Effects of roadside trees three-dimensional morphology characteristics on traffic-related PM2.5 distribution in hot-humid urban blocks

Urban plants play a significant role in improving the deterioration of air quality caused by extremely heavy traffic. This study aims at exploring the effects of three-dimensional morphology characteristics of road-trees on traffic-related PM_2.5 distribution through ENVI-met simulations. Firstly, this study verifies the performance of ENVI-met pollutants dispersion model by comparison with field measurements. Then a typical high-density block model is built and thirteen simulation groups under varied morphology parameters of height of tree, height of trunk and width of crown are designed in ENVI-met. The results indicate that planting 10 m-height roadside trees is effective for pollutant dilution in the vertical direction, and the trees with high trunk height have a strong capacity in clearing the traffic-related PM_2.5 at pedestrian level and promote the diffusion of PM_2.5, but excessive crown width would cause PM_2.5 escaping from the block hardly and increase pedestrian exposure levels. A practical range of plant morphological parameters is recommended and applied in landscape design that takes seasonal requirements into account.