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There are two main groups of air pollutants:
 Primary and Secondary
 Primary pollutants are those emitted directly into the air. They include particulates,sulfur dioxide, carbon monoxide, nitrogen oxides, and hydrocarbons.
Secondary pollutants are pollutants produced through reactions between primary pollutants and normal atmospheric compounds. For example, ozone forms over urban areas through reactions of primary pollutants, sunlight, and natural atmospheric gases. Thus ozone is a secondary pollutant that is produced on bright, sunny days in areas where there is much primary pollution. This phenomenon has been particularly well documented for cities in southern California, such as Los Angeles, but occurs worldwide under appropriate conditions.
 Again, the primary pollutants that account for nearly all air pollution problems are carbon monoxide, particulates, hydrocarbons, nitrogen oxides, and sulfur oxides. Each year well over a billion metric tons of these materials enters the atmosphere from human-related processes. About half is carbon monoxide; the other four pollutants listed account for a few percent each. At first glance this amount of pollutants appears to be very large. If the air pollutants were uniformly distributed in the atmosphere, the concentration would be Only a few parts per million by weight.Unfortunately, pollutants are not uniformly distributed but tend to be released, produced, and concentrated locally or regionally. For example, in large cities weather and climatic conditions combine with the high density of people and industry to produce air pollution problems.
The major air pollutants Occur either in gaseous forms or as particulate matter (PM). The gaseous pollutants include sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), ozone (O3), hydrocarbons (HC), hydrogen sulfide (H2S), and hydrogen fluoride (HF). Particulate matter pollutants are particles of solid or liquid substances and may be organic or inorganic.

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