Numerical investigation on the effect of avenue trees on PM2.5 dispersion in urban street canyons

Abstract

The Reynolds-averagedNavier-Stokes (RANS)model and revised generalized drift fluxmodel were used to investigate the characteristics of airflow fields and PM2.5 dispersion in street canyons with a variety setting on tree crown morphologies (i.e., conical, spherical, and cylindrical), leaf area densities (LADs = 0.5, 1.5, and 2.5 m2/m3), and street canyon aspect ratios (H/W= 0.5, 1.0, and 2.0). Results were as follows: (1) airflow fields were reversed in the presence of trees and enhanced with higher LAD; (2) air velocity decreased negligibly when LAD increased from 1.5 to 2.5, but significantly when LAD increased from0.5 to 1.5; (3) tree crownmorphologies, building aspect ratios, and LADswere interrelated. The comparison of PM2.5 showed that themost critical situations inH/W= 0.5, 1.0, and 2.0 corresponded to LAD = 0.5 with a conical canopy; (4) the H/W = 1.0 and LAD = 1.5 scenario was identified as the most efficient combination for PM2.5 capture; (5) the maximum PM2.5 reduction ratio was ordered from low to high in the sequence of conical, spherical, and cylindrical canopies. At predestinated LADs and aspect ratio, Populus tomentosa with cylindrical crown morphology exhibited the best efficiency on PM2.5 capture with a reduction ratio of 75% to 85% at pedestrian height.