 |
NARSTO Workshop on Innovative Methods for Emission Inventory Development and Evaluation University of Texas, Austin October 14-17, 2003 |
 |
Traffic-Related Emission Factors in Two Major Freeways in Southern California
By Rong Chun Yu, Yifang Zhu, William C. Hinds, John R. Froines
Southern California Particle Center and Supersite,
University of California at Los Angeles, Los Angeles, CA 90095
Despite great efforts have been made, uncertainties remain to be a major issue for the methodology used to estimate motor vehicle emission factors. This study is sought to apply measurements of pollutants in the vicinity of major freeways to estimate traffic-related emission factors. Carbon monoxide (CO) and black carbon (BC) were measured upwind and downwind at various distance from two major freeways I-405 and I-710 in Southern California, one mostly occupied by gasoline-powered cars and the other dominantly by heavy-duty diesel trucks,. A line-source Gaussian dispersion model was applied to fit these data. We adopted the concept of mixing zone, a region directly over the freeway as a zone of uniformly mixed emissions and turbulence, in CALINE4 model and a power function to model vertical dispersion parameter. The method of least squares of errors between model predictions and observations was used to simultaneously estimate the coefficients of the vertical dispersion as well as the emission factors. We found the emission factors of CO were 35.5 and 10.6 g/km vehicle for I-405 (diesel < 5%) and I-710 (diesel > 25%), respectively, consistent with the fact that gasoline-powered vehicles emitted more CO than diesel-powered trucks. The emission factors of BC were estimated to be 0.075 and 0.048 g/km vehicle for I-710 and I-405, respectively, again, consistent with the fact that diesel-powered trucks emitted more soot particles than gasoline-powered vehicles. These values are comparable with those values estimated by published other studies using different methodology. It is concluded that the horizontal profile of airborne pollutions near freeways and the line-source Gaussian dispersion model are successfully used to estimate emission factors for aggregate vehicular traffic in major freeways. This methodology is demonstrated to be an excellent tool to re-examine the uncertainties of emission factors estimated by traditional methods.