Development of a microscale emission factor model for CO for predicting real-time motor vehicle emissions

The U.S. Environmental Protection Agency's (EPA) National Exposure Research Laboratory has initiated a project to improve the methodology for modeling human exposure to motor vehicle emissions. The overall project goal is;to develop improved methods for modeling the source through the air pathway to human exposure in significant microenvironments of exposure. This paper pr esents the technical description of a newly developed model for CO emission s. The sensitivity analysis and evaluation of this emission model is presen ted in a companion paper titled "Sensitivity Analysis and Evaluation of Mic roFacCO: A Microscale Motor Vehicle Emission Factor Model for CO Emissions. "(1) The MOBILE models (used in the United States, except California) and E MFAC models (used in California only) used to estimate emissions are suitab le for supporting mostly regional (county)-scale modeling and emission inve ntory because of their dependence on vehicle-miles-traveled aggregate data. These emission models are not designed to estimate real-time emissions nee ded for human exposure studies near roadways. A number of independent studi es have found that current mobile source emission factor models are not ver y reliable at estimating microscale emissions and are, therefore, inappropr iate for use with microscale modeling necessary to estimate human exposures near roadways. A microscale emission factor model for predicting real-world real-time moto r vehicle CO emissions (MicroFacCO) has been developed. It uses available i nformation on the vehicle fleet composition. The algorithm used to calculat e emission factors in MicroFacCO is disaggregated based on the on-road vehi cle fleet. The emission factors are calculated from a real-time fleet rathe r than from a fleet-wide average estimated by a vehicle-miles-traveled weig hting of the emission factors for different vehicle classes. MicroFacCO use s the same database used to develop the MOBILE6 model. As compared with MOB ILE emission models, MicroFacCO requires only a few input variables, which are necessary to characterize the real-time fleet being modeled. The main i nput variables required are on-road vehicle fleet, time and day of year, am bient temperature, and relative humidity.