Impacts of Lowered PM_2.5 NAAQS to Take Shape in 2025 and other NAAQS Related Updates

On February 7, 2024, the United States Environmental Protection Agency (U.S. EPA) finalized the particulate matter (PM) National Ambient Air Quality Standards (NAAQS) reconsideration process and lowered the annual PM less than 2.5 microns (PM_2.5) NAAQS from 12 micrograms per cubic meter (mg/m³) to 9 mg/m³ (check out my 4TR article here on the lowered annual PM_2.5 NAAQS). A change to any NAAQS triggers a regulatory process for implementation of the revised NAAQS that has a number of milestones starting at the promulgation date. There are several critical implementation milestones for the lowered annual PM_2.5 NAAQS occurring in 2025 that will provide more clarity on how the lowered annual PM_2.5 NAAQS may impact your operations, including an opportunity for the public to provide comments on the implementation process. 

State and local regulatory authorities are currently reviewing ambient PM_2.5 monitoring data collected during 2021 – 2023 to support their initial designation determinations, which are due to U.S. EPA by February 7, 2025. As part of the initial designation determination, state and local regulatory authorities can remove any ambient monitoring data that meets the criteria of an exceptional event. An exceptional event is defined as “unusual or naturally occurring that can affect air quality but are not reasonably controllable using techniques that agencies may implement in order to attain and maintain the NAAQS.” Exceptional event determinations associated with wildfires are proving to be critical for potentially avoiding PM_2.5 nonattainment area designations across the country. I am currently aware of exceptional event determinations in Georgia, Alabama, and North Carolina (who have published their initial destination determination for public review) that have enabled those states to avoid proposing PM_2.5 nonattainment areas.  

The next milestone in the PM_2.5 NAAQS implementation process is October 9, 2025, when U.S. EPA will notify states of any modifications to their initial designation determinations, which initiates a 30-day public comment period. U.S. EPA will be relying on 2022 – 2024 ambient monitoring data for their notice. The 2022 – 2024 annual PM_2.5 design values will be officially certified and available in June of 2025. States will have 60 days from when U.S. EPA publishes any modifications to their initial designation determinations to respond with additional information. U.S. EPA is then required to finalize the annual PM_2.5 designation determinations by February 7, 2026. For areas that are designated as PM_2.5 nonattainment, this is the “drop dead” date for Prevention of Significant Deterioration (PSD) permitting actions to be finalized by (i.e., final permit issued) and after which, the Nonattainment New Source Review (NNSR) permitting program begins to be implemented for direct PM_2.5 and PM_2.5 precursors emissions. Sulfur dioxide (SO₂) and oxides of nitrogen (NO_X) are automatically considered PM_2.5 precursors and ammonia (NH₃) and volatile organic compounds (VOC) are determined on a case-by-case basis if they are significantly contributing to the nonattainment status and regulated as PM_2.5 precursor emissions under the state or local agency’s NNSR permitting program.  

As you may recall, the PM NAAQS reconsideration process was implemented at the beginning of the Biden administration because all of the PM NAAQS were retained, without revision, at the end of the first Trump administration, during the routine PM NAAQS review process. What impacts will a second Trump administration have on the lowered annual PM_2.5 NAAQS? The National Association of Manufactures (NAM) has already provided a letter to the Trump administration that, among other things, has identified that the lowered PM_2.5 NAAQS has the potential to “block critical infrastructure and investments in facility modernization and harm competitiveness, jobs and economic growth”. It is ALL4’s opinion that the annual PM_2.5 NAAQS will not likely be relaxed. However, we anticipate that the Trump administration will do everything they can to make it easier to administer the lowered annual PM_2.5 NAAQS through updated and revised policies. ALL4 has already evaluated existing air quality modeling policies that, if updated, could remove overly conservative practices currently in place for air quality modeling demonstrations. A couple of examples include more flexibility in model receptor placement that takes into the pollutant averaging period being modeled (i.e., 1-hour versus annual) or utilizing Monte Carlo statistical modeling approaches to account for emissions variability. ALL4 also expects more leeway in accounting for exceptional event determinations when developing background datasets for air quality modeling demonstrations to and as part of the PM_2.5 NAAQS designation process to minimize the number and size of PM_2.5 nonattainment areas throughout the country. 

While the PM_2.5 NAAQS is currently on center stage, several other NAAQS are currently under review or are scheduled to be reviewed. The ozone NAAQS is scheduled to be reconsidered in 2025. Under a second Trump administration, we expect the current 70 parts per billion (ppb) ozone NAAQS to be retained without revision.  The lead NAAQS is currently undergoing the routine review process. U.S. EPA provided a notice of an update to the “Integrated Science Assessment for Lead” availability in November 2024, which U.S. EPA will use to make their recommendation on a revision to the lead NAAQS, if needed. The last time the lead NAAQS was lowered was over 17 years ago in 2008 when it was reduced by a factor of 10 from 1.5 mg/m³ to 0.15 mg/m³. Initial indications were that a reduction to 0.1 mg/m³ was being considered. However, it’s likely the lead NAAQS to be retained without revision if the review process is finalized in the next four years.  

In other “NAAQS adjacent” news for 2025, on December 23, 2024, U.S. EPA proposed to extend the third implementation period for the Regional Haze Rule (RHR) from July 31, 2028 until July 31, 2031 to allow more time for States to consider the potential emissions reductions from implementing other near-term regulatory programs (such as the lowered annual PM_2.5 NAAQS) when developing their Regional haze State Implementation Plans (SIP). The RHR protects visibility in Class I Areas (i.e., certain National Parks, National Wildness Areas, and National Wildlife Area established before 1977 depending on their size) with the goal of reasonable progress toward natural visibility levels by 2064. The first implementation period included visibility modeling requirements as part of Best Available Control Technology (BACT) analyses, and the second implementation period included four-factor (FFA) analyses. Both types of analyses evaluate the cost-effectiveness and feasibility of controls on certain sources of visibility impairing pollutants which include PM_2.5, PM less than 10 microns (PM₁₀), NO_X, and SO₂. The public comment period for the proposed extension ends on February 6, 2025.  

In addition to the proposed extension, we expect U.S. EPA to revise the RHR in 2025 based on comments from states to streamline the process for states that are ahead of the “glidepath” to achieve natural visibility levels by 2064.  Specifically, states have requested a revised RHR to provide clarity for obligations when the “glidepath” is being met, to include an “effectively controlled” off-ramp to be included in the rule and not just guidance, for final decisions related to FFA and cost effectiveness to be made by the states, and for the required contents of the SIP to be streamlined so that requirements can be enforceable without being detailed within a SIP.  

As you head into 2025, I suggest considering participating in industrial association advocacy efforts related to the PM_2.5 NAAQS implementation process and NAAQS air quality modeling guidance. If it appears that you will be in or near a PM_2.5 nonattainment area, I’d suggest reviewing the designation determination to ensure all possible exceptional events were excluded and consider commenting on U.S. EPA’s designation determinations if not, because it could provide valuable information to change the outcome. ALL4 has experience in exceptional event evaluation for both designation and background air quality modeling demonstrations if you need assistance. In addition, if you end up in or near a PM_2.5 nonattainment area ALL4 is one of the few companies that has extensive experience in NNSR PM_2.5 air permitting. For up-to-date information on the PM_2.5 NAAQS designation process, check out ALL4’s PM_2.5 NAAQS Resource webpage or reach out to Dan Dix at 610.422.1118 or ddix@all4inc.com to discuss how ALL4 can help.