Updates on the Lowered PM2.5 NAAQS
Posted: September 10th, 2024
Authors: Dan D.
It’s been four months since the lowered annual particulate matter less than 2.5 microns (PM2.5) National Ambient Air Quality Standard (NAAQS) became effective on May 6, 2024, and seven months since my last 4TR article on the topic. I thought it was a good time to provide an update on what has transpired in the last four months and where regulators are in the process of meeting regulatory milestones related to implementing the lowered annual PM2.5 NAAQS, which now sits at 9 micrograms per meter cubed (mg/m3).
On April 30, 2024, right before the lowered annual PM2.5 NAAQS became effective, United States Environmental Protection Agency (U.S. EPA) released supplemental guidance that lowered the annual PM2.5 Class II Significant Impact Level (SIL) from 0.2 mg/m3 to 0.13 mg/m3 and lowered the Class I SIL from 0.05 mg/m3 to 0.03 mg/m3. The reduction was not surprising because U.S. EPA utilized the same statistical approach that relied on quantifying a statistically significant deviation from the design value utilizing PM2.5 measurements from the entire U.S. ambient monitoring network to establish the SIL. The analysis was updated from the original 2014-2016 dataset to a more recent 2020-2022 dataset and the more recent dataset is lower on average, both because of a general reduction of PM2.5 emissions nationally in the intervening time period and the inclusion of 2020, which was impacted by the COVID pandemic, and therefore resulted in a lower calculated SIL. Keep in mind the SIL is established as part of guidance and not by regulation. Therefore, states and local jurisdictions are free to utilize the SIL or propose their own SIL threshold. Because the initial annual PM2.5 SIL has been established there have now been three different annual SILs (0.3 mg/m3, 0.2 mg/m3, and 0.13 mg/m3) and ALL4 is aware of each of the three SIL values currently being utilized in a least one or more states across the U.S.
Another more recent update related to the PM2.5 NAAQS was U.S. EPA’s posting the 2023 design values report on August 9, 2024. The 2023 design values report is a spreadsheet that contains the 2021-2023 design values for all monitoring stations in the U.S. and is the design value that states, and local jurisdictions are utilizing to make initial PM2.5 NAAQS designation determinations which are due to U.S. EPA by February 7, 2025.
There was a delay in posting the 2023 PM2.5 design values report (usually posted in June) because of a network data alignment of Teledyne T640 and T640X PM2.5 measured concentrations. Specifically, the Teledyne T640 and T640X had been identified in peer-reviewed literature to be reporting generally high bias relative to other Federal Reference Method (FRM) and Federal Equivalency Method (FEM) monitors, which lead to the correction of reported PM2.5 concentrations utilizing the Teledyne monitor back to 2017. The Teledyne bias adjustment reduced measured PM2.5 concentrations by 5-20%, depending on the site’s temperature and relative humidity conditions. For information on whether your area could be designated in attainment or nonattainment with the lowered annual PM2.5 NAAQS, ALL4 has updated our PM2.5 NAAQS Resource Webpage to include maps that utilized the finalized 2021-2023 design values. Keep in mind that the 2023 design values report doesn’t exclude exceptional events and that final U.S. EPA designation determinations will be based on the 2022-2024 design values.
As part of their initial designation recommendations to U.S. EPA, states can provide justification for removing exceptional events to help reduce the 2021-2023 design values to below the lowered annual PM2.5 NAAQS in order to not have to establish PM2.5 nonattainment areas. U.S. EPA will have to review and approve the exceptional event determinations as part of their approval of states’ designation determinations. Some states will provide a comment period for the public to provide feedback on their initial designation determinations. If your facility is located in an area that may be established as a PM2.5 nonattainment area, this is your opportunity to provide comments and suggestions on the approach utilized to make the proposed designation determination. You’ll get another opportunity to comment on designation determinations when U.S. EPA proposes their final designations (based on the states’ proposals) by February 7, 2026. If necessary, states can ask for a one-year extension to complete exceptional and atypical event determinations, which would push back any new potential PM2.5 nonattainment areas until 2027. ALL4 is aware of at least one state, Pennsylvania, that has had this request approved by U.S. EPA.
Removal of exceptional event data can only be completed if it reduces the design value below an established NAAQS. For example, if the 2021-2023 annual PM2.5 NAAQS design value is 8.9 mg/m3 then no reduction is allowed under U.S. EPA’s Exceptional Event guidance. This is important because design values are also utilized in NAAQS air quality modeling demonstrations. However, on a case-by-case basis applicants can provide justification to remove atypical events when establishing a background concentration specifically for air quality modeling demonstrations. Atypical events can be the same as those classified under U.S. EPA’s Exceptional Event guidance as well as more localized events that may not have the same scale as some events identified as exceptional. Some states (i.e., Iowa and Georgia) are developing background concentration databases for air quality modeling demonstrations that remove exceptional and atypical events in an effort to streamline the permitting process.
If you are located in an area that has the potential to be established as a PM2.5 nonattainment area reach out to ALL4 to see how we can help in reviewing and commenting on states’ initial proposals to U.S. EPA. If your county does end up being designated as nonattainment, we have experience with PM2.5 nonattainment new source review permitting, so we can help with that as well. In addition, if you’re considering a project in an attainment area that triggers PM2.5 air quality modeling ALL4 can assist in evaluating options for refining emissions data and making physical changes that will result in modeling below the lowered annual PM2.5 NAAQS. Please contact Dan Dix at ddix@all4inc.com or (610) 422-1118 for more information or for assistance with your project.
