The pollutants of the atmosphere are simple gases, hydrocarbons and particulate material which can be disposed of easily by biotic agencies.

However, the biosphere occurs only of earth’s surface while most of these pollutants escape high up in the atmosphere. Therefore, abiotic processes, which involve water vapours, the constituent gases of the atmosphere, solar radiations and chemical constituents of particulate material, are responsible for causing their chemical transformations.

In the atmosphere carbon monoxide is oxidized to carbon dioxide. Most of the hydrocarbons are also oxidised to yield carbon dioxide and water. Similarly nitrous and nitric oxides are changed to nitrogen dioxide. These oxides dissolve in minute droplets of water or the fine film of water on aerosol surface to yield the corresponding acids.

If ammonia is present in the atmosphere it also dissolves in water to form ammonium hydroxide. A variety of chemical reactions occur on aerosol surface in the atmosphere between these reactants. Aerosols containing other constituents such as chlorides, carbonates and other salts also take part in the atmospheric reactions. The metallic constituents of the aerosols, usually, catalyse these reactions.

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Depending upon the quantities of various constituents present in the atmosphere and the extent of atmospheric reactions aerosols bearing various acids such as carbonic, nitric and sulphuric acids; and their salts such as carbonates, nitrates and sulphates are formed. These are brought down in bulk with rains, dew or snow. Gases present in amounts larger than those needed to saturate the carrrying medium (water) are left in the atmosphere.