Atmospheric deposition and exceedances of critical loads from 1800−2025 for the conterminous United States

Abstract

Atmospheric deposition of nitrogen (N) and sulfur (S) has increased dramatically over pre-industrial levels, with many potential impacts on terrestrial and aquatic ecosystems. Quantitative thresholds, termed "critical loads" (CLs), have been developed to estimate the deposition rate above which damage is thought to occur. However, there remains no comprehensive comparison of when, where, and over what time periods individual CLs have been exceeded. We addressed this knowledge gap by combining several published data sources for historical and contemporary deposition, and overlaying these on six CL types from the National Critical Loads Database (NCLDv2.5; terrestrial acidification, aquatic acidification, lichen, nitrate leaching, plant community composition, and forest-tree health) to examine exceedances from 1800 to 2011. We expressed CLs as the minimum, 10th, and 50th percentiles within 12-km grid cells. Minimum CLs were relatively uniform across the country (200-400 eq·ha ·yr ), and have been exceeded for decades beginning in the early 20th century. The area exceeding minimum CLs peaked in the 1970s and 1980s, exposing 300,000 to 3 million km (depending on the CL type) to harmful levels of deposition, with a total area exceeded of 5.8 million km (~70% of the conterminous United States). Since then, deposition levels have dropped, especially for S, with modest reductions in exceedance by 2011 for all CL types, totaling 5.2 million km in exceedance. The 10th and 50th percentile CLs followed similar trends, but were not consistently available at the 12-km grid scale. We also examined near-term future deposition and exceedances in 2025 under current air quality regulations, and under various scenarios of climate change and additional nitrogen management controls. Current regulations were projected to reduce exceedances of any CL from 5.2 million km in 2011 to 4.8 million km in 2025. None of the additional N management or climate scenarios significantly affected areal exceedances, although exceedance severity declined. In total, it is clear that many CLs have been exceeded for decades, and are likely to remain so in the short term under current policies. Additionally, we suggest many areas for improvement to enhance our understanding of deposition and its effects to support informed decision making.

Growth and survival relationships of 71 tree species with nitrogen and sulfur deposition across the conterminous U.S.

“…Therefore, a predicted future change in growth or survival depends on the direction of change in atmospheric S and/or N deposition; regions with positive instantaneous growth responses would have a predicted decrease in growth if atmosphere deposition decreases in the future, as observed in some regions of the conterminous U.S. [22]. …”

“…However, tree species differ widely in sensitivity to S deposition [20, 21], potentially based on differences in physiology, local soil conditions, or other factors. Even though S deposition over much of the U.S. has significantly decreased in the past few decades [16], it remains elevated over pre-industrial levels [22]. Consequently, it is critical to disentangle the separate effects of N and S deposition on trees, if critical loads are developed for both N and S deposition.…”

Key Components and Contrasts in the Nitrogen Budget Across a U.S.‐Canadian Transboundary Watershed

“…Loss of over 50% of N inputs to the environment, primarily as volatilized ammonia and hydrologic N exports to surface water and groundwater, has a strong potential to adversely affect human health and the environment (Townsend et al, 2003). Ammonia, predominantly from losses related to housing and storage of manure, can contribute to regional smog and odor problems (Barthelmie & Pryor, 1998; Kotchenruther & Taylor, 2014) and can harm human respiratory health (Paulot & Jacob, 2014) Enhanced N deposition resulting from elevated N emissions can cause significant damage to terrestrial and aquatic ecosystems, including cation leaching, altered nutrient stoichiometry in streams and lakes, and changes in biodiversity (Clark et al, 2018; McMurray et al, 2013).…”

Automotive wastes

“…Atmospheric levels of nitrogen and sulfur have increased since the industrial revolution. For decades, nitrogen and sulfur levels have exceeded the deposition rates at which ecological damage is thought to occur, and deposition rates will likely remain hazardous in the short-term given current policies (Clark et al, 2018). However, in China, SO 2 emissions have decreased 10% between 2005 and 2010 due to flue gas desulfurization improvements and NOx emissions have been falling since 2011 due to end-of-pipe control measures (Sun et al, 2018).…”