Toward the improvement of total nitrogen deposition budgets in the United States

Abstract

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 (NO_x) emissions continue to decline, reduced forms of inorganic nitrogen (NH_x = 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 NH_x 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.

Keywords: Ammonia; Dry deposition; Organic nitrogen; Oxidized nitrogen; Reactive nitrogen; Wet deposition.

Figure 1.

Summary of TDep maps (NADP, 2018) for the continental U.S., averaged over the period 2014–2016, of (A) total N deposition, (B) percentage of total N deposition attributed to dry deposition, (C) percentage of total deposition attributed to reduced N (NH_x = NH₃ + NH₄⁺), and (D) percentage of total deposition attributed to dry deposition of N species not measured at CASTNET sites (i.e. “Other N”). “Other N” comprises NO, NO₂, HONO, N₂O₅, and organic N.

Figure 2.

Summary of TDep maps (NADP, 2018) of total oxidized (left hand) and reduced (NH₃ + NH₄⁺, right hand) N deposition averaged over the periods 2000 – 2002 (top) and 2014 – 2016 (bottom).

Figure 3.

Exceedance of herbaceous richness critical load based on TDep total N deposition 2013–2015. Negative values of exceedance indicate that the total deposition is less than the critical load. Exceedance data from Simkin et al., 2016. Figure from Walker et al. (2019b).

Figure 4.

Map of AMoN NH₃ monitoring sites (stars) and county-scale NH₃ emissions (all categories) from the U.S. EPA 2014 (V2) National Emissions Inventory (U.S. EPA, 2014). Exploded view of Sampson County, NC showing locations of animal production facilities (NC DEQ, 2019) categorized by # of animals (circles) with location of AMoN site NC35 shown (star).

Figure 5.

Spatial scales of deposition budgets used in critical loads assessments. Adapted from Walker et al. (2019b).

Figure 6.

Resistance diagrams for generalized unidirectional dry deposition model for gases (A) and bi-directional modeling framework for NH₃ (B, adapted from Nemitz et al., 2001).

Figure 7.

CMAQ (V5.3 with STAGE) land use specific and grid average fluxes during July, 2011 for three locations in Tennessee (TN), North Carolina (NC), and Georgia (GA) in the southeastern U.S. Positive fluxes indicate deposition.

Figure 8.

Schematic showing the science and policy issues associated with the “Linkage between agricultural emissions and Nr deposition” (top row) and examples of stakeholders (ovals).