Observed and modeled trend of sulfate and nitrate in precipitation in eastern North America

The trend of annual average sulfate and nitrate ion concentrations in precipitation at 17 stations in eastern North America over 7 years 1979–1985 is estimated and compared with a modeled trend. The observed concentrations fluctuate from year-to-year about a mean value with a standard deviation of 12.7% on average. The average slope of a linear regression line at the 17 stations is −2.8% a⁻¹ for sulfate and −0.4% a⁻¹ for nitrate. The modeled trend at these stations, which assumes constant meteorology but a year-to-year varying emission inventory, is −2.4% a⁻¹ for sulfate and −0.9% a⁻¹ for nitrate.