Numerical Simulation-Based Investigation of the Effects of Street Trees on the Microclimate and Air Quality of High-Density Urban Street Valleys Under Different Street Valley Patterns

Greening is an effective measure to improve microclimate and air quality in high-density urban street valleys. In this paper, different valley patterns and greening are coupled to comprehensively analyze the effects of both on the thermal environment of buildings and pollutant dispersion. The results show that street trees have a dual effect on the valley microclimate and thermal comfort, and the effect of street trees on the valley microclimate gradually decreases from the ground to the roof, and the planting of street trees in the east–west oriented valley with the height-to-width ratio of 1 and the length-to-width ratio of 4 has the best effect on improving thermal comfort. However, street trees significantly hindered the diffusion of particulate matter, especially in the east–west oriented street valley, and the effect of street trees on particulate matter concentration was greater at night than during the day. In the north–south valley, with a height-to-width ratio of 1 and a length-to-width ratio of 2, street trees had the least effect on the diffusion of particulate matter in the outdoor pedestrian layer. Based on the study, the most promising options for improving the urban microenvironment could be effectively selected.