The Increasing Importance of U.S. Reduced Nitrogen Deposition

Jeffrey L. Collett, Jr.1, Yi Li1, Bret A. Schichtel2, John T. Walker3, Donna B. Schwede3, Xi Chen3, Christopher M.B. Lehmann4, Melissa A. Puchalski5, and David A. Gay4

1Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA,
2National Park Service, Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado, USA
3U.S. Environmental Protection Agency, Office of Research and Development, Research Triangle Park, North Carolina, USA
4National Atmospheric Deposition Program, University of Illinois Urbana-Champaign, Champaign, Illinois, USA
5U.S. Environmental Protection Agency, Clean Air Markets Division, Washington D.C., USA.


U.S. reactive nitrogen emissions increased greatly in the last century due to rapid increases in fossil fuel combustion and development of agriculture.  One result was excess nitrogen deposition to many natural ecosystems. Successful policies in the last two decades to reduce nitrogen oxides emissions have substantially decreased nitrate wet deposition. Levels of wet ammonium deposition, however, have increased in many regions. Overall, the balance between oxidized and reduced nitrogen deposition has shifted from a nitrate-dominated situation in the 1980s to an ammonium-dominated situation today. Although gaseous ammonia has not historically been routinely measured in the U.S., a recent expansion in observations, combined with ongoing measurements of nitric acid and fine particle ammonium and nitrate, provides new insight into the balance of oxidized and reduced nitrogen in the total (wet + dry) U.S. nitrogen deposition budget. Observations reveal that reduced nitrogen contributes approximately 65 percent, on average, of the total inorganic nitrogen deposition budget. Dry deposition of ammonia plays an especially key role in nitrogen deposition.  While U.S. emissions of nitrogen oxides are expected to continue to decline in the foreseeable future, ammonia emissions are projected to grow.  Continued progress toward reducing U.S. nitrogen deposition will be increasingly difficult without new efforts to reduce ammonia emissions.