Trends in and relationships between ambient air SO2 and sulfate concentrations at 48 urban and 27 nonurban sites throughout the United States between 1963 and 1974 have been analyzed. Large decreases in SO2 concentrations at urban sites in the eastern and midwestern United States have been accompanied by modest decreases in sulfate concentrations. Large variations in SO2 emissions among air quality control regions also result in much smaller variations in sulfate concentrations. Large changes in the patterns of SO2 emissions have little impact on sulfate concentrations in most air quality regions. Comparisons of air quality regions with similar SO2 emission levels and patterns of emissions in the eastern and western United States and of SO2, sulfate, and vanadium relationships between urban-suburban and urban nonurban sites lead to the same conclusion. Long-distance SO2 transport with chemical conversion of SO2 to sulfates over ranges of hundreds of kilometers or more provides a consistent explanation for all of the observed results. This conclusion has been suggested earlier, and the present analysis strongly supports previous discussions. Reduction of sulfate concentration levels will require strenuous efforts to control SO2. Also, large new additions to utility capacity in western areas may lead to significant increases in western sulfate concentration levels. The types of research activities required to quantitate crucial experimental parameters are discussed.

"EPA-600/3-77-054." "June 1977." Includes bibliographical references (pages 28-31.