Daytime variations of PM2.5 concentration in three street canyons in Xi’an were measured during June 11–15, 2010. Weather conditions, including the ambient wind condition and the street canyon air temperature, were also recorded. These measurements were conducted on weak-wind summer days, when the mean ambient wind velocity was 1.26 m/s. The results show that, under weak-wind conditions, there were no good correlation between PM2.5 concentrations and vehicle flux inside the street canyons. The vehicle related PM2.5 concentrations accumulated in the daytime inside the street canyons. The PM2.5 concentrations at the pedestrian level were of near uniform horizontal distribution. These results indicate the absence of the typical primary air flow re-circulation inside the street canyons under weak-wind conditions, and that the dispersion and transportation of vehicle exhausts inside and/or outside the street canyons are influenced by vehicle induced air flow and thermal induced buoyancy.