Particulate material present in the atmospheric air may include very fine and light particles, heavier dust particles, various types of fumes, vapours, pollen grains, fungal spores, algae, bacteria etc. Very small particles which are so light that turbulence of air keeps them afloat almost permanently are known as aerosols.
Much of the smoke derived from the combustion of organic matter, coal, petroleum and natural gas, forest fires, volcanic emissions etc. comes under the category of very fine particles or aerosols. From the point of view of quality of atmospheric air aerosols are of tremendous importance.
Particles larger than aerosols are usually classed as dust particles and for liquid droplets the term mist is often used. There is no hard and fast rule demarkating the aerosols from dust particles or mist. The grouping is purely arbitrary. Some authorities consider only those particles which are 10 µm in diameter or smaller as aerosols while others include particles upto 500 µm in diameter in the category.
Possibly the main criterion which distinguishes aerosols from dust particles is their weight. Aerosols are so light that they keep afloat almost permanently. They do not settle down under influence of gravity while dust particles are heavier and as such they settle out rather quickly before winds can disperse them too far off places.
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In India about 230 million tons of coals were consumed in 1990-91, more than half of which, about 118.7 million tons, were burned by Thermal Power Generation Sector to produce electricity Fly-ash content of Indian coal ranges between 28-42% and about 12% of the total ash content in coal escapes as smoke, fly-ash or aerosols. With the present rate of consumption of coal we are introducing about 13-16 million tons of fly-ash into the atmosphere. Automobile exhausts are another important source of particulate pollution.
While petrol driven vehicles emit only 1.8 kg of aerosols forever 1000 litres of fuel used, diesel powered vehicles inject about 14 kg of aerosols into the air for the same volume of diesel used. The total consumption of petroleum products in India was around) Million tons in the year 1990-1991, much of which was kerosene, petrol and diesel.
Apart from these, various mining and processing operations, processing of agricultural produce, handling and processing fibres like cotton, jute, hemp, coir etc. and most of the large or small-scale industrial establishments generate their share of aerosols. This should serve to illustrate the enormous magnitude of aerosols which we are introducing in the atmosphere in India alone.
Yet the Indian contribution to the global load of atmospheric pollution is a mere trifle as compared to the developed countries. The minority which constitutes the developed countries of the world is responsible for injecting almost two-third of the entire bulk of pollutants while the developing countries including India which represent major part of the global population contribute only one- third of the total burden of atmospheric pollution.
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In late seventies so much fine particulate material was introduced into the atmosphere that satellite pictures of earth under conditions of still weather showed a wide ring of dust and smoke encircling the globe around the latitudes of USA, Europe and Japan (MIT Report, 1970).