Primary pollutants
1. Sulphur dioxide (S02) produced from burning of coal mainly in thermal power plants. It causes smog and acid rain.
2. A Nitrogen oxides (N02) causes smog and acid rain. It is produced from burning fuels including petrol, diesel and coal.
3. Carbon monoxide (CO) is a product by incomplete combustion of fuel such as natural gas, coal or wood.
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4. Chlorofluorocarbons (CFCs), released mainly from refrigeration. These gases combine with few other gases, which leads to a reduction of the ozone layer that protects the earth from the harmful ultraviolet rays of the sun.
5. Carbon dioxide (C02), a greenhouse gas emitted from combustion.
6. Suspended particulate matter (SPM), include smoke, dust and vapour that can remain suspended for extended periods and can introduced in the environment as primary as well as secondary pollutants.
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7. Toxic metals, such as lead, cadmium and copper can enter to the environment through petrol, hair dye products, paints, batteries etc.
8. Volatile organic compounds (VOC), such as hydrocarbon fuel vapors and solvents.
9. Ammonia (NH3) emitted from agricultural processes.
10. Odors, such as from garbage, sewage, and industrial processes
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11. Radioactive pollutants produced by nuclear explosions and war explosives, and natural processes such as radon.
Secondary pollutants
1. Particulate matter formed from gaseous primary pollutants and compounds in photochemical smog, such as nitrogen dioxide.
2. Ground level ozone (03) formed from NO-, and VOCs.
3. Peroxyacetyl nitrate (PAN) similarly formed from N02 and VOCs.
Effects of air pollution (A) On biological systems
(a) On animals
Air pollution is a major alarming environmental problem affecting the developing and developed countries adversely.
The health effects caused by air pollutants may
range from biochemical and physiological changes to difficulty in breathing, wheezing, coughing.
All pollutants that are inhaled have serious impact on human health affecting the lungs and the respiratory system. Individual reactions to air pollutants depend on the type of pollutant, the degree of exposure, the individual’s health status and genetics.
These pollutants are also deposited on soil, plants and in the water, further contributing to human exposure. The most harmful of the gases and pollutants that emitted are particulate matter, carbon dioxide, polycyclic organic matter, and formaldehyde.
(b) On plants
Photochemical smog has a deleterious effect on plants. Chronic exposure to ozone may weaken plants and make them susceptible to disease and reduce crop yield. Acids (acid rain) enhances the uptake of toxic heavy metals by the plants
(B) On non-biological systems
(a) Climatic effects
(i) Green house effect
In the environment, carbon dioxide is confined exclusively in troposphere. In dense concentration, it can act as serious pollutant.
The temperature of the earth surface has been maintained by the energy of the sun’s rays that strike the planet and heat that gets radiated back into the space.
Some of the rays that penetrate carbon dioxide layer are able to strike earth and get converted into heat.
The heated earth is able to reradiate this absorbed energy as radiations of longer wavelength. At higher CO, concentrations, much of heat gets absorbed by the CO? And water in the atmosphere and adds to the heat that has been already present. Thus the earth’s atmosphere heats up. This phenomenon of heating of earth’s atmosphere is termed as green house effect.
(ii) Global warming
Since Chlorofluorocarbons and N01 deplete ozone layer, they allow the penetration of harmful UV radiation to strike the earth’s surface. CFCs can hang in the atmosphere for decades.
They slowly diffuse into the stratosphere, where UV-radiation decomposes them into atomic chlorine that triggers a lengthy CLOx chain reaction forming various oxides. This chain eats away the ozone. This led UV rays to enter into the atmosphere. This warms up the troposphere to an uncomfortable level and raise surface temperature causes global warming.
(iii) Acid rain
Acid rain means the presence of excessive acids in rain water. Sulphur dioxide (S02) and nitrogen dioxide (N02) are highly soluble in water when present in the atmosphere in higher amount. During rain these react with water vapour of the atmosphere to form acids like sulphuric acid, sulphurous acid, nitric acid and nitrous acid.
(b) Corrosive and soiling effects
Building, bridges and other manmade structures can be soiled and damaged by air pollution. The accumulation of dust can alter the appearance of a building. Sulphur dioxide in smog chemically transforms marble into gypsum, causing it to crack and flake off.
(c) Ozone depletion
Ozone is present at all altitudes in the atmosphere mainly in the stratosphere (extending from 12 kms to 35 kms). This layer protects the earth’s atmosphere from ultraviolet radiations coming from solar system. In nature, there is equilibrium between ozone and oxygen
Ozone depletion (decrease in concentration of ozone) involves the conversion of atmospheric nitrogen into N02 due to solar activity or anthropogenic processes. N02 and CFCs are ozone eaters.
Ozone depletion causes passing of harmful cosmic rays and ultraviolet rays and other harmful rays to the troposphere and these rays affect human life. This will results in increase of skin cancer and increasing temperature.
Air quality standards
In general, there are two types of air quality standards
i) National Ambient Air Quality Standards set maximum atmospheric concentrations for specific pollutants e.g. industrial, residential and sensitive areas.
ii) Air Quality Index takes the form of a scale with various thresholds. The scale may or may not distinguish between different pollutants.