A METHOD FOR EVALUATION OF ACIDIC SULFATE AND NITRATE IN PRECIPITATION* D. L. SISTERSON Center for Environmental Research, Biological, Environmental and Medical Research Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A. (Received May 9, 1988; revised December 5, 1988) Abstract. A simple method is presented and used to estimate the portions of SO 2 - and NO3- that contribute to the strong acidity in weekly precipitation samples collected at three NADP sites in the eastern United States. The method assumes that, in general, the difference between SO ]- and NH 2 represents acidic sulfate and the difference between NO3- and soil-derived materials (the sum of Ca 2 +, Mg 2 +, and K ÷ ) represents acidic nitrate. Acidic sulfate and nitrate are considered to be the predominant source of H + (determined from laboratory pH) in the weekly precipitation samples. Most of the acidity for all three sites was attributed to acidic sulfate. The highest fraction of acidic SO 2- to H + wet deposition values was for the east-central Tennessee site (0.95) and the northeastern Illinois site (0.90), and the lowest fraction occurred at the central Pennsylvania site (0.75). The Tennessee site had the greatest acidic fraction of sulfate (0.84) and the Pennsylvania site had the greatest acidic fraction of nitrate (0.59). I. Introduction Sulfur and nitrogen compounds in the atmosphere are primarily responsible for the acidity of precipitation (Galloway et al., 1976). but the equivalents of SO4 a - and NOf far exceed those for H + in precipitation in the eastern United States. The acidic portions of sulfates and nitrates in precipitation cannot be determined analytically. The chemical relationships between ions in precipitation have been investigated using statistical regression (e.g., Wagner and Steele, 1987), factor analysis (e.g., Crawley and Sievering, 1986), or cluster analysis (e.g., Gotham etal., 1984), providing qualitative results. Therefore, the apportionment of SO 2 - and NO; that contribute to the pH of precipi- tation is not resolved. The approach used in this paper is based on the premise that chemical relationships of ions in precipitation should be largely determined by those in the ambient atmosphere. An investigation of the literature on aerosol chemistry and wet removal processes allows chemical relationships between ions in precipitation typical of the eastern half of the United States to be estimated. The method and rationale to determine those ionic relationships in precipitation are presented in this study. This study then uses the developed relationships of ions in precipitation to investigate the contributions of acidic SO 2- and acidic NO; to pH in precipitation samples for three National Atmospheric Deposition Program (NADP) sites in the eastern half of the United States for the period 1979-1983. These sites are used to show potential * This work has been funded as part of the National Acid Precipitation Assessment Program by the Office of Health and Environmental Research of the U.S. Department of Energy under Contract W-31-109-ENG-38. Water, Air, and Soil Pollution 43: 61-72, 1989. © 1989 Kluwer Academic Publishers. Printed in the Netherlands.