Carbon monoxide distribution in street canyons under various geometry's, atmospheric stability, temperature gradient induced buoyant effect and traffic load were simulated. We intend to use the simulation results to evaluate pedestrian pollutant exposure and to provide external conditions for evaluating indoor air quality of the buildings beside urban street. In this study, we consider several factors those may affect the pollutant distribution in street canyons. The considered factors include geometry of the street canyons, atmospheric stability, traffic load and turbulent buoyant effect. A two- dimensional domain that includes suburban roughness and urban street canyons was considered as the simulation domain. The considered influential factors were imposed into the simulation through the associated boundary conditions. The simulated results show that serious pollutant accumulation occurred at where double vortices encountered and the situation only occurred at specific aspect ratios of the street canyons. Usually, the stagnation zone formed at where the encounter of two vortices that reduce the transport of pollutant and the pollutant tent to accumulate at the region. Buoyant effect to the vortices formation was found that the double vortices occurred at smaller aspect ratio of the canyons with buoyancy than those without buoyancy. The buoyant effect also change the pollutant transport pattern in the canyons. The pedestrian exposure to the street air pollutant under various traffic loads and atmospheric stability were evaluated. This study concluded that the patterns of pollutant distribution in different street canyons are mostly affected by their aspect ratios where the affect of buoyancy and atmospheric stability played less role. The pedestrian exposure to the street air pollution is considered to be serious under specific conditions when pollutant concentrations at 3m above ground level are very high. Those specific conditions were also found in this study.