Seasonal effects of street trees on particulate matter concentration in an urban street canyon

Abstract The net influence of trees on local air quality in urban street canyons, negative or positive, has rarely been evaluated by field investigations. To quantify the influence of street trees on particulate matter (PM) and total suspended particle (TSP) concentration near ground level, a seasonal field investigation was carried out in a street canyon with an aspect ratio of 0.52 in Shenyang, China. The greening tree species in this street were Sophora japonica and Ulmus pumila. We monitored PM1, PM2.5, PM4, PM10, TSP concentration and the microclimatic conditions at locations with or without trees from before the leaf expansion season until the end of the defoliation season. Concentration of PM2.5 was higher at the tree-stand location, whereas concentration of PM10 and TSP were higher at the tree-free location during the leaf-on season. Street trees were beneficial to coarse particle removal during the leaf-off and leaf-expansion season, but PM pollution increased during defoliation. Wind direction was more uniform at the tree-stand location. Street trees weakened the impact of the wind on PM dispersion near ground. This indicates that street trees might pose a health threat to pedestrians because fine PM can enter the human respiratory system more easily than coarse PM.