A National Assessment of the Ecological Effects of Nitrogen and Sulfur Oxides

Tara Greaver*, Kris Novak, Jeff Herrick, Jeff Arnold, Lingli Liu and Jean-Jaques Dubois
Office Research and Development
National Center for Environmental Assessment
U.S. EPA, Mail Drop B-243-01
RTP, NC 27711

Publically released in December 2008, the Integrated Science Assessment (ISA) for Oxides of Nitrogen and Sulfur (NOx and SOx)-Ecological Criteria is the scientific foundation for the review of the secondary National Ambient Air Quality Standards (NAAQS) for NOx and SOx. Over 3400 scientific publications, newly available since the last assessments of NOx and SOx (1993 and 1982, respectively), were reviewed. The key conclusions of the 2008 ISA were that acidification (due to NOx, NHx and SOx) and nitrogen enrichment (due to NOx and NHx) continue to occur in ecosystems across the U.S. under current loads of atmospheric deposition.

Assessment of the atmospheric sciences showed that the highest deposition levels of N and S in the U.S were 21 kg S/ha/yr and 9 kg N/ha/yr in the states of Ohio and Indiana, as reported by routine monitoring networks (2004-2006). However, national-scale networks that routinely monitor N deposition are inadequate to characterize the full range of reduced and oxidized forms of N deposition and the substantial regional heterogeneity across the U.S. Model-predicted values for N deposition are greater than 20 kg N/ha/yr in some regions of the Adirondacks and are greater than 32 kg N/ha/yr in a region of S. California.

Assessment of the ecological sciences indicated that the effects of acidifying deposition on ecosystems have been well studied over the past several decades, leading to the identification of vulnerable regions and the development of robust ecological models used for predicting soil and surface water acidification. Chemical and biological indicators of acidification were identified and threshold values discussed. In contrast to acidification, a broadly applicable and well-tested predictive model of the ecological effects of N deposition is not available. However, there is substantial empirical information for specific ecosystems and endpoints. Chemical and biological indicators of N enrichment were identified and values for critical loads were discussed. Publications typically did not include data on reduced vs. oxidized N, thus causality determinations were made on the effects of total N deposition. Additional ecological effects included mercury methylation caused by sulfate deposition and direct phytotoxic effects of gas-phase NOx and SOx. Few publications on ecosystem services and valuation associated with NOx and SOx air pollution are available. These were summarized and considered in terms of the categories outlined by the Millennium Ecosystem Assessment.

The conclusions of the NOx and SOx ISA serve as the scientific basis for decisions to retain or revise the NOx and SOx Secondary NAAQS.

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