Toward ${\mathrm{PM}}_{2.5}$ Distribution Patterns Inside Symmetric and Asymmetric Street Canyons: Experimental Study

Distribution patterns of particulate matter 2.5 (${\mathrm{PM}}_{2.5}$) inside urban street canyons varies according to the street canyon enclosure ratio (SCER) and traffic flow states, and therefore they affect people in different positions of street canyons to different extents. This paper studied ${\mathrm{PM}}_{2.5}$ distribution patterns in both symmetric and asymmetric street canyons by using the computational fluid dynamics technique. Simulation experiments were conducted in a series of isolated street canyons in Beijing with five SCERs and three types of traffic flow states. The results of the simulation experiments were verified with on-site data. The results are threefold: (1) ${\mathrm{PM}}_{2.5}$ emitted by the vehicles gathers around the corners of leeward buildings of street canyons, and this phenomenon tends to be aggravated by heavy traffic flow states in most scenarios; (2) of five SCERs, it was the most difficult for ${\mathrm{PM}}_{2.5}$ to spread out of the canyons with SCER = 2:1:2, whereas it was the easiest for ${\mathrm{PM}}_{2.5}$ to spread out of the canyons with SCER = 2:2:1; and (3) pedestrians and residents on the leeward side of asymmetric street canyons are exposed to the highest ${\mathrm{PM}}_{2.5}$ concentration. The findings of this research will assist in designing street canyons, controlling the traffic flow, and developing measures to reduce the harm of ${\mathrm{PM}}_{2.5}$ to the public.