Background concentrations (BCs) for project-level “hot-spot” analyses for fine particulate matter (PM2.5) included in NEPA documentation are typically high relative to the national ambient air quality standard (NAAQS) set by EPA, which can make demonstrating compliance with the NAAQS very challenging for proposed transportation projects. The challenge is significantly increased if there are “atypical events” (AEs) such as wildfires that contribute to higher monitored concentrations, and greatly exacerbated if EPA coincidentally and substantially reduces the NAAQS as that makes not only compliance with the NAAQS much more challenging but also significantly increases the number of states that will have to do PM analyses in the future to the new more stringent standard. That is the current state of affairs for state DOTs; 2023 was a year in which wildfires across North America contributed to widespread higher concentrations of PM2.5 across the nation, and EPA recently (on February 7, 2024) made a dramatic reduction (25%) to the annual primary NAAQS for PM2.5. The challenges will be ongoing as wildfires occurring in most states and other AEs (fireworks, dust storms etc.) may reasonably be expected to continue to occur in the future and negatively bias air quality monitoring data for PM. The proposed research would help DOTs avoid unnecessary, time consuming and resource intensive PM analyses and focus their limited resources on higher priority projects.

One way to address these ongoing challenges is to take advantage of a new option provided by EPA in their 2019 update to their guidance for hot-spot analyses for PM (both PM2.5 and PM10). The new option would allow “monitoring data exclusion, selection, or adjustment” (termed AEs here) to be excluded from BCs used in PM analyses. Note, although EPA did not provide step-by-step instructions for how to first identify AEs and then adjust BCs to exclude them, the guidance appears to reflect the recommendations for “exceptional type” events (ETEs) from the NCHRP 25-25 Task 89 study. It is unknown how many (if any) state DOTs have made use of the new AEs option provided by EPA, what methodology they used to identify the AEs and then adjust the BCs to exclude them, and even whether their attempts were successful. A more general question is what the best approach is to identify AEs and as appropriate adjust BCs in PM analyses for proposed transportation projects in the future.

The proposed study would therefore first review the EPA 2019 guidance and, as available, state DOT experience in its application. Then, building from the step-by-step process for identifying ETEs and adjusting BCs provided in NCHRP 25-25 Task 89, it would develop a primer for state DOTs on how to best identify and designate AEs and adjust BCs as appropriate for PM analyses. It would also, as funding permits, conduct pilot applications of the new primer and assist state DOTs with the identification of AEs and adjustment of BCs as appropriate for select projects in their respective jurisdictions. It would also assist (as funding permits) in the associated inter-agency consultation for conformity related to the adjustment of BCs for AEs. The final version of the primer would be revised as needed to reflect the lessons learned from the pilot applications including the conformity consultation.

For background, on February 7, 2024, EPA issued a final rule for a revision of the PM2.5 NAAQS that will reduce the annual primary standard by 25 percent, i.e., from 12 to 9 micrograms per cubic meter. This is expected to substantially increase the number of regions in the nation that will fall into nonattainment and therefore will have to conduct PM2.5 conformity hot-spot analyses for inclusion in NEPA assessments. The EPA news release for the new standards stated that: “Of the 119 counties with 2020-2022 design values above 9 ug/m3, 59 counties are totally or partially contained in nonattainment areas for current PM2.5 standards.” This represents over a 100 percent increase in impacted counties. Note even more counties (and states) may be expected to be nonattainment for the new standard than this preliminary estimate by EPA, as the final designations by EPA for the new NAAQS will be based on data for 2022 to 2024 and so will be affected by the AEs caused by wildfires in 2023. The implications for state DOTs is stark: as BCs for PM2.5 are typically high relative to the NAAQS, the substantially reduced NAAQS will make it that much harder (if not impossible) to show compliance with the NAAQS for proposed transportation projects, with or without mitigation measures. If they are also not able to show build design values are less than no build (which is very challenging for capacity additions), they may even be prevented from implementation of the proposed projects.

NCHRP 25-25 Task 89 “Establishing Representative Background Concentrations for Quantitative Hot-spot Analyses for Particulate Matter” (2014) provided an overview for state DOTs of both exceptional events (EEs) as provided by EPA and “exceptional type” events (ETEs) that it proposed as a research option. ETEs differ from EEs primarily in that ambient data influenced by a wildfire or other event may be excluded even if the NAAQS was not exceeded or if was exceeded but not approved by EPA as an EE. EEs are applied by EPA and state air agencies for the determination of (unbiased) design values used to categorize attainment status for a region, while ETEs would be applied (by state DOTs preferably) for the determination of unbiased background concentrations (BCs) to be proposed for use in hot-spot analyses for particulate matter (PM). Note BCs for the annual primary PM2.5 standard are based on three years of monitoring data, so the effect of the 2023 wildfires on BCs for PM2.5 would last three years, and the effect of future wildfires or other AEs would also last three years each. More generally, the challenges posed by ETEs/AEs are ongoing and a means to mitigate their effects in PM analyses is needed.

Finally, in 2019, EPA updated its guidance for PM analyses and provided an option that appears to be similar or functionally equivalent to the ETEs proposed in NCHRP 25-25 Task 89 (although that NCHRP study was not specifically referenced). The 2019 EPA guidance states that “Specifically, monitoring data exclusion, selection, or adjustment may be considered for the following types of determinations and analyses: …[1d] Preparing any required particulate matter (PM) “hot-spot” analysis for a transportation conformity determination for certain projects under 40 CFR Part 93 and relevant guidance.” This option is informally called here “atypical events” (AEs) and would apply for BCs used for PM2.5 and PM10 analyses included in NEPA documentation by state DOTs.