This paper reports a computational fluid dynamics simulation of airflow and fine particle (PM_2.5) dispersion in the street canyon in Hong Kong, using large eddy simulation. An aspect ratio (AR) of 2.7 and a Reynolds number of 5 × 10⁶ with a one main vortex, were used. This study focused mainly on the vehicle-induced momentum source and PM_2.5 concentrations were measured at 10 altitudes near the leeward wall in the street canyon, to provide high resolution measurements for model validation. The simulated PM_2.5 concentrations agreed well with the measurements, (R = 0.85). The concentration was higher at the lower part near the leeward wall than the upper part. Higher concentration was found near the roof level. A near-uniform vertical dispersion of PM _2.5 near the windward wall was demonstrated; and the average concentrations were lower than found near the leeward wall. The intermittent escape of the PM_2.5 above the canyon at the roof level occurred mainly at the centre and near windward wall areas. The results demonstrated that a reduction in the AR can be conducive to pollutant dispersion in street canyon planning. The findings of this research would inform building designers to formulate effective strategies such as positioning of ventilation air intake, for the control of ingress of PM_2.5 into building environments.