Impact of an urban street canyon's greening configurations on its traffic-related particulate matter

Roadside greening is gaining attention because it can mitigate street air pollution. To investigate the effects of greening configurations along bicycle lanes and sidewalks on exposure to traffic-related particulate matter (PM), we monitored in situ concentrations (PM10, PM2.5, PM1) in a shallow street canyon in Xi'an, China, and numerically simulated the people's exposure at common street greening scenarios with a microclimate model (ENVI-met). Field measurements showed that greenery in late winter had a higher PM removal effect than in late spring, but late spring's greenery had an overall negative effect on PM concentrations. Notably, greening is more effective at removing coarse particles (PM10) than fine particles (PM2.5, PM1). Simulation results revealed that when trees are planted at different locations on the roadway, the ranking of their PM-removal effect is sidewalk > bicycle lane > traffic lane; however, for planted shrubs, the ranking changed to bicycle lane > traffic lane > sidewalk. PM removal from simultaneous planting of greenery (shrubs, trees, tree-shrub mixes) on sidewalks and bicycle paths is not satisfactory. The greening configuration of “planting shrubs on bicycle lanes and trees on sidewalks” performed best at removing PM. These findings can provide a practical reference for optimizing street greening designs.