Exploring Sustainable Street Tree Planting Patterns to Be Resistant against Fine Particles (PM2.5)

Recent health threats from fine particles of PM2.5 have been warned by various health organisations including the World Health Organisation (WHO) and other international governmental agencies. Due to the recognised threats of such particulate materials within urban areas, counter measures against PM2.5 have been largely explored; however, the methods in the context of planting types and structures have been neglected. Therefore, this study investigated and analysed the concentration levels of PM2.5 in roads, planting areas, and residential zones within urban areas. Moreover, the study attempted to identify any meaningful factors influencing the reduction of PM2.5 and their efficiencies. After surveying PM2.5 in winter and spring season, there were serious reductions of PM2.5 concentrations within the areas of pedestrian paths, planting, and residential areas compared to other urban areas. In particular, a significant low level of PM2.5 concentrations was shown in the residential areas located behind planting bands as green buffer. This research also found that three-dimensional volumes and quantity of planting rows play a critical role in reducing PM2.5. A negative correlation was shown between the fluctuated concentration rate of PM2.5 and quantity of planting rows—single row of trees showed fluctuated concentration rate of PM2.5, 84.77%, followed by double rows of trees 79.49%, and triple rows of trees 75.02%. Especially, trees need to be planted at certain distance to allow wind to diffuse fine particles rather than dense planting. Finally, planting shrubs also significantly reduces the concentration level of PM2.5—the fluctuated concentration rate of the single layer showed 88.79%, while the double layer and the multi-layer showed 81.16% and 68.93%, respectively—since it increases three-dimensional volume of urban plantings.