Characterization of Nitrogen and Sulfur Deposition in and near Selected Sensitive Ecosystems for the NOx/SOx Secondary NAAQS Review

Anne Rea*, Norm Possiel and Adam Reff
Office of Air Quality Planning and Standards
U.S. Environmental Protection Agency

The U.S. Environmental Protection Agency (EPA) is currently conducting a joint review of the existing secondary (welfare-based) National Ambient Air Quality Standards (NAAQS) for oxides of nitrogen (NOx) and sulfur (SOx). EPA is jointly assessing the science, risks, and policies relevant to protecting the public welfare associated with nitrogen and sulfur due to both their atmospheric interactions and ecological effects. This review used a case study analysis approach to address aquatic and terrestrial effects associated with ambient air concentrations of NOx and SOx as they relate to atmospheric deposition in areas known to be sensitive to acidification and nutrient enrichment. The air quality analyses for this review encompass the current emissions sources of nitrogen and sulfur, as well as atmospheric concentrations, estimates of deposition of total nitrogen, policy-relevant background, and non-ambient loadings of nitrogen and sulfur to ecosystems. Annual total emissions for 2002 from the National Emissions Inventory (NEI) were used to characterize the magnitude and spatial patterns in emissions of NOx, NH3, and SO2 nationwide. Air quality model predictions are taken from applications of the Community Multiscale Air Quality (CMAQ) model. CMAQ was used to simulate concentrations and deposition for 2002 using meteorology and emissions.

Atmospheric deposition is the link between ambient concentrations and ecological effects. Spatial fields of deposition were created by using wet deposition measurements from the National Atmospheric Deposition Program (NADP) National Trends Network and dry deposition estimates from the Community Multiscale Air Quality (CMAQ) model for the year 2002. The spatial patterns of total nitrogen deposition reflect the combination of the deposition from the reduced and oxidized nitrogen components. The case study analyses examined the magnitude of nitrogen and sulfur deposition in sensitive areas, as well as the relative contributions of oxidized vs reduced forms of nitrogen. Overall, nitrogen deposition varied among the case study areas, and was higher in the East (ranging from 8-15 kg N/ha-yr) than the West (ranging from 3-11 kg N/ha-yr). Sulfur deposition also varied among the case study areas and was greater in the East (7-24 kg S/ha-yr) than the West (~2 kg S/ha-yr or less). Year-to year variation ranged from 1-3 kg/ha yr or less between 2002 and 2005 for both pollutants, with larger differences in the Kane Experimental Forest (PA) for sulfur. This analysis indicated that 2002 was generally representative of current levels of deposition for both pollutants, and was consistent with trends associated with 24 NADP/CASTNet sites in the East

* Office of Air Quality Planning and Standards, U.S.EPA, 109 TW Alexander Dr. MC C504-04, RTP, NC 27711, USA; Phone: 919-541-0053; Fax: 919-541-0480; Email: