An improved model to simulate pollutant dispersion from roadways

An improved dispersion model, UCD 2001, designed to estimate pollutant concentrations near roadways was developed and its performance evaluated. The UCD model internally represents a highway link as a three-dimensional array of point sources which simulates a roadway mixing zone that extends 2.5m above a highway link. Dispersion from each point source is estimated with the Huang dispersion equation. The Huang equation is a simplified solution to the semi-empirical advection diffusion equation (SEADE) whose derivation requires that the vertical profiles of wind speed and eddy diffusivity in the boundary layer be approximated by power law functions.; Several independent studies concluded that the atmosphere is well mixed and neutrally stratified immediately downwind of a roadway with significant vehicular activity. The UCD 2001 model was calibrated with one-half of the General Motors (GM) SF₆ tracer study database. This calibration resulted in a selection of eddy diffusivity parameters that did not vary with ambient meteorology which suggests pollutant mixing immediately downwind of an active roadway depends more on vehicle activity than upon ambient meteorology. Consistent with these findings, UCD 2001 assumes a neutral downwind profile and does not compensate for ambient stability effects downwind of an active highway.; UCD 2001 model performance was evaluated and compared to the CALINE3 and CALINE4 dispersion models using the GM database. UCD 2001 adequately simulates near parallel, low wind speed (less than 0.5 m/s) meteorological conditions, whereas the CALINE models significantly over predict most receptor concentrations for these conditions. The UCD 2001 model results in approximately 80 to 90 percent reduction in least squared residual error when compared to the CALINE3 and CALINE4 models. In addition, the UCD 2001 model shows better agreement in simulating the top forty observed concentrations than either CALINE model. Lastly, the UCD 2001 model requires less user input and modeler expertise than most roadway dispersion models and should result in more consistent and robust pollutant field estimations.