Atmospheric nitrogen in the Mississippi River Basin } emissions, deposition and transport

Atmospheric deposition of nitrogen has been cited as a major factor in the nitrogen saturation of forests in the north-eastern United States and as a contributor to the eutrophication of coastal waters, including the Gulf of Mexico near the mouth of the Mississippi River. Sources of nitrogen emissions and the resulting spatial patterns of nitrogen deposition within the Mississippi River Basin, however, have not been fully documented. An assessment of atmospheric nitrogen in the Mississippi River Basin was therefore conducted in 1998]1999 to: 1. evaluate the forms in which nitrogen is deposited from the atmosphere; 2. quantify the spatial distribution of atmospheric nitrogen deposition throughout the basin; and 3. relate locations of emission sources to spatial deposition patterns to evaluate atmospheric transport. Deposition data collected through the NADPrNTN National Atmospheric Deposi- tion ProgramrNational Trends Network. and CASTNet Clean Air Status and Trends Network. were used for this analysis. NOx Tier 1 emission data by county was obtained for 1992 from the US Environmental Protection Agency Emissions Trends Viewer CD, 1985]1995, version 1.0, September 1996. and NH3 emissions data was derived from the 1992 Census of Agriculture US Department of Commerce. Census of Agriculture, US Summary and County Level Data, US Department of Commerce, Bureau of the Census. Geographic Area series, 1995:1b. or the National Agricultural Statistics Service US Department of Agriculture. National Agricultural Statistics Service Historical Data. Accessed 7r98 at URL, 1998. http:rrwww.usda.govrnassrpubsrhisdata.htm.. The highest rates of wet deposition of NOy3 were in the north-eastern part of the basin, downwind of electric utility plants and urban areas, whereas the highest rates of wet deposition of NHq4 were in Iowa, near the center of intensive agricultural activities in the Midwest. The lowest rates of atmospheric nitrogen deposition were on the western windward. side of the basin, which suggests that most of the nitrogen deposited within the basin is derived from internal sources. Atmospheric transport eastward across the basin boundary is greater for NOy3 than NHq4 , but a significant amount of NHq is likely to be transported out of the basin through the formation of NH . SO and NH NO particles } a 4 4 2 4 4 atmospheric transport of nitrogen from the Midwest to upland forest regions in the North-East, such as the western Adirondack region of New York, where NHq4 constitutes 38% of the total wet deposition of N. 3 process that greatly increases the atmospheric residence time of NHq4 . This process is also a likely factor in the