Air is available in abundance in the surface of the earth. It is never found clean in nature. A number of gases are continually released into the atmosphere through natural activity. The tiny particles of solids or liquid are distributed throughout the air by winds, volcanic explosions and similar other natural sources. Man-made pollutants resulting from chemical and biological processes are also released into the atmosphere. These impurities lead to pollution of air.
Air pollution may be defined as any atmospheric condition in which certain substances are present in such concentration that they can produce undesirable effects on man and environment. These substances are gases such as sulphur oxides, nitrogen oxides, carbon monoxides, hydrocarbons and particulate matter such as smoke, dust, fumes, aerosols as well as radioactive materials etc.
Classification of Air Pollutants:
Air pollutants are classified on the basis of origin of pollutants, chemical composition and states of matter. These are:
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(I) Primary Pollutants:
The primary pollutants are emitted directly from the sources e.g., Ash smoke, dust, fumes, dust and spray, sulphur dioxides, hydrogen sulphide, nitric oxide, ammonia, carbon monoxide, hydrogen fluoride, olefinic, aromatic hydrocarbons and radioactive compounds.
(II) Secondary Pollutants:
The primary pollutants discharged into the atmosphere undergo chemical changes in the presence of water, oxygen and ultraviolet rays of sunlight to form secondary pollutants.
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The emissions of air pollutants depend upon the method of combustion and the type of fuel used. The combustion of all fuels results in exothermic oxidation of carbon, hydrogen, sulphur and nitrogen. In case of complete combustion, carbon dioxide, water vapour, nitrogen oxides, volatile and non-volatile trace metals such as Arsenic, Cadmium, Lead and Mercury would be the principal emission.
However, in practice complete combustion does not occur and as a result of additional particulate and gaseous pollutants result in the production of secondary pollutants. Certain pollutants undergo atmospheric oxidation to form acidic precipitation.
In addition, indoor air pollution is also considered to be of great importance in terms of personal exposure. The principal air pollutants are divided from combustion of stoves, heaters, cigarettes, building materials and furnishing air products. House hold dust is also believed to be of importance in the indoor air pollution. The impact of indoor emissions depends upon the strength of the source and the ventilation rate.