somewhat larger than the oxidation products of sulfur dioxide (Kellogg et al. 1972). If so, sea-salt sulfate particles should be less important toxicologically, because they will tend to settle out of the atmosphere more quickly and will not penetrate as deeply into the respiratory tract. Almost all the sulfur oxide emitted from the stationary combustion of fossil fuels is in the form of the dioxide, but about 2–5 percent is directly released as sulfuric acid.

Fossil fuels used in automobiles generally contain relatively small amounts of sulfur and are thus insignificant as sulfur dioxide sources. However, the advent of catalytic converters will result in the oxidation of gasoline to sulfuric acid which will be released in the exhaust. This could conceivably lead to a considerable ground level of sulfuric acid, particularly in heavily traveled city streets which are often poorly ventilated. This potential problem requires careful consideration before mandating the use of catalytic converters (USEPA 1975).

A number of authors have assessed the total atmospheric sulfur balance, including sources, atmospheric residence time, and eventual sinks. At present, these are estimates, inasmuch as important pertinent information is lacking (Hill 1973). Of particular relevance to the problem of sulfur oxide control is the extent to which natural and man-made sources contribute to airborne sulfur dioxide and sulfate concentrations in the United States. It appears that the bulk of airborne sulfur dioxide and sulfate in industrialized and heavily populated areas, such as the northeastern United States, is derived from fossil-fuel combustion (Altshuller 1973). However, further refinement of the data is necessary.

The atmospheric residence times of the various sulfur oxides are not precisely known. Estimates range from hours to weeks for sulfur dioxide and longer for fine particulate sulfates. Residence time depends on such factors as rainfall, windspeed, and temperature. It should be noted that climatologic conditions affect the deposition of gaseous sulfur dioxide



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