For state departments of transportation (DOTs), comparisons of alternative build and no-build scenarios in project-level air quality analyses typically focus on comparisons of emissions and/or ambient concentrations of specific pollutants, including greenhouse gases, mobile source air toxics, particulate matter, and carbon monoxide. However, the emission and concentration estimates between pollutant classes are not directly comparable and cannot be directly aggregated. This constrains decision-making on NEPA alternatives, as it limits the ability to consider potential multi-pollutant air quality impacts, which may differ substantially by alternative.

One possible solution to this problem is to translate the results for each pollutant class into a common risk-based metric that would allow for aggregation and comparison. Potential synergistic effects could also be considered as part of the aggregation. NEPA alternatives could then be compared on a common basis, i.e., the potential air quality impacts for each alternative in terms of aggregate risk. Risk assessments incorporated into NEPA documentation would enhance transparency and communication to stakeholders, such as environmental justice (EJ) populations, and support informed decision-making on preferred alternatives for which the potential multi-pollutant air quality impacts may differ substantially.

The objective of this research is to develop a simplified means to assess and report the relative risk of potential transportation air quality impacts for NEPA alternatives, including those for EJ populations. The research would produce a simplified, practical, and affordable approach to air quality risk assessment that could be applied at state DOT discretion in NEPA studies.