Pollutants enter the atmosphere primarily from natural sources and human activity. This pollution is called primary pollution, in contrast to secondary pollution, which is caused by chemical changes in substances in the atmosphere. Sulfur dioxides, nitric oxides, and hydrocarbons are major primary gaseous pollutants, while ozone is a secondary pollutant, the result of atmospheric photochemistry between nitric oxide and hydrocarbons.
Pollutants do not remain unchanged in the atmosphere after release from a source. Physical changes occur, especially through dynamic phenomena, such as movement and scattering in space, turbulent diffusion, and changes in the concentration by dilution.
Changes also result from the chemistry of the atmosphere. These changes are often simple, rapid chemical reactions, such as oxidation and changes in temperature to condense some gases and vapors to yield mist and droplets. After a long residence of some gaseous pollutants in the atmosphere, these gases convert into solid, finely dispersed substances. Solar conditions cause chemical reactions in the atmosphere among various pollutants and their supporting media. Figure 5.4.1 shows simplified schemes of the main chemical changes of pollutants in the atmosphere.
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