INTRODUCTION The Clean Air Act of 1967 and its 1970 Amendments established restrictions on the ambient concentration of several pollutants to maintain standards of public health and welfare. At that time@ the literature review in the National Criteria Document (U.S.NAPCA@ 1969) recognized sulfur dioxide (SO2) only as a surrogate for pollutant impact. Of the sulfur oxides in air@ it was suspected that airborne particles containing certain specific sulfates associated with SO2 were more significant hazards to human health than the gas itself. More recent work of the U.S. Environmental Protection Agency's (EPA) Community Health Environmental Surveillance System (CHESS) provided evidence that appeared to support earlier suspicions about the importance of airborne sulfate@ measured as the water soluble sulfate fraction of particulate material collected on glass fiber filter substrates (U.S. EPA@ 1974). These results have been criticized@ and partly discredited@ but they remain part of a strong motivation to determine whether sulfate should be regulated separately from the present sulfur oxide standard (e.g.@ U.S. EPA@ 1975). Evidence clarifying the role of sulfates is being sought by several new toxicological and epidemiological studies. Regardless of the ambiguities in health consequences of airborne sulfate@ there is evidence that such material is closely related to widespread visibility degradation@ materials corrosion@ and rain acidity in the greater eastern United States. Such considerations in combination with a suspected health impact have formed a basis for possible implementation of a National Ambient Sulfate Standard (NASS) by the early 1980's (U.S. EPA@ 1975). As a result of this pressure@ substantial efforts are being committed by industry and government to improve knowledge in atmospheric sulfate origins@ behavior@ and health and welfare impact prior to promulgation of a new standard@ and a consequent abatement strategy.