Atmospheric Pollution
Atmospheric Pollution
The
atmosphere that surrounds the earth is not of the same thickness at all
heights. There are concentric layers of air or regions and each layer has
different density. The lowest region of atmosphere in which the human beings
along with other organisms live is called troposphere. It extends up to the
height of ∼10 Km from sea level. About the troposphere, between 10 and 50 Km
above sea level lies stratosphere. Troposphere is a turbulent, dusty zone
containing air, much water vapor and clouds. This is the region of strong air
movement and clouds formation. The stratosphere, on the other hands, contains
dinitrogen, dioxygen, ozone and little water vapor.
Atmospheric
pollution is generally studied as tropospheric and stratospheric pollution. The
presence of ozone in the stratosphere prevents about 99.5 per cent of the sun’s
harmful ultraviolet (UV) radiations from reaching the earth’s surface and
thereby protecting humans and other animals from its effect.
Tropospheric Pollution
Tropospheric
pollution occurs due to the presence of undesirable solid or gaseous particles
in the air. The following are the major gaseous and particulate pollutants
present in the troposphere:
- Gaseous air pollutants: These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.
- Particulate pollutants: These are dust, mist, fumes, smoke, smog etc.,
Gaseous air pollutants
(a) Oxides of
Sulphur: Oxides of sulphur are produced when sulphur containing fossil fuel is
bunt. The most common species, sulphur dioxide, is a gas that is poisonous to
both animals and plans. It has been reported that even a low concentration of
sulphur dioxide causes respiratory diseases e.g., asthma, bronchitis, emphysema
in human beings. Sulphur dioxide causes irritation to the eyes, resulting in
tears and redness. High concentration of SO2 leads to stiffness of
flower buds which eventually fall of from plants. Uncatalysed oxidation of
sulphur dioxide is slow. However, the presence of particulate matter in
polluted air catalyses the oxidation of sulphur dioxide to sulphur trioxide.
2SO2
(g) +
O2 (g) → 2SO3 (g)
The reaction
can also be promoted by ozone and hydrogen peroxide.
SO2
(g) +
O3 (g) → SO3 (g) + O2(g)
SO2
(g) +
H2O (I) → H2SO4 (aq)
(b) Oxides
of Nitrogen: Dinitrogen and dioxygen are the main constituents of air. These
gases do not react with each other at a normal temperature. At high altitudes
when lightning strikes, they combine to form oxides of nitrogen. NO2
is oxidised to nitrate ion, NO3 which is washed into soil, where it
serves as a fertilizer. In an automobile engine, (at high temperature) when
fossil fuel is burnt, dinitrogen and dioxygen combine to yield significant
quantities of nitric oxide (NO) and nitrogen dioxide (NO2) as given
below:
N2
(g) +
O2 (g) (Heat 148 K) → 2NO (g)
NO reacts
instantly with oxygen to give NO2
2NO (g) + O2
(g) → 2NO2 (g)
Rae of
production of NO is faster when nitric oxide reacts with ozone in the stratosphere.
NO(g) + O3(g) → NO2(g) + O2(g)
The irritant
red haze in the traffic and congested places is due to oxides of nitrogen.
Higher concentrations of NO2 damage the leaves of plants and retard
the rate of photosynthesis. Nitrogen dioxide is a lung irritant that can lead
to an acute respiratory disease in children. It is toxic t living tissues also.
Nitrogen dioxide is also harmful to various textile fibers and metals.
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Hydrocarbons: Hydrocarbons are composed of hydrogen and carbon only and are
formed by incomplete combustion of fuel used in automobiles. Hydrocarbons are
carcinogenic, i.e., they cause cancer. They harm plans by causing ageing,
breakdown of tissues and shedding of leaves, flowers and twigs.
(d) Oxides of Carbon
(i)
Carbonmonoxide: Carbon monoxide (CO) is one of the most serious air pollutants.
It is a colorless and odorless gas, highly poisonous to living beings because
of its ability to block the delivery of oxygen to the organs and tissues. It is
produced as a result of incomplete combustion of carbon. Carbon monoxide is
mainly released into the air by automobile exhaust. Other sources, which
produce CO, involve incomplete combustion of coal, firewood, petrol, etc. The
number of vehicles has been increasing over the years all over the world. Many
vehicles are poorly maintained and several have inadequate pollution control
equipment resulting in the release of greater amount of carbon monoxide and
other polluting gases. Do you know why carbon monoxide is poisonous? It binds
to haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable
than the oxygen-haemoglobin complex. In blood, when the concentration of
carboxyhaemoglobin reaches about 3-4 per cent, the oxygen carrying capacity of
blood is greatly reduced. This oxygen deficiency, results into headache, week
eyesight, nervousness and cardiovascular disorder. This is the reason why
people are advised not to smoke. In pregnant women who have the habit of
smoking the increases CO level in blood may induce premature birth, spontaneous
abortions and deformed babies.
(ii) Carbon
dioxide: Carbon dioxide (CO2) is released into the atmosphere by
respiration, burning of fossil fuels for energy, and by decomposition of
limestone during the manufacture of cement. It is also emitted during volcanic
eruptions. Carbon dioxide gas is confined troposphere only. Normally it forms
about 0.03 per cent by volume of the atmosphere. With the increased use of
fossil fuels, a large amount of carbon dioxide gets released into the atmosphere.
Excess of CO2 in the air is removed by green plants and this
maintains appropriate level of CO2 in the atmosphere. Green plans
require CO2 for photosynthesis and they, in turn, emit oxygen, thus
maintaining the delicate balance. As you know, deforestation and burning of
fossil fuel increases the CO2 level and disturb the balance in the
atmosphere. The increased amount of CO2 in the air is mainly
responsible for global warming.
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