More PM2.5 NAAQS Related Updates

There have been more guidance updates and rule making in the last month that will impact both air quality modeling and the designation process for the lowered annual particulate matter less than 2.5 microns (PM_2.5) National Ambient Air Quality Standard (NAAQS) that officially went into effect on May 6, 2024.

On April 30, 2024, the U.S. EPA published a “Supplement to the Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program.” As expected, the supplemental guidance reduced the annual Class II Significant Impact Level (SIL) from 0.2 micrograms per cubic meter (µg/m³) to 0.13 µg/m³. In addition, the Class I annual PM_2.5 SIL was also lowered from 0.05 µg/m³ to 0.03 µg/m³. There were no changes to the 24-hour PM_2.5 Class I or Class II SILs nor to the 8-hour ozone SIL. The annual PM_2.5 SIL was expected to be lowered based on how the original SIL was calculated. In 2018 U.S. EPA established the PM_2.5 SIL based on the 50 percent confidence interval (CI) of observed ambient air data across all U.S. PM_2.5 ambient monitoring data from 2014-2016 to quantify the bounds of change in air quality that can be considered an “insignificant impact”. This same approach was updated utilizing the 2020-2022 PM_2.5 ambient monitoring data. Because the U.S. wide average measured PM_2.5 ambient monitoring data has been decreasing over time it was expected that the updated statistical approach would reduce the annual PM_2.5 SIL. The lowered annual SIL will obviously make it more difficult to “screen out” of multi-source NAAQS air quality modeling demonstration that are also more difficult with the lowered annual PM_2.5 NAAQS and small amount of available headroom.

Because of the changed annual Class I and Class II PM_2.5 SIL, U.S. EPA also had to publish a Clarification on the Development of Modeled Emission Rates for Precursors (MERPs) as a Tier 1 Demonstration Tool for Ozone and PM2.5 under the PSD Permitting Program and update data contained on U.S. EPA MERPs View Qlik webpage. Updates were necessary to remove the published MERP emissions threshold values because the MERPs were tied to the SIL. Instead, U.S. EPA left the hypothetical modeled PM_2.5 concentrations for applicants to calculate site specific PM_2.5 concentrations resulting from PM_2.5 precursor emissions of sulfur dioxide (SO₂) and nitrogen oxide (NO_X). The lowered annual PM_2.5 SIL effectively also rachets down the MERP values.

On April 30, 2024 U.S. EPA also published an “Effective Tools for Fine Particulate Matter under the Prevention of Significant Deterioration Permitting Program Fact Sheet” that includes a list of policy and technical guidance, models, and tools to assist stakeholders in evaluating all the currently available options for obtaining a Prevention of Significant Deterioration (PSD) permit with the lowered annual PM_2.5 NAAQS. While the Fact Sheet is a good resource that summarizes all the pertinent information related to PSD permitting policy and guidance it does not include any updated guidance and/or relief from conservative modeling approaches since the lowered annual PM_2.5 NAAQS was finalized.

Finally, on May 16, 2024 U.S. EPA published a final rule in the Federal Register (FR) for the “Update of PM2.5 Data from T640/T640X PM Mass Monitors.” The rulemaking finalizes U.S. EPA’s intent to make a network data alignment correction of PM_2.5 concentrations from Teledyne T640/640X monitor measurements that had been identified in peer-reviewed literature as reporting on average 20 percent high bias relative to Federal Reference Method (FRM) and other Federal Equivalency Method (FEM) PM_2.5 monitors. The Teledyne T640/640X PM_2.5 monitors are used at almost a third of ambient monitoring stations across the U.S. On average the correction will result in a 20 percent reduction in measured PM_2.5 concentrations. However, that number will change from station to station because the correction is based on hourly temperature and relative humidity. This correction is important for both air quality modeling demonstrations that may be relying on Teledyne T640/640X PM_2.5 monitored values for background concentrations and as states begin the process of making their initial annual PM_2.5 designation recommendations which are due to U.S. EPA by March 6, 2025. It should be noted that industry associations have identified other PM_2.5 FEM monitors that have a positive bias when compared to FRM monitors. However, U.S. EPA has indicated that they don’t plan to make any other network data alignment corrections, nor would they approve an adjustment on a case-by-case basis to background concentrations in a PM_2.5 NAAQS modeling demonstration.

The multifaceted aspects of the lowered annual PM_2.5 NAAQS have made it difficult for ALL4 to broadly identify how the change will impact the regulated community. The lowered PM_2.5 NAAQS will impact facilities differently depending on the type of source they are (e.g., major Title V versus minor), where they’re located in the U.S., and what the state regulatory requirements are with respect to air quality modeling demonstrations. In addition, it will be important to evaluate whether the current measured PM_2.5 concentrations were measured by a Teledyne 640/640X monitor and whether those concentrations were impacted by an exceptional event such as a wildfire in order to determine if areas will be designated as attainment or nonattainment and what a potential background concentration would be for a PM_2.5 NAAQS air quality modeling demonstration.

ALL4 is here to help – we can evaluate all aspects of how the lowered annual PM_2.5 NAAQS will impact your operations now and over the next 10 years whether you have an air permitting project or not and concisely summarize it for you and your company’s leadership team. In addition, ALL4 can make recommendations on what can be done now to better position yourself for when the lowered PM_2.5 NAAQS will impact your facility. Please contact Dan Dix at ddix@all4inc.com or (610) 422-1118 to discuss how we can help.