Workshop on Agricultural Air Quality Trends of Nitrogen and Sulfur in U.S. Precipitation — the Roles of Combustion-Related and Agricultural Emissions

For more than a quarter century, the National Atmospheric Deposition Program- National Trends Network (NADP-NTN) has measured the acids, nutrients, and base cations in U.S. precipitation. NADP-NTN data are used to evaluate regional and seasonal variations and long-term trends in precipitation chemistry. Trends result from the combined effects of changes in natural and human-induced pollutant emissions, physical and chemical transformations, and weather. Results of a Seasonal Kendall Trend test of 1985- 2004 NADP/NTN data demonstrate that statistically significant trends have occurred in the chemistry of U.S. precipitation. Over this 20-year period, dissolved inorganic nitrogen concentrations (nitrate-N + ammonium-N) decreased in the northeastern U.S. and increased in mid-continental states from eastern Washington, Oregon, and California to the Mississippi River Valley. In these mid-continental states, both ammonium, largely from agricultural sources, and nitrate, largely from combustion sources, increased at most sites. In contrast, sulfate concentrations decreased throughout much of the continental U.S. This paper will examine the roles of combustion-related and agricultural emissions, as well as the changing relationships between airborne nitrogen and sulfur species in affecting these trends