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Science needs for continued development of total nitrogen deposition budgets in the United States
Walker, Johnt, G. Beachley, A. Helen, 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, P. Templar, T. Thompson, D. Tong, G. Wetherbee, T. Whitlow, Z. Wu, Z. Yu, L. Zhang, AND R. Scheffe. Science needs for continued development of total nitrogen deposition budgets in the United States. U.S. Environmental Protection Agency, Office of Research and Development, National Center for Environmental Assessment, Washington, DC, EPA/600/R-18/383, 2019.
Critical load frameworks for limiting ecosystem exposure to nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). Determination of the amount of deposition in excess of the ecosystem critical load (i.e. “exceedance”) requires an estimate of total 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, improvement in the completeness, accuracy, and spatial representativeness of total Nr deposition budgets remains limited by key data and knowledge gaps. 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, it is becoming more important to understand 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. 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 report details the state of the science of Nr deposition budgets in the U.S. and identifies research priorities to improve deposition budgets to support critical load frameworks and the secondary NAAQS.
This report describes the state of the science with respect to total Nr deposition budgets in the United States and the research needed to improve these budgets from both a measurement and modeling perspective. The document is intended to serve as a roadmap for research activities but also, more broadly, to provide program managers, natural resource managers, policy makers and scientists with an understanding of (1) the need for complete and accurate Nr deposition budgets to protect ecosystem health and human welfare, and (2) the linkages between the underlying policy-relevant science questions and the specific knowledge and data gaps needed to improve Nr deposition budgets.