he Reynolds-Averaged Navier-Stokes(RANS)model and revised generalized drift flux model were utilized to investigate the characteristics of airflow fields and PM2.5 dispersion in street canyons with different 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 showed that: 1)airflow fields were reversed in the presence of trees and enhanced with increased LAD; 2)air velocity did not significantly decrease when LAD increased from l.5 t0 2.5,but significantly decreased when LAD increased from 0.5 t0 1.5; 3)tree crown morphologies,building aspect ratios and LADs interacted with each other.The comparison of PM2.5 showed that the most critical situations in H/W=0.5,1.0,and 2.0 correspond to LAD=0.5 with conical canopy; 4)the H/W=1.O and LAD=1.5 scenario was identified as the most appropriate combination for PM2.5 capture; 5)the maximum PM2.5 reduction ratio was ranked from lowest to highest in the order of conical,spherical and cylindrical canopies.At predestinated LADs and aspect ratio,Populus tomentosa with cylindrical crown morphology was the most effective at PM2.5 capture with a reduction ratio of 75%t0 85%at pedestrian height.