The pollutants in the atmosphere mainly come from automobile exhausts, smoke from burning of fuels like-coal, wood, etc, and chemical industries.
- Automobile Exhaust
in big cities, about 60% of atmospheric pollution is caused by internal combustion engines like scooters, cars, trucks, etc.
The major pollutants released through automobile exhaustS include monoxide (77.2%), oxides of nitrogen (7.76) and hydrocarbons (13.79%). 1he peuy eum used in automobiles also contains tetraethyl lead and tetramethyl lead. This is release in the form of particulate compounds.
- Industrial Pollutants
The gaseous air pollutants released by industries include SOg. CO CO. H hydrocarbons and nitrogen oxides. These are produced by the burning of coal and petroleum. Chemical industries also release, oxides of zinc, lead, arsenic, chlorine etc.
- Ionizing Radiations
1onising radiations mainly come from nuclear reactors or due to nuclear bomb explosion etc. These include alpha, beta and gamma rays. These are highly dangerous.
Types of Air Pollutants
Air pollutants can be divided into two categories,
- Primary air pollutants.
- secondary air pollutants.
Primary Air Pollutants
These pollutants enter the atmosphere directly from various sources and directly affect the air quality.
These include particulate matter, gases like CO, SO. nitrogen oxides hydrocarbons (HCs) etc.
Secondary Pollutants
These are formed during chemical reactions between primary air polutants and other atmospheric constituents, such as water vapours.
Secondary air pollutants include-photochemical smog, acids (which cause aad rain) etc.
These reactions mainly occur in the presence of sunlight.
Some important air pollutants have been discussed below
(a) Particulate matter
Particulate matter consists remain suspended in air.
Examples of particulate matter are soot, smoke, dust, asbestos fibres ,pesticides, some metals (including Hg, Pb,Cu and Fe), and also biological agents like tiny dust mites and flower pollen.
In the upper atmosphere (stratosphere), the accumulated particulate mar may significantly reduce the thermal radiation and lower the temperature the earth’s surface.
The freon gas used in refrigerators and aerosol sprays (containing chloro of fluoro carbons), reduce the ozone layer allows more ultraviolet rays to reach the earth. UV rays cause cancer and increase mutation rates due to their greater penetration power
(b) Carbon monoxide
It is a major environmental pollutant. High amount of carbon monoxide in the air causes difficulty in breathing resulting in headache. This gas affects the mucous membrane.
Carbon monoxide gets dissolved in hemoglobin of the blood and reduce its oxygen carrying capacity. This results in asphyxiation and ultimately causes death of the organism. If the concentration of carbon monoxide becomes more than 100 ppm, it causes dizziness and lassitude.
(c) Hydrocarbon (HCs) or volatile organic carbon (VOCs)
Hydrocarbons are compounds, composed of hydrogen and carbon. These are produced naturally during decomposition of organic matter and also by certain types of plants (e.g.. pine trees). HCs are also generated during the burning of fossil fuels (coal and petroleum).
Methane (CH,), the most abundant hydrocarbon in the atmosphere. It s evolved from soil in flooded rice fields and swamps.
Benzene and its derivatives, such as benzaldehyde are carcinogeni (Cause cancer).
Formaldehyde emitted from indoor sources, such as newly-manulacu tured carpeting, causes indoor pollution.
Unburnt hydrocarbons, like benzpyrene reaches the lungs and cancer. Ethylene causes premature leaf fall, flower bud shedding, curling of petals and discolouration of sepals.
(d) Sulphur dioxide (SO2)
It is a major constituent in the effluents released, when sulphur containing coal is burnt.
Ore smelters and oil refineries also emit significant amounts of SO,. Sulphur di oxide may cause respiratory problems.
This gas also forms sulphuric acid. As a result, metals, electrical equipments, clothes, paints and varnishes, leather, etc., get corroded.
Sulphur dioxide also damages the membrane system of living cells. Sensitive plants like alfalfa, barley, cotton, wheat, apple, etc., are affected at even 0.1 ppm concentration of sulphur dioxide.
Lichens are very sensitive to sulphur dioxide pollution and, therefore, serve as bioindicator for SO,. The number of lichens (and mosses also) decreases with increase in sulphur dioxide concentration.
(e) Nitrogen oxides
These are the compounds which are formed by the combination of nitrogen
and oxygen gases in different proportions.
These are produced mainly during the combustion of fossil fuels at high temperatures in automobile engines.
NO includes a mixture of NO and NO.
Excess of nitrogen oxides, make the air brownish in colour. Such an air is called brown air.
The higher amounts of nitrogen oxides reduce the visibility. It also reduces the yield of crop plants.
Nitrogen oxides directly cause heart and lung problems. They may also cause cancer.
In upper atmosphere, the oxides of nitrogen may combine with water vapours to form nitric acid, which results in acid rain.
(f) Photochemical smog
It is a secondary air pollutant.
It is formed when oxides of nitrogen react with hydrocarbons in the present of UV radiation.
Photochemical smog = nitrogen oxides + ozone + PAN
it is formed when oxide of nitrogen react with hydrocarbons in the present of UV radiation.
Automobile exhaust is the main source of smog, as it contains hydrocarbons as well as nitrogen oxides.
A simple set of photochemical reactions, occurring inside an engine, which lead to the formation of smog, is shown below
Reaction, inside engine-
Ng +Og
2NO
Reactions, in atmosphere
2NO+O
2NO
NO2
UV radiation NO + O
O+Og
NO +O
HC + NO +Og
NO + O
NO, + PAN
The smog in areas of intense solar radiations forms brown air, while in areas of low solar radiation, it forms grey air.
The ozone, in the smog, may adversely affect the plant and animal life. It causes lung diseases. Being an effective oxidant, it corrodes the heritage buildings and
other cultural monuments.
PAN is highly sensitive to many plant species. It destroys the chlorophyll and reduces photosynthetic efficiency of the plants. It directly interferes with light reaction of photosynthesis. It inhibits ETS (during respiration) and reduces metabolism.
PAN causes acute irritation of eyes and may damage the corneal surface in human beings.
(g) Acid rains
Oxides of nitrogen (NO, NO,) and sulphur (SOg, SO,) are two main gaseous air pollutants produced by burning of fossil fuels.
These gases are highly reactive. In the atmosphere, these oxidised to form nitric and sulphuric acid.
These acids may get dissolve in atmospheric water and precipitate as acid rain. This is called wet deposition.
Sometimes, the acids do not precipitate, instead they get mixed with blowing air and get settled over ground. This is called dry deposition.
Acid rains increase acidity of soil lakes, streams, etc. This in turn affects land and aquatic flora and fauna.
Besides these, it also corrodes buildings, monuments (notable example 1s 1 Mahal at Agra), statues, bridges, etc.
Tags: Sources of Air Pollutants