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The Increasing Importance of Deposition of Reduced Nitrogen in the United States
Citation:
Li, Y., B. Schichtel, Johnt Walker, D. Schwede, X. Chen, C. Lehmann, M. Puchalski, D. Gay, AND J. Collett, Jr. The Increasing Importance of Deposition of Reduced Nitrogen in the United States. PNAS (PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES). National Academy of Sciences, WASHINGTON, DC, 113(21):5874-5879, (2016).
Impact/Purpose:
The chemistry of atmospheric nitrogen deposition across the U.S. is now dominated by reduced (NH3 + NH4) rather than oxidized (NOx) forms of nitrogen. Future regulations in support of ecosystem health will need to consider reductions in ammonia emissions to address this shift in type and amount of nitrogen deposition.
Description:
Rapid development of agricultural activities and fossil fuel combustion in the United States has led to a great increase in reactive nitrogen (Nr) emissions in the second half of the twentieth century. These emissions have been linked to excess nitrogen (N) deposition (i.e. deposition exceeding critical loads) in natural ecosystems through dry and wet deposition pathways. U.S. efforts to reduce nitrogen oxides (NOx) emissions since the 1970s have substantially reduced nitrate deposition, as evidenced by decreasing trends in long-term wet deposition data. These decreases in nitrate deposition along with increases in wet ammonium deposition have altered the balance between oxidized (nitrate) and reduced (ammonium) nitrogen deposition. Across most of the U.S., wet deposition has transitioned from being nitrate dominated in the 1980s to ammonium dominated in recent years. Because ammonia has not been a regulated air pollutant in the U.S., it has historically not been commonly measured. Recent measurement efforts, however, provide a more comprehensive look at ammonia concentrations across several regions of the U.S. These data, along with more routine measurements of gas phase nitric acid and fine particle ammonium and nitrate, permit new insight into the balance of oxidized and reduced nitrogen in the total (wet + dry) U.S. inorganic reactive nitrogen deposition budget. Utilizing two years of N-containing gas and fine particle observations from 37 U.S. monitoring sites, we estimate that reduced nitrogen contributes, on average, approximately 65 percent of the total inorganic N deposition budget. Dry deposition of ammonia plays an especially key role in N deposition compared with other N deposition pathways, contributing from 19% to 65% in different regions. With reduced N species now dominating the wet and dry reactive N deposition budgets in much of the country, the U.S. will need to consider ways to reduce ammonia emissions if it is to continue progress toward reducing N deposition to sustainable levels defined by ecosystem critical loads.
