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Updates to Guidance on Ozone and Fine Particulate Matter Permit Modeling Issued by U.S. EPA

Posted: October 8th, 2021

Authors: Dan D. 

U.S. EPA’s Office of Air Quality Planning and Standards (OAQPS) published the “Revised DRAFT Guidance for Ozone and Fine Particulate Matter Permit Modeling” (Revised Draft Guidance) on September 20, 2021.  The Revised Draft Guidance replaces the initial “DRAFT Guidance for Ozone and Fine Particulate Permit Modeling” (Initial Draft Guidance) guidance issued by U.S. EPA on February 10 ,2020.  The Revised Draft Guidance introduces one important change intended to address public comments received on the Initial Draft Guidance, consistent with January 20, 2021 Executive Order 13990 (Protecting Public Health and the Environment and Restoring Science to Tackle the Climate Crisis).  U.S. EPA published the Revised Draft Guidance to make revisions based on their current policy as it relates to which regulated New Source Review (NSR) pollutants should be included in a Prevention of Significant Deterioration (PSD) air quality assessment for ozone (O3) and particulate matter less than 2.5 microns (PM2.5).

The Initial Draft Guidance and the Revised Draft Guidance provide recommendations on demonstrating compliance with O3 and PM2.5 National Ambient Air Quality Standards (NAAQS) and PSD increments for PM2.5 as part of PSD permit applications.  Both outline a two-tiered approach for assessing primary and secondary PM2.5 precursor pollutants [e.g., nitrogen dioxide (NOX) and sulfur dioxide (SO2)] impacts and secondary O3 precursor pollutants [e.g., NOX and volatile organic compounds (VOC)] impacts.  The Tier 1 approach utilizes Modeled Emissions Rates for Precursors (MERPs) thresholds developed through photochemical air dispersion modeling that represent a precursor pollutant emissions threshold that will result in an impact below the PM2.5 and O3 Significant Impact Level (SIL).  A Tier 2 approach would rely on the use of a photochemical dispersion model to predict secondary impacts from PM2.5 and O3 precursor pollutants.  To date, ALL4 is not aware of any application where it was necessary to utilize the Tier 2 approach because of the ability to refine the MERPS to be site specific.

The Initial Draft Guidance recommended that only NSR precursor pollutants that exceed the PSD Significant Emissions Rates (SERs) be included in the Tier 1 or Tier 2 impact evaluation.  For example, if as part of a PSD permit application you were projecting an increase in NOX emissions greater than the PSD SER, but below the PSD SERs for all remaining NSR pollutants, you would only have been required to evaluate NOX as a precursor to PM2.5 and NOX as a precursor to O3.  You would not have had to consider SO2 as a precursor to PM2.5 or VOC as a precursor to O3.  The Revised Draft Guidance now recommends that if any precursor pollutant exceeds the PSD SER that all precursor pollutants be included in a Tier 1 or Tier 2 impact evaluation.  More importantly, the Revised Draft Guidance also explicitly recommends that direct PM2.5 emissions be cumulatively evaluated along with PM2.5 precursor impacts.  Therefore, if you were to trigger the PSD SER for NOX you would be required to evaluate secondary PM2.5 impacts from NOX and SO2 (PM2.5 precursors) through the Tier 1 or Tier 2 impact evaluation approach and evaluate direct PM2.5 emission impacts through direct PM2.5 air dispersion modeling (such as with AERMOD).  This means that applicants will no longer be able to avoid direct PM2.5 modeling through PM2.5 PSD avoidance if NOX or SO2 PSD permitting is triggered.  The requirement to evaluate the cumulative impacts of precursor emissions and direct PM2.5, along with U.S. EPA’s announcement to reconsider the previous administration’s decision to retain the PM2.5 NAAQS, continues to make PSD permitting projects that include increases in PM2.5 emissions (and now precursor emissions) difficult.  It should also be noted that while the Revised Draft Guidance is still considered a draft, state agencies have historically utilized “draft” guidance for permitting and some draft guidance issued by U.S. EPA has never been finalized, despite being routinely utilized.  Therefore, depending on your state’s policy, this Revised Draft Guidance could be relied on immediately for your next project.

U.S. EPA plans to hold a webinar on October 14, 2021 at 3 pm EDT (which ALL4 plans to attend) to provide an overview of the Revised Draft Guidance and public comments on the Revised Draft Guidance will accepted until November 19, 2021.  If you have any questions about how a potential project that includes PM2.5 emissions will be impacted by the Revised Draft Guidance, or wish to discuss submitting comments on the guidance, please contact Dan Dix at ddix@all4inc.com or at 610.422.1118.


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