Pollutant dispersion near roadways — Experiments and modeling

Since the 1970's, many field and wind tunnel experiments have been conducted to study pollutant dispersion from roadways. For an at-grade situation, field experiments have revealed that mechanical mixing dominates effects due to ambient stability, that plume rise is important under very low crossroad winds, that regions of large shear enhance the mixing volume, and that the wake region grows rather slowly in the vertical direction. The models that have been developed based on recent experimental results are briefly described. For the street canyon situation, both field and wind tunnel experiments have revealed that ambient stability does not play an important role, that corner vortices near an intersection cause an increase in pollutant concentrations near the bottom corners of the leeside buildings, that in the midsection of a street block the vortex circulation causes high pollutant concentrations to be advected toward and up the leeside wall. No general street canyon models are available except an empirical model for the midsection of the street block. The complicated flow field must first be ascertained before a reliable concentration model can be developed.