The U.S. Environmental Protection Agency (U.S. EPA) revised the annual National Ambient Air Quality Standard (NAAQS or standard) for particulate matter 2.5 micrometers or less in diameter (PM2.5) from 12.0 µg/m3 to 9.0 µg/m3 on February 7, 2024. The State of California is required to submit recommendations for nonattainment area designations and boundaries to U.S. EPA by February 7, 2025. The deadline for U.S. EPA to finalize designations is February 6, 2026.
On January 23, 2025, the California Air Resources Board (CARB) approved the initial nonattainment areas recommendations for the new NAAQS. These initial areas were based on comparing the design value, a three-year average of annual average concentrations, to the level of the standard. CARB’s recommendations are based on PM2.5 air quality monitoring data for the years 2021 to 2023 as well as contributing sources. CARB recommended PM2.5 nonattainment designations and boundaries for the following nine areas:
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- Mendocino County (partial) – This area has never been designated nonattainment for any of the previous PM2.5 standards.
- Plumas County (partial)
- Yuba City-Marysville
- Sacramento County
- San Francisco Bay Area
- San Joaquin Valley
- Los Angeles-South Coast Air Basin
- San Diego County
- Imperial County (partial)
It is to be noted that the 2024 PM2.5 monitor data for Sacramento County and Yuba City-Marysville has improved and both areas may attain the standard. However, the 2024 data is currently going through the certification process which is expected to conclude by May 2025. If the certified data shows the same concentration reduction trend, the list of nine areas above will be reduced from nine to seven by excluding Sacramento County and Yuba City-Marysville. The nonattainment designation process can also exclude “Exceptional Events” such as wildfires, high wind dust events, fireworks, prescribed fires, stratospheric ozone intrusions, and volcanic and seismic activities, and agencies can develop exceptional events demonstrations to exclude the effects of such events on the data evaluation for nonattainment designation purposes. The recent California fires may be considered “Exceptional Events” that may affect this initial list of area designations and boundaries. The new EPA administration may attempt to reverse the 2024 PM2.5 NAAQS changes in the next few months and work though the associated legal process and challenges. Therefore, the PM2.5 nonattainment designations continue to be a fluid matter and an area of focus.
The nonattainment designations will continue the regulatory pressures on industry operating in these geographies. These pressures include more prevalent Nonattainment NSR (NNSR) permitting in the nonattainment areas above, new and revised Reasonably Available Control Technology (RACT) requirements for existing sources, and a possible reduction on the major source thresholds for industrial operations. RACT, or “prohibition”, rules typically contain emission limits on operations such as heaters, boilers, flares, tanks, manufacturing processes, etc. Possible reductions in these emission limits will impact future capital expenditures (CapEx) and operational expenditures (OpEx) costs. Similarly, NNSR permitting thresholds for Best Available Control Technology (BACT) and emission offsets may be reduced, which can increase CapEx and OpEx due to the addition of high-performance emission controls. Several of these geographics have limited emission reduction credits (ERCs), which may complicate the permitting process.
In preparing for permitting and compliance under the potentially lower PM2.5 NAAQS, industrial operations can refine many of the historically conservative approaches for estimating emissions of PM2.5. The largest and most impactful refinement is abandoning the historical assumption for many industrial sources that “PM = PM10 = PM2.5”. While this assumption may have been applicable many years ago, the body of PM2.5 emission factors and publications about the fractions of PM that is PM2.5 has grown significantly in the past few years. This data suggests that PM2.5 emissions are a very small percentage of PM emissions, therefore the assumption that “PM = PM10 = PM2.5” is very conservative in most cases. Another consideration for refining PM2.5 emissions profile is associated with fugitive emissions such as storage piles and roads. For instance, road emission calculations typically require an estimate of silt content and many industrial operations use default U.S. EPA silt content which tend to be conservative. Site-specific silt content measurements are generally a very cost-effective way to reduce these silt content estimates and greatly reduce the PM2.5 emission profile associated with industrial roads. Industrial operations can use these refined factors, fractions, and silt content to refine historic PM2.5 emission estimates and potentially reduce their profile below the thresholds of the various permitting hurdles noted above.
For more information, please contact Eddie Al-Rayes or your local Trinity Consultants California Office.
