Numerical investigation on the coupled effects of building-tree arrangements on fine particulate matter (PM2.5) dispersion in housing blocks

A numerical simulation using Reynolds-Averaged Navier-Stokes (RANS) model and revised generalized drift flux model was conducted to investigate the coupled effects of different building-tree arrangements on outdoor PM2.5 dispersion in housing blocks. Results showed that: 1) trees reduced wind speed according to canopy height and disrupted the airflow field within close vicinity to the canopy, which led to an apparent decrease in PM2.5 concentrations within the height of tree crown; 2) horizontal vortex circulation along the building facades produced lower pedestrian-level concentrations behind buildings; 3) buildings that experienced wind deflection on one facade and airflow separation on the other one could get more airflows and benefit from lower PM2.5 along vertical building facades; 4) aerodynamic effects of trees did not conquer over the deposition effects in all investigated cases due to the different tree-building arrangements; and 5) the configuration that had dispersed trees and two parallel rows of buildings, which were perpendicular to the prevailing wind, could form more vortex circulations as a result of decreasing concentrations. These results provided an evaluation index for the architectural and tree planting design process and for feedback of sustainable projects to maximize particle removal capability.