Fast Air Quality Clearances Using EPA Near-Road Monitoring Data

Focus Area

Air Quality


Air Quality






1-2 years

Research Idea Scope

    Air quality project-level analyses (PLAs) that state Departments of Transportation (DOTs) are required to prepare for purposes of the National Environmental Policy Act (NEPA) and the EPA transportation conformity rule typically include modeling for carbon monoxide (CO) and may also include modeling for fine particulate matter (PM2.5). This modeling would however be unnecessary for most freeway projects if practical screening criteria based on readily-available ambient monitoring data could be developed. This study would develop practical monitoring-based screening criteria for PM2.5 and CO that, once subjected to inter-interagency consultation, could be applied to quickly and easily clear freeway projects for NEPA  (for CO) and conformity (both CO and PM2.5) without time-consuming and expensive project-specific air quality modeling. Pilot tests would be conducted in several states to prove and as needed refine the new approach for air quality clearances before the new screening criteria are finalized.

       The Transportation Pooled Fund (TPF) for Near-Road Air Quality ( recently demonstrated the great potential for this approach in a paper pending publication*. The study compared background concentrations for PM2.5 to data from the recently implemented national near-road air quality monitoring network, and determined a typical upper limit for the contribution from high-volume freeways (up to approximately 600 thousand “fleet-equivalent AADT”) of approximately 2.12 ± 0.5 micrograms per cubic meter (µg/m3). This implies that a monitoring data-based screening criterion for freeways could potentially be implemented in which all freeways that do not exceed that volume (which would be most freeways) in areas in which PM2.5 background concentrations are lower than the annual primary national ambient air quality standard (NAAQS) of 12 µg/m3 by more than 2.12 µg/m3 (or more conservatively 2.62 µg/m3) would reasonably be expected to meet the annual NAAQS and therefore could be cleared for PM2.5 without project-specific air quality modeling. 

       An extension of the methodology to create tiered levels of potential roadway contributions (or “criteria”) covering a range of highway facility configurations and operating conditions – rather than just the upper limit or maximum potential impact of the largest facilities in the nation as done in the TPF study – would provide the greatest flexibility to state DOTs. The overall objective for this proposed study is therefore to develop a tiered set of criteria that would provide the greatest coverage of roadway facilities and operating conditions across the nation, and that have been demonstrated in pilot studies to be supported in inter-agency consultation for use as criteria in screening projects as applicable for NEPA and conformity. Criteria would be developed for both PM2.5 and CO . Additionally, recommendations for inclusion of the new monitoring-based criteria for CO into existing programmatic agreements for clearing project-level analyses for CO should be provided. 

       Note the criteria for PM2.5 would apply only in nonattainment and maintenance areas that are subject to EPA conformity requirements, as project-level PM2.5 analyses are not required for purposes of NEPA. States and regions at their own discretion may still of course apply the PM2.5 criteria developed for purposes of conformity in this proposed study to their respective jurisdictions for purposes of NEPA and/or to meet state requirements. Criteria for CO would be applied for both conformity and NEPA.

Determine if any PM10 monitors across the nation meet the general criteria for near-road monitor (within 50 m of a high-traffic corridor.  For any PM10 monitor that meets criteria, determine if tiered levels of potential roadway contributions can also be developed for PM10. 

1.Determine if any PM10 monitors nationwide are within 50 m of a high traffic corridor.  If yes, analyze 3 years of data for such monitors to determine if they offer a set of highway configurations and operating conditions to develop a tiered set of criteria. 
2. Develop a tiered set of criteria covering a range of highway configurations and operating conditions using data from the national near-road monitoring network. Separate tiers for CO and PM2.5 and PM10 (if possible) for high volume facilities should be developed to cover major facilities in areas in which background concentrations are sufficiently low to allow its application. A tier for lower volume facilities for PM2.5 is also needed for areas in which background concentrations are higher, and may be combined with criteria based on the examples provided in EPA guidance for projects of potential air quality concern. An intermediate tier for PM2.5 may be recommended as well. The criteria can be in the form of a spreadsheet table or check-list, or even a web-based tool. To the extent feasible:
       a) Optimize the selection of highway configurations and operating conditions to maximize the potential coverage of the tiered criteria, i.e., for all states and regions across the nation for CO, and limited to nonattainment and maintenance areas for PM2.5 and PM10 (if possible). For this purpose, considering as needed data from the Highway Performance Monitoring System (HPMS) and, for background concentrations, ambient air quality monitoring network data for states and regions across the nation that would be representative of areas in which the proposed criteria would be applied. 
       b) Develop monitoring-based criteria for other facility types including interchanges and arterial street intersections using available near-road and other ambient monitoring data. Include consideration of the historic long-term nation-wide downward trends in motor vehicle CO emission rates (due to more stringent emission and fuel quality standards) and ambient concentrations.
       2. Identify 5-7 states and regions for pilot studies for implementing the proposed CO and PM2.5 and PM10 (if possible) criteria following as applicable inter-agency consultation for: a) conformity for CO and PM2.5 and PM10 (if possible), and b) for NEPA for CO with FHWA Division offices. Document results.
       3. Refine the proposed criteria as needed for both CO and PM2.5 and PM10 (if possible) based on the feedback received in inter-agency consultation.
       4. Develop recommendations and guidance for state DOTs and regions to customize as needed and implement the CO criteria for NEPA and conformity, and the PM2.5 and PM 10 (if possible) criteria for conformity. Alternatively, if all parties are amenable, work with FHWA/FTA and EPA as needed to implement the criteria on a national basis. In either case, include recommendations to incorporate the new criteria for CO into the template programmatic agreement as updated in NCHRP 25-25 Task 104 (

       * A. Mukherjee  et al., “Influence of roadway emissions on near-road PM2.5: Monitoring data analysis and implication for screening transportation projects of local air quality concern (POAQC)”, Draft Report, March 18, 2019

Urgency and Payoff

The potential payoff is substantial: freeway improvement projects may be much more quickly, reliably and cost-effectively cleared for both CO and PM2.5 than is possible through other traditional project-level modeling or even programmatic agreements that require periodic updating (as models change primarily, and to expand coverage to more project types and configurations), may be more complex to apply, and may also be limited to projects of relatively smaller scope.

Suggested By

Christopher Voigt Virginia DOT 804 371 6764

[email protected]