Atmospheric contaminants, in dissolved, gaseous, or particulate form, enter stormwater runoff through either the process of precipitation or as dustfall; gases also enter by direct absorption at the earth's surface. The deposition rates of particulate atmospheric contaminants in US urban areas vary from 3.5 to over 35 tons/km2/month , and the higher rates are found in congested industrial areas and business districts. In addition to particulate matter, many other contaminants are contained in, transported by, or deposited from atmospheric fallout, such as nitrogen and phosphorus, sulfur dioxide, toxic heavy and trace metals, pesticides and insecticides, fungi and pollen, methane and mercaptans, fly ash, and soil particles. Dustfall rates vary significantly from area to area and are largest in the central United States, with the geometric means of dustfall
values ranging from 2.8 to 144 tons/mi2/month . Although contaminant concentrations in rural dustfall are related closely to soil conditions, urban dustfall is related more to local air pollution problems. Fly ash from industrial coal-burning activities and disintegration of urban litter is one more important source of atmospheric contaminant contribution, especially in the vicinity of industrial and urban centers .
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