The influence of street tree parameters on PM_2.5 diffusion in street canyons? Evidence from a parametric study

Traffic-related PM_2.5 worsens street air quality and affects pedestrian health, and the benefits of street trees in reducing air pollution within street canyons are still controversial. To clarify the influence of street tree parameters on PM_2.5 dispersion, tree height (TH), tree spacing (TS) and leaf area density (LAD) were selected from three different categories to represent the common characteristics of street tree species. Field measurements were conducted to validate ENVI-met model. We carried out 64 simulation scenarios with different parameters (TH, TS and LAD) and a treeless scenario in idealized street canyons (aspect ratio = 1:1) under oblique wind direction. The results showed that the 3D surface plots can effectively display the spatial distribution of PM_2.5 within street canyons. PM_2.5 concentration increased after planting trees; the leeward side showed higher concentrations than windward side, and the downstream showed higher concentration than upstream area. TH and LAD significantly influenced PM_2.5 concentration, while TS had less influence, and their effects differed in spatial position. PM_2.5 reduction efficiency (RE) varied with different street tree parameters on the windward side and leeward sides. Sky view factor (SVF) was significantly negatively correlated with PM_2.5 concentration, while green coverage ratio (GCR) and green plot ratio (GPR) were inversely correlated. TH = 6 m, TS = 6 m and LAD = 1.0 m² m^-3 could be a suitable parameter combination for street tree planting or pruning. These results could provide practical suggestions for urban planning and landscape design to improve street air quality and promote sustainable urban development.