Past and Present Acid Deposition Measurements as a Basis for Decision Making

Analysis of historical precipitation chemistry data shows substantial uncertainty in the pH values derived from them. Consideration of climatological influences on precipitation chemistry suggests that the relatively low acidity inferred for precipitation in large areas of the United States during 1955 to 1956 may have been the result of elevated concentrations of alkaline soil elements. Thus, data from this period may not be suitable for inclusion in any analysis of historical trends in precipitation acidity due to pollutant emissions. Data from scattered sites operated during the 1920s to 1940s show higher sulfate concentrations and lower nitrate concentrations than those measured recently at nearby sites. Because of data limitations in terms of continuity of measurements, their spatial coverage, and their quality, no strong case can be made either for or against long term trends in precipitation acidity and its areal extant in the eastern United States.