ENV-11-04 Challenges for Modeling Compliance With Short-term SO2, NO2, and PM2.5 Ambient Standards

Description:

U.S. EPA’s recently promulgated revisions to National Ambient Air Quality Standards1 (NAAQS) for SO2 and NO2 provide a host of implementation challenges regarding the application of dispersion models to assess compliance. Given that these new NAAQS are substantially lower than the standards they replace, the customary degree of overestimation that is intentionally designed into the present Modeling Guideline2 (40 CFR Part 51, Appendix W) methods limits the ability of regulators and applicants to conduct accurate and unbiased air quality compliance assessments. Due to the shrinking margin of compliance associated with these new standards, the skill of regulatory models such as AERMOD3 and CALPUFF4 is more severely tested, and refined methods to incorporate background concentrations and deal with the need to model numerous background emission sources are needed. So far, EPA implementation of modeling procedures for the new standards has resulted in significant permitting challenges. The authors have been involved in actual modeling applications and development of new modeling tools and approaches for several projects that involve modeling analyses that address the new NAAQS. Due to the release on March 1, 2011 of a lengthy EPA guidance document5 on this subject and a March 24, 2011 EPA document6 regarding modeling for compliance with the SO2 NAAQS, we focus some of our discussion on interpreting and commenting upon this new guidance and determining its effect upon mitigating the modeling challenges. We also present our recommendations for refined approaches to consider for routine modeling applications.