Draft Scope and Methods Plan for Risk/Exposure Assessment: Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

摘要

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 nitrogen dioxide (NO2) and sulfur dioxide (SO2). We recognize that this is the first time that we have conducted a joint, multi-pollutant review of a secondary standard separate from the review of the primary standard. As discussed in the Integrated Review Plan (U.S. EPA, 2007a), this was done in recognition of the important linkages between ambient nitrogen and sulfur leading to deposition of ambient particles that can have significant impacts on the environment. We further recognize that a fully comprehensive assessment of such linkages and impacts is very complex and will extend beyond the time available in this review, as constrained by our court-ordered schedule. Thus, this Scope and Methods Plan is more narrowly focused on key aspects of the evolving scientific understanding to provide timely results to meet our court-ordered schedule. Our plan for the current review is to focus on the identification of sensitive ecosystems, the predominant linkages between ambient levels of nitrogen and sulfur, and the levels of deposition that create adverse effects in those ecosystems, building directly from the key findings of our Integrated Science Assessment (ISA). To the degree possible, our risk and exposure assessment will attempt to evaluate whether ecosystem damage is occurring in specific ecosystems under current ambient concentrations, and, if so, what alternative levels of ambient nitrogen and sulfur might be expected to allow various degrees of recovery of impacted systems and prevention of further damage.