Effects of urban tree planting on thermal comfort and air quality in the street canyon in a subtropical climate

Planting trees is considered to relieve the thermal load. However, trees may "pollute" the air quality. Tree net effects should receive more attention, but only a few studies have simultaneously addressed thermal comfort and air quality. By computational fluid dynamics (CFD) simulations, we created an evaluation model of trees and investigated the overall effects of tree planting inside street canyons. We considered the following four parameters by comparing seven common tree species in Hong Kong (subtropical climate): three tree morphological indicators (leaf area index (LAI), tree height (Htree), and crown spread (Scrown)) and tree planting density (Ptree). The results demonstrated that under a high ambient wind speed, higher trees with a lower near-ground leaf area density are better options, which more greatly reduces the physiological equivalent temperature (PET) by up to 1.1 K but causes the least pollutant accumulation. Bigger-crown trees on the windward side are advocated, while smaller ones are suitable for the leeward side. In such a way, there was at least a 0.7 K decrease in PET on the tree-planted side while maintaining better air quality. Thermal comfort can be improved by increasing LAI or Ptree, but a higher LAI or Ptree causes a greater accumulation of pollutants. Increasing Ptree can cause a 1.1 K reduction in PET, while the effect of changing LAI is relatively limited. When the length of a street is reduced, varying tree factors have a comparable effect. As canyon depth increases, the effect of trees is limited.