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 emissio ns and the resulting spatial patterns of nitrogen deposition within the Mis sissippi River Basin, however, have not been fully documented. An assessmen t of atmospheric nitrogen in the Mississippi River Basin was therefore cond ucted in 1998-1999 to: (1) evaluate the forms in which nitrogen is deposite d from the atmosphere; (2) quantify the spatial distribution of atmospheric nitrogen deposition throughout the basin; and (3) relate locations of emis sion sources to spatial deposition patterns to evaluate atmospheric transpo rt. Deposition data collected through the NADP/NTN (National Atmospheric De position Program/National Trends Network) and CASTNet (Clean Air Status and Trends Network) were used for this analysis. NOx Tier 1 emission data by c ounty was obtained for 1992 from the US Environmental Protection Agency (Em issions Trends Viewer CD, 1985-1995, version 1.0, September 1996) and NH3 e missions data was derived from the 1992 Census of Agriculture (US Departmen t 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 Agric ulture. National Agricultural Statistics Service Historical Data. Accessed 7/98 at URL, 1998. http://www.usda.gov/nass/pubs/hisdata.htm). The highest rates of wet deposition of NO3- 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 NH4+ were in Iowa, near the center of intensive agricultural activities in the Midwest. The lowest rates of atmospheric nit rogen deposition were on the western (windward) side of the basin, which su ggests that most of the nitrogen deposited within the basin is derived from internal sources. Atmospheric transport eastward across the basin boundary is greater for NO3- than NH4+, but a significant amount of NH4+ is likely to be transported out of the basin through the formation of (NH4)(2)SO4 and NH4NO3 particles - a process that greatly increases the atmospheric reside nce time of NH4+. This process is also a likely factor in the atmospheric t ransport of nitrogen from the Midwest to upland forest regions in the North -East, such as the western Adirondack region of New York, where NH4+ consti tutes 38% of the total wet deposition of N. (C) 2000 Elsevier Science B.V. All rights reserved.