PM emissions emanating from limited-access highways

Motor vehicle contributions to primary particulate matter (PM) emissions include exhaust, tire wear, brake and clutch wear, and resuspended road dust. Relatively few field studies have been conducted to quantify fleet-average exhaust emissions for actual on-road conditions. Therefore, direct measurements of motor vehicle-related PM emissions are warranted. In this study, PM10 and PM2.5 mass concentrations were measured near two major highways in the St. Louis area over the period from February-April 1997. Samplers were deployed both upwind and downwind of the roadways to capture the transport and dispersion of PM with distance from the roadway The observed microscale concentration fields were compared to estimates using the PARTS emission factor model together with the CALINE4 highway dispersion model. Traffic-induced PM mass concentrations observed downwind of the roadway were always less than PART5/CALINE4 predictions; average percent differences for observed traffic-induced mass concentrations compared to predicted values were -34% for PM2.5 and -70% for PM10. In most cases, the observed PM concentration decay with increasing distance from the roadway was steeper than predicted by dispersion modeling. Motor vehicle-induced emission factors were reconstructed by fitting CALINE4 to the observed concentration data with the emission factor as the sole adjustable parameter. Reconstructed fleet-average motor vehicle emission factors for the urban interstate highway were 0.03-0.04 g/VMT for both PM2.5 and PM10, while the fleet-average emission factors for the rural interstate highway were 0.2 and 0.3 g/VMT for PM2.5 and PM10, respectively.