Abstract
Air pollutions in big cities are still serious issues in all countries. The nitrogen oxides and diesel smokes from automobiles especially diesel engine cars are main concern in urban street canyons, where tall buildings along the street block wind and ventilation is very poor. Moreover, in some urban street in Japan, toll roads are running over them and ventilation mechanism may be worsened more. In this study, air flow and diffusion of pollutant in street canyons were simulated numerically, and the effects of the upper road on pollutant diffusions were examined. The numerical simulations showed that a stable clear vortex is formed in the street canyon when the street width and building height were almost equal. Where as when the building height is higher than the street width the canyon vortex became weak and dim. The effects of elevated road on gas diffusion from ground levels street was estimated to be weak.
