Hanna, Steven R. Environmental Research and Technology, Concord, Massachusetts.
Samson, Perry J. Department of Atmospheric, Oceanic, and Space Science, Space Physics Research Laboratory, University of Michigan, Ann Arbor, Michigan.
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Natural and artificial substances in the atmosphere that affect human health or well-being, or the well-being of any other organisms in the environment. Air pollution also applies to situations in which contaminants affect structures and artifacts or esthetic sensibilities (such as visibility or smell). Most artificial impurities are injected into the atmosphere at or near the Earth's surface (see illustration). The lower atmosphere (troposphere) cleanses itself of some of these pollutants in a few hours or days as the larger particles settle to the surface and soluble gases and particles encounter precipitation or are removed through contact with surface objects. Removal of some particles (such as sulfates and nitrates) by precipitation and dry deposition results in acid deposition, which may cause serious environmental damage. Also, mixing of the pollutants into the upper atmosphere may dilute the concentrations near the Earth's surface, but can cause long-term changes in the chemistry of the upper atmosphere, including the ozone layer. See also: Atmosphere; Troposphere
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