particulate matter

September 18, 2023

The other night, I woke up in a bit of a panic and thought that I must have missed an announcement by the USEPA regarding the PM_2.5 NAAQS reconsideration! Did I?

The answer, thankfully, is no! However, that is not any reason to relax and take no action.

By way of background, the Clean Air Act requires that the USEPA set National Ambient Air Quality Standards (NAAQS) for each of the criteria air pollutants, including particulate matter (PM). These NAAQS are based on the best available science and aim to protect human health and welfare. In June of 2021, the USEPA announced that they would reconsider the 2020 PM NAAQS final action under the prior administration that did not lower any standards. Then, in January 2023, the USEPA published the proposed rule to revise the PM_2.5 primary annual standard down from 12 micrograms per cubic meter (µg/m³) to within a range of 9-10 µg/m³. In that proposal, the agency retained the 24-hour standard of 35 µg/m³.

The comment period for this proposed rule has since closed, and the agency is now working through over 5,000 submitted comments. It is unclear when the USEPA will issue the final rule (it could be any day now) and if the new annual standard will be set at 9 or 10 or something in between.

What does this mean for me as a regulated source? What should you do? SCS Engineers recommends the following actions:

• If you have any air quality modeling analyses in progress for PM_2.5, review the annual PM_2.5 analysis and ensure the results are at or below 10, or ideally 9 µg/m³.
  • NOTE: Once the proposed rule becomes final, required air dispersion modeling assessments must demonstrate compliance with the new standard before issuing a permit, even when submitting the application before the new NAAQS standard.
• Review and understand local ambient air monitoring data and trends to develop permitting strategies depending on whether your airshed will become a non-attainment area, or if the background values will be very close to the new standard, leaving little room for regulated source emissions.
• Review current emission inventories and calculation methodology to determine if there is room to modify conservative assumptions to reduce PM_2.5 emissions.
• Investigate potential emission reduction projects. The investigation is especially important if your area will become a non-attainment area so that you can generate offsets for future permitting needs or emission reduction banking.

Additional Resources:

• USEPA’s Rulemaking Process
• Clean Air Act Services

About the Author: Rafe Christopherson, PE, is a project director and SCS’s industrial Clean Air Act leader. He is an air quality professional with a wide variety of experience over his 25-year career. His expertise includes consulting, working at an air quality regulatory agency and with industry. His expertise includes semiconductors, biofuels, pulp and paper, hardwoods, power generation, refineries, and general manufacturing. If you are interested in more information on this PM_2.5 NAAQS reconsideration process and what it might mean for your business, you may reach Rafe at or via LinkedIn.