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On May 30, 2019, the U.S Environmental Protection Agency (EPA) released a final guidance document for Fine Particulate Matter (PM2.5) Precursor Demonstrations for analyzing contributions of PM2.5 precursors to ambient concentrations of PM2.5 in areas designated as nonattainment for the PM2.5 National Ambient Air Quality Standards (NAAQS). Chemical reactions involving sulfur dioxide (SO2), oxides of nitrogen (NOX), volatile organic compounds (VOC), and ammonia cause the formation of secondary PM2.5. These emissions are considered precursor emissions for PM2.5 and are required to be evaluated in all PM2.5 nonattainment area State Implementation Plans (SIPs).

This guidance is published to assist air agencies in the development of optional PM2.5 precursor demonstrations as permitted by the PM2.5 SIP Requirements Rule. This rule establishes that PM2.5 precursor emissions must be evaluated for potential control measures in any PM2.5 attainment plan, and any nonattainment new source review (NNSR) program. This rule allows air agencies to submit an optional precursor demonstration, such as a concentration-based analysis or sensitivity-based analysis, showing that emissions of one or more of those precursors would not contribute significantly to PM2.5 levels that exceed the standard. If the EPA approves a precursor demonstration for a particular nonattainment area, the attainment plan and NNSR program can exclude the precursor from certain control requirements.

This guidance provides awareness of different approaches that can be made when working in a PM2.5 nonattainment area. This has been a highly anticipated guidance document and Trinity meteorologists and modelers will continue to track developments and implementation of PM2.5 precursor demonstrations for nonattainment planning purposes. For additional information and a link to EPA's announcement, click here or contact your local Trinity Consultants office at (800) 229-6655.