A review on particulate matter removal capacity by urban forests at different scales

Urban forests play a vital role in terms of environmental quality related to particulate matter (PM) and studies on this area are increasing. This review discusses how urban forests influence atmospheric PM at three scales, including the single tree scale, stand scale and regional scale. Additionally, the PM analysis was divided into vertical and lateral directions at the stand scale. As individuals, trees capture particles mainly by the leaves, the extent of which is determined by the characteristics in foliar structure such as hair, trichomes, wax, stoma, shape and others. At the stand level, the effects of urban forest vegetation showed differences in the two directions (vertical and lateral) and the predicted values of deposition velocity in the vertical direction differed with various models, which resulted from the input of numerous influencing factors (e.g., meteorological factors, plant species, building design and others). At the regional scale, the removal capacity of atmospheric particles by urban forests was dependent on the pollution level, vegetation coverage area, leaf area index and underlying surface type. The PM removal capacity of different vegetation types was usually in the following order: coniferous forest > evergreen forest > deciduous forest. The analyses at different scales indicated that the roles of urban forests on PM could not be simply deemed positive or negative, because they need to be considered by combining various factors (e.g., plant species, meteorological condition and city planning and design) at different scales. Therefore, multi-scale analyses on the effects of atmospheric particles could help to better understand the roles of urban forests as a complex system and provide the foundation for future studies.