The atmosphere, participating in biological processes, acquires trace concentrations of many organic and inorganic gases. It also acquires trace gases through earth erosion, volcanic action, and other inanimate processes. Since excessive amounts of these trace gases are generally detrimental to human welfare, they are properly called pollutants. As they are transported through the atmosphere, these pollutants are subject to photolysis, oxidation, and reaction with other trace gases. Reactions involving the pollutants are most pronounced at the highest altitudes. Above the ozonosphere, the strength of the ultraviolet sunlight is sufficient to break up any molecule.

The processes of gas transport, photolysis, and chemical reaction create a condition in the atmosphere in which the concentrations of pollutants vary with altitude, weather, terrain, and time of day. This is the normal or steady-state condition of the atmosphere to which man has adapted himself in an ecologically balanced world. Unfortunately, the increasing population, increasing human consumption, and the increasing use of fossil fuels are perturbing the ecological balance. It is hoped that the self-cleansing of the atmosphere still exceeds the rate of emission of pollutants, but there is no doubt that in the vicinity of large cities the self-cleansing has been overtaxed. Government-sponsored monitoring programs are attempting to following the changes in atmospheric trace gas concentrations.

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