Furthermore, MDVs and HDVs are responsible for a large proportion of PM2.5 exposure from exhaust emissions, accounting for 63.1% (30.4% + 32.7%) of the average daily exposure (Table ). Similarly, they are responsible for a significant proportion of PM2.5 exposure from nonexhaust emissions, accounting for 21.4% (12.7% + 8.7%) from brake wear and 15.2% (7.2% + 8.0%) from tire wear of the average daily exposure Table . These results highlight the significant role both exhaust and non-exhaust emissions play in overall air quality, particularly for MDVs and HDVs. At the county level, there is considerable variation in emissions exposure based on vehicle type and regional characteristics. While LDVs generally contribute more significantly to emissions exposure compared to MDVs and HDVs, certain counties-particularly those along interstate highways and major arterial roads-show a higher percentage of emissions from MDVs and HDVs (Figure ). Notably, counties in states such as Utah, Oregon, Oklahoma, and South Carolina exhibit a considerably larger proportion of emissions from MDVs. In some areas, emissions exposure from MDVs is more than double that from LDVs. This can be attributed to the high volume of MDVs on long-distance freight corridors and the relatively lower urban density in these regions. Similarly, HDVs contribute disproportionately to emissions exposure in many counties in the central U.S., further emphasizing the need to evaluate emissions at the county level and consider the role of specific vehicle types in shaping regional air quality. Additionally, we examine the relative contributions of different emission processes to PM2.5 exposure (See Table ). HDVs and MDVs are overwhelmingly responsible for PM2.5 exposure from exhaust emissions, with 90.2% of the total daily PM2.5 exposure from HDVs and 86.7% from MDVs. LDVs also contribute significantly to exhaust emissions, but to a lesser extent (52.5%), with a larger share of their PM2.5 exposure coming from non-exhaust sources like brake and tire wear. Brake wear is a major contributor to PM2.5 exposure for LDVs, accounting for 34.4% of their total daily exposure. This indicates that for LDVs, non-exhaust emissions (particularly from braking) play a larger role compared to MDVs and HDVs, where brake wear is a relatively smaller contributor (11.1% for MDVs and 7.4% for HDVs). Tire wear is a minor source of PM2.5 exposure across all vehicle types, contributing a smaller share compared to exhaust and brake wear (13.1% for LDVs, 2.2% for MDVs, and 2.4% for HDVs).