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A three-dimensional computational fluid dynamics(CFD) model with the renormalization group(RNG) k-εtur-bulence model is used to examine the effects of difference in building height on ow and pollutant dispersion inasymmetric street canyons. Three numerical experiments with different street canyons formed by two isolated build-ings are performed. In the experiment with equal building height, a portal vortex is formed in the street canyon anda typical recirculation zone is formed behind the downwind building. In the experiment with the downwind build-ing being higher than the upwind building, the ambient flow comes into the street canyon at the front of the down-wind building and incoming flow diverges strongly in the street canyon. Hence, pollutants released therein arestrongly dispersed through the lateral sides of the street canyon. In the experiment with the upwind building beinghigher than the downwind building, a large recirculation zone is formed behind the upwind building, which is dis-turbed by the downwind building. Pollutants are weakly dispersed from the street canyon and the residue concen-tration ratio is largest among the three experiments. This study shows that the difference in upwind and downwindbuilding height signicantly inuences flow and pollutant dispersion in and around the street canyon.