Mobile Source Contributions to Ambient Ozone and Particulate Matter in 2025

The contribution of precursor emissions from 17 mobile source sectors to ambient ozone and fine particulate matter levels across the U.S. were evaluated, using the CAMx photochemical model, to identify which mobile source sectors are projected to have the largest impacts on air pollution in 2025. Both onroad and nonroad sectors contribute considerably to projected air pollution across much of the country. Summer ozone season ozone contributions between 2 and 5 ppb, which are among the highest levels presented on the maps of mobile source sectors, are largely found in the southeast United States from the onroad sectors, most notably light-duty and heavy-duty vehicles, and along the coastline from the Category 3 (C3) marine sector. Annual average PM_2.5 contributions between 0.5 to 0.9 microg/m³, which are among the highest levels presented on the maps of mobile source sectors, are found throughout the Midwest and along portions of the east and west coast from onroad sectors as well as nonroad diesel and rail sectors. Additionally, contributions of precursor emissions to ambient ozone and PM_2.5 levels were evaluated to understand the range of impacts from precursors in the various mobile source sectors. For most mobile source sectors, in most locations, NO_X emissions contributed more to ozone than VOC emissions, and secondary PM_2.5 contributed more to ambient PM_2.5 than primary PM_2.5. The largest ozone levels on the maps showing contributions from mobile source NO_X emissions tended to be between 2 and 5 ppb, while the largest ozone levels on the maps showing contributions from mobile source VOC emissions tended to be between 0.9 and 2 ppb, except for southern California where ozone contributions from VOC emissions from onroad light duty vehicles were between 2 and 5 ppb. The largest contributions to ambient PM_2.5 on the maps showing primary and secondary contributions from mobile source sectors tended to be between 0.1 and 0.5 microg/m³. The contribution from primary PM_2.5 extended over localized areas (urban-scale) and the contribution from secondary PM_2.5 extended over more regional (multi-state) areas.