Three-dimensional spatial inhomogeneity of traffic-generated urban PM2.5 in street canyons

Abstract

Traffic emissions within densely populated communities would result in substantial variations in PM2.5 exposures of citizens. The objectives of this study were to (1) validate the Graz Lagrangian model (GRAL) at a 5-m resolution horizontally and vertically and (2) investigate the three-dimensional spatial inhomogeneity of traffic-induced PM2.5 within the street canyons of an urban community in Northern Taiwan. By comparing with observations, it was shown that the GRAL performed well in both the horizontal and vertical (up to the 5th floor) PM2.5 distributions under a wind speed of >0.2 m s−1. The simulation results revealed an inhomogeneous dispersion of PM2.5 on the main street and inner alley at street level was approximately eight times within the community. The street-level PM2.5 concentration doubled when the winds were perpendicular to the street canyon as opposed to when they were parallel. The PM2.5 tended to accumulate on the street near the upwind side of the tall buildings under perpendicular wind, at the center of streets on the lee side of tall buildings, and in the entrance of alleys when the wind was parallel to the street canyon. The PM2.5 concentration 26 m above the ground was approximately one-quarter that at street level under a wind speed of 0.5 m s−1, regardless of the wind's movement over streets or alleys. These results demonstrate a substantial spatial inhomogeneity of PM2.5 in urban street canyons, with the extent of the inhomogeneity depending heavily on the wind–building configurations.