Citation:

Walker, J., G. Beachley, H. Amos, J. Baron, J. Bash, R. Baumgardner, M. Bell, K. Benedict, X. Chen, D. Clow, A. Cole, J. Coughlin, K. Cruz, R. Daly, S. Decina, E. Elliott, M. Fenn, L. Ganzeveld, K. Gebhart, S. Isil, B. Kerschner, B. Larson, T. Lavery, G. Lear, T. Macy, A. Mast, K. Mishoe, K. Morris, p. Padgett, R. Pouyat, M. Puchalski, H. Pye, A. Rea, M. Rhodes, C. Rogers, R. Saylor, B. Schichtel, D. Schwede, G. Sexstone, B. Sive, R. Sosa, P. Templar, T. Thompson, D. Tong, G. Wetherbee, T. Whitlow, Z. Wu, Z. Yu, AND Z. Leiming. Published Paper: Toward the improvement of total nitrogen deposition budgets in the United States. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 691:1328-1352, (2019). https://doi.org/10.1016/j.scitotenv.2019.07.058

Impact/Purpose:

Critical loads are an approach for quantifying the amount of reactive nitrogen (Nr) deposition that can be harmful to ecosystems. Determination of the amount of deposition in excess of the ecosystem critical load (i.e. “exceedance”) requires an estimate of total Nr deposition, which is typically derived from gridded chemical transport models or a combination of measurements and models. While much progress has been made in developing total Nr deposition budgets for the U.S. over the past several years, further improvement in the completeness, accuracy, and spatial representativeness of total Nr deposition budgets is needed. For example, lack of dry deposition measurements imposes a reliance on models for this component of deposition budgets, resulting in a much higher degree of uncertainty relative to wet deposition. As NOx emissions continue to decline, understanding the spatial and temporal patterns of NHx (NH3 + NH4+) deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems is becoming more important. Urban and suburban areas are hotspots of oxidized nitrogen deposition. However, current monitoring networks that support deposition research were designed to characterize non-urban environments. Finally, monitoring networks do not capture some forms of Nr that are known to be important to total deposition budgets, particularly organic nitrogen. Total deposition budgets are therefore incomplete with respect to speciation of the atmospheric Nr pool. This journal article summarizes the state of the science of Nr deposition budgets in the U.S. and identifies research priorities to improve deposition budgets to support critical loads and the secondary NAAQS.

Description:

Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (>90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NOx) emissions continue to decline, reduced forms of inorganic nitrogen (NHx = NH3 + NH4+) now contribute >50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NHx deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.

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