Chemical reactions, producing secondary pollutants

Many of the chemical reactions that create photochemical smog take place in aerosol particles. These reactions produce secondary pollutants— pollutants that are not directly released from some source but are formed by reactions with other components in the air. 

Air pollutants are divided into two broad categories primary and secondary. Primary pollutants are those emitted directly into the air, in contrast to secondary pollutants, which are created in the atmosphere by the reactions among the primary pollutants, usually in the presence of sunlight. Specifically, a variety of chemical or photochemical reactions (catalyzed by light) produce a wide range of secondary pollutants, especially in urban air. A prime example is the formation of ozone in smog. 

Atmospheric pollution cannot be controlled so long as the nature and the mechanism of formation of its deleterious constituents remain unknown. While many chemical constituents of polluted atmospheres have been identified, their presence or concentration does not seem to follow a regular pattern. On the other hand, ozone is always present in polluted outdoor atmospheres. Its concentration consistently rises from a normal value of a few parts per hundred million to many times this value during periods of severe contamination. Whether ozone is the primary cause of pollution or is a secondary effect of the reaction of other substances is not entirely clear, but it appears to be an important link in the chain of chemical reactions which produce atmospheric pollution. Very likely, a knowledge of the variations of ozone concentration in atmospheres would permit a study of the influence of the various parameters, and this knowledge may eventually furnish a lead to an explanation of the mechanism of formation and the effects of pollutants. 

Chemicals can be labeled as either a primary air pollutant or secondary air pollutant. Primary air pollutants are those such as carbon monoxide and sulfur dioxide that enter the atmosphere directly as a result of human or natural events. Carbon monoxide s primary source in the atmosphere is the incomplete combustion of gasoline. Hundreds of different chemicals are present in gasoline. The combustion of octane, C Hj, can be used to represent the general reaction of hydrocarbons in an automobile engine to produce energy ... 

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