The results of numerical simulation of pollutant transport over a surface with large-scale roughness are presented. The problem was solved with the use of the Reynolds-averaged Navier-Stokes equations with the Boussinesq closure. Turbulent parameters were determined on the basis of the modified k- -model of turbulence and the Launder-Spalding's method of wall functions. The problem is solved numerically by the finite-volume method. The influence of urban vegetation on air pollution and the effect of traffic-induced turbulence on the pathways of air pollutants in a street canyon were investigated.