The United States Environmental Protection Agency's (EPA) National Exposure Research Laboratory is developing improved methods for modeling the pollutant sources through the air pathway to human exposure in significant microenvironments of exposure. As a part of this project, the authors developed MicroFacPM, a microscale emission factor model for predicting real-world real-time motor vehicle particulate matter (PM(sub 10)) and (PM(sub 2.5)) emissions. MicroFacPM uses available information on the vehicle fleet composition. The main input variables required are the characterization of on-road vehicle fleet, time and day of the year, ambient temperature, relative humidity and percentage of smoking vehicles. Using the fleet information, MicroFacPM estimates a Composite Emission Factor (milligrams per mile). This paper presents the use of MicroFacPM to calculate the contribution of PM(sub 2.5) from motor vehicle sources along an example roadway as input to a roadway air dispersion model. The contribution of PM(sub 2.5) is presented per vehicle class (light, heavy duty), vehicle age, fuel type (gasoline, diesel), brake wear and tire wear sources.