Are Green Walls Better Options than Green Roofs for Mitigating PM 10 Pollution? CFD Simulations in Urban Street Canyons

To examine the effect of green roofs (GRs) and green walls (GWs) on coarse particle (PM 10 ) dispersion in urban street canyons, a computational fluid dynamics (CFD) simulation was conducted with a Reynolds-averaged Navier-Stokes (RANS) model and a revised generalized drift flux model. Simulations were performed with different aspect ratios (H/W = 0.5, 1.0, and 2.0), greenery coverage areas (S = 300, 600, and 900 m 2 ), and leaf area densities (LADs = 1.0, 3.5, 6.0 m 2 /m 3 ). Results indicate that: (1) GRs and GWs all had the reduction ability of PM 10 at the pedestrian level; (2) Averaged concentrations of PM 10 in GWs and GRs varied little as LAD changed in H/W = 0.5 and 1.0. When H/W = 2.0, the aerodynamic effects of GRs increased since airflow was enhanced within street canyons, resulting in the increasing concentrations in GRs compared with non-greening scenarios; (3) Given equal greenery coverage area and aspect ratio, GWs are more effective in reducing street-canyon PM 10 , and the averaged concentrations declined with increasing LADs and greenery coverage areas, especially the H/W; (4) At the pedestrian level, the reduction ratio of GRs is greater than that of GWs with the maximum value of 17.1% for H/W = 0.5. However, where H/W = 1.0 and 2.0, the concentrations within GWs are lower than GRs, with maximum reduction ratios of 29.3% and 43.8%, respectively.