concentration of ozone above Polar Regions because the rate of removal is slower than near the equator. The absorption of UV and infrared radiation in the stratosphere means that the temperature in the stratosphere is higher than the temperature at the top of the troposphere. This temperature inversion stabilizes air movements in the stratosphere and controls weather patterns in the stratosphere.

The stratospheric air above the Antarctic encloses extremely little water vapour (4 - 6 ppm via volume). Consequently, nucleation and condensation to form clouds only take place at very low temperatures. When the temperature drops below -75^oC, nitric acid trihydrate (HNO₃.3H₂O), NAT, particles, begin to condense to form kind 1 polar stratospheric cloud (Type 1 PSCs). Their formation eliminates nitrogen oxide species from the gas phase and traps them in the clouds as solid nitric acid. Nitrogen dioxide can break the ozone, destroying the cycle initiated through chlorine via reacting through the chlorine monoxide,

ClO + NO₂    CIONO₂

But this is prevented when the nitrogen species become locked up in the clouds. To worsen the condition, any chlorine nitrate, CIONO₂ that is present in the stratosphere can be destroyed by reactions that occur on the surface of the solid particles, but don't take place in the gas phase.

CIONO₂ + H₂O HOCI + HNO₃

CIONO₂ + HCI CI₂ + HNO₃

Such reactions raise the quantity of the potentially reactive chlorine species Cl (from Cl₂) and HOCI and CINO₂. The raised employ of chlorine compounds as refrigerants, foaming agents, solvents and in aerosol-spray cans has generated a rapid increase in the concentration of chlorine in the atmosphere. Natural levels of chlorine are about 0.6 ppb via volume. In the year 1992, concentrations had reached 3.5 ppb and might increase further to 4 ppb before a decline is supposed.

The chlorofluorocarbons (CFCs) such as CCI₃F and CCI₂F₂ were expanded as inert, non-toxic compounds that could be securely utilized through industry and in home. Unfortunately, they are so inert that when they escape into the atmosphere they gradually pass unchanged through the troposphere and enter the stratosphere. Here, they are subjected to photochemical decomposition under the influence of UV radiation to provide up free chlorine atoms that catalyse the decomposition of ozone for instance.

                     uv                       O₃                       O                     O

CCI₂F₂ CI O₂+CIOO₂+CIOO₂+CI (Can react again)                                                                                        

Dichlorodifluoro            Activated               Chlorine oxide

methane                    Chlorine atom

The CFCs have long atmospheric lifetimes (65 years for CCI₃F and 130 years for CCI₂F₂). There are enough of these compounds in the atmosphere to support the elevated stratospheric chlorine concentrations until 2100. Since anthropogenically introduced chlorine and bromine levels will remain high for so long, it is expected that there will raise in dangerous UV radiations reaching the earth's surface. Before ozone depletion began, 70 - 80 per cent of the dangerous UV radiation was absorbed before reaching the earth surface. A 10 per cent decrease in stratospheric ozone concentration would diminish the amount of dangerous UV being absorbed to 55 - 65 per cent. This could have main consequences on susceptible organisms these as plankton and land plants.

Effects on humans would comprise enhanced risk of skin cancer and eye cataracts. Such direct results on human are less worrying than the interference through food production that could happen. The enhanced utilize of fertilizers is causing a raise (0.25% per year) in the quantities of N₂O being produced through micro-organisms. Like CFCs, N₂O is extremely steady and passes up through the troposphere and lastly enters the stratosphere. Here, (Stratosphere), is broken down under the pressure of UV radiation into a mixture of about 95 per cent N2 and 5 per cent NO. NO is one of the ozone destroying compounds. N₂O

                UV

2N₂O      N₂  + 2NO

                         (95%)

NO + O₃   NO₂ + O₂

NO₂ + O  NO + O₂

                          (5%)

The consequences of enhanced N₂O concentrations are comparatively tiny and might well be balanced via enhances in methane (CH₄) that is capable of rising or preserving ozone concentrations via reacting through chlorine in the stratosphere.

The mesosphere

The mesosphere is a layer of earth's atmosphere straight above the stratosphere and below the thermosphere.  It expands from about 50kilometres to 85kilometres above the earth surface. The stratosphere and mesosphere mutually are sometimes termed to as the middle atmosphere.  Throughout the mesosphere, temperatures decreases with height; the coldest temperatures (approx. - 900C) inside the Earth's atmosphere are originate near the top of the mesosphere for example within the mesopause.

Much about the mesosphere is still mysterious since, it is hard to obtain instrumental measurements of the mesosphere straight Weather balloons and other aircraft can't fly elevated sufficient to reach the mesosphere and satellites orbit above it; it can't as well directly calculate traits of this layer. Scientists utilize the instruments on sounding rockets to example the mesosphere directly, but these flights are brief and infrequent.

Most meteors vaporize in the mesosphere. Some materials from meteors linger  in  the  mesosphere  such  that  this  layer  has  a  relatively  high concentration of iron and other metal atoms. Very strange, high altitude clouds called 'noctilucent clouds' or 'polar mesospheric clouds' sometimes form in the mesosphere near the poles. Such peculiar clouds form at a much higher altitude than other kinds of clouds. Odd electrical discharges alike to lightning termed 'sprites' and 'ELVES' infrequently appear in the mesosphere dozens of kilometers above thunderclouds in the troposphere below. Inside and below the mesopause, gases made of dissimilar kinds of atoms and molecules are thoroughly blended mutually through turbulence in the atmosphere.

The Thermosphere

Above the mesosphere is the thermosphere. It extends for about 90 kilometres above the earth surface. It is the layer of the Earth's atmosphere that is 1^st exposed to the sun's radiation. In the thermosphere, gases elements collide so infrequently and obtain sorted into strata depend on the molecular mass and kinds of chemical elements they enclose. Inside this layer, UV radiation causes ionisation of the atmospheric particles, enabling radio waves to bounce off and be obtained beyond the horizon.

Thermospheric temperatures enhance through altitude and can rise to 1500⁰C and above due to absorption of extremely energetic solar radiation via the tiny amount of residual oxygen still present. The air within the thermosphere is so thin that a tiny enhance in energy can translate to a huge temperature raise. Since of the thin air in the thermosphere, scientists can't calculate the temperature straight. They compute the density of the air via pressure drag it puts on satellites and then utilize the density to find the temperature.

The ionosphere is the very outer edge of the thermosphere. It is not a separate layer as such; it is a place where gas atoms drift into space from here. It is called ionosphere because in this part of the atmosphere where the sun's radiation is ionized, or pulled apart as  it travels  through the earth's magnetic fields to the north and south poles. This pulling apart is seen from the Earth as auroras. The colorful displays of auroras  are called the 'Northern  Lights' or "Aurora Borealis" in the Northern Hemisphere, the 'Southern Lights' or 'Aurora Australis' in the Southern Hemisphere.

The dynamics of the lower thermosphere (below 120 kilometres) are dominated through atmospheric tide, which is driven in part, via the extremely important diurnal heating. The atmospheric tide dissipates above this level since molecular concentrations don't support the coherent motion required for fluid flow.

The exosphere

The exosphere is the highest layer of the atmosphere. Together through the ionosphere, it could be considered as part of the thermosphere extending from 500 to about 1000 kilometers (it might extend to about 10,000 kilometres above the earth's surface). This, certainly, is the upper limit of the earth's atmosphere where the atmosphere turns or merges into space.

The extremely diluted gas in this layer can reach 2500^oC during the day; even though the temperature is so high, one would not feel hot in the thermosphere; since it is so near vacuum that there isn't sufficient contact through the few atoms of gas to move much heat. A usual thermometer would read significantly below 0^oC due to the energy lost via radiation overtaking the energy obtained from the atmospheric gas via direct contact.

In this region of the atmosphere, hydrogen and helium are the prime components and are only present at very low densities. This is the zone where many satellites orbit the Earth.

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