Effects of green infrastructure on the dispersion of PM2.5 and human exposure on urban roads

Urban green infrastructure (GI) has been widely demonstrated to effectively improve air quality in the built environment. However, due to the lack of comparative studies of the effects of different GI forms on PM2.5 dispersion, optimal GI designs suitable for different urban road types currently remain unclear. In this study, we adopted different roadside GI types in Hangzhou city as case studies and used the ENVI-met model to compare the effects of the different GI forms on PM2.5 dispersion and human exposure to PM2.5. The results indicated that 1) In open roads, the concave-shaped GI type could effectively reduce PM2.5 aggregation and human exposure on motorways, and the all-tree GI type performed the best in terms of sidewalk PM2.5 purification. 2) In street canyons, green roof and green screen were highly conducive to PM2.5 concentration reduction under commuter exposure compared with traditional green solutions. 3) There were trade-offs in the GI-PM2.5 interaction. GI types which can reduce pedestrian exposure tend to increase exposure in motorways. The same GI type deployed along the two different road types could yield completely opposite dispersion effects. Novel GI types had better environmental performance and relatively high economic cost. All decision-making should be based on the trade-offs between the advantages and disadvantages of GI. Our study also highlights the importance of comprehensive consideration of GI and road types and local wind conditions in future urban road planning and GI applications.