Critical Loads as a Policy Tool: Highlights of the NOx/SOx Secondary National Ambient Air Quality Standard Review

Anne Rea1*, Jason Lynch2, Randy Waite1, Ginger Tennant1, Jennifer Phelan3 and Norm Possiel1

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. As discussed in the Clean Air Act (CAA), the purpose of a secondary NAAQS is to “protect the public welfare from any known or anticipated adverse effects associated with the presence of such air pollutants in the ambient air”. This review has focused on the ecological effects due to ambient air concentrations of NOx and SOx as they relate to atmospheric deposition of nitrogen and sulfur: aquatic and terrestrial acidification and aquatic and terrestrial nutrient enrichment. Atmospheric deposition is the link between ambient concentrations and these 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.These data were used in all the ecological analyses.

For the ecological endpoint of acidification, we have used a critical loads approach to assess whether adverse effects are occurring under current atmospheric loadings of nitrogen and sulfur. For aquatic acidification, the results indicated that 28-48% modeled lakes in the Adirondacks exceed their critical load for ANC ranging from 20-100 µeq/L. These data were scaled up to 1842 lakes in the region, indicating that 13-51% of the regional lake population exceeded critical loads for ANC ranging from 20-100 µeq/L. A similar analysis showed that 72-92% of the modeled streams in the Shenandoahs exceed their critical loads for ANC ranging from 20-100 µeq/L. For terrestrial acidification, the results indicated that 3-75% of sugar maple plots (n=4992 plots in 24 states) and 3-36% of red spruce plots (n=763 plots in 8 states) exceed their critical load for Bc:Al ranging from 0,6 to 10. An update on the status of the review will include highlights from the Risk and Exposure Assessment, policy implications, and the science-policy issues anticipated for the next review.

The Risk and Exposure Assessment and other documents associated with this review are available at:

1 Office of Air Quality Planning and Standards, U.S. EPA
1*109 TW Alexander Dr. MC C504-04, RTP, NC 27711, USA; Phone: 919-541-0053; Fax: 919-541-0480; Email:
2 Office of Atmospheric Programs, U.S. EPA
3 Research Triangle Institute, International