Effects of roadside green infrastructure on particle exposure: A focus on cyclists and pedestrians on pathways between urban roads and vegetative barriers

Abstract Roadside green infrastructure (GI) has attracted worldwide attention for its potentials to alleviate local air pollution. However, previous studies have not clearly characterized the effects of roadside GI on personal exposure levels to vehicular emissions, particularly for cyclists and pedestrians on the pathways between urban roads and vegetative barriers. In this study, field experiments were implemented to measure and compare the concentration levels of particulate matter (PM) and black carbon (BC) on pathways (e.g., bike lanes and sidewalks) and near residential buildings with and without roadside GI. Results show that the presence of GI significantly elevated the particle concentrations over bike lanes and sidewalks. Compared with the GI-free case, an increase of up to 20% and 28% was separately observed for BC and PM on the pathways before GI. This is strongly associated with the impact of GI on local wind fields that impedes the dispersion of traffic-emitted particles on the roadways. However, roadside GI brought a significant reduction of particle concentrations near residential buildings, separately decreasing by about 8% for BC and 6% for PM. Then Computational Fluid Dynamics software was adopted to simulate the diffusion process of traffic-emitted particles in the typical urban scenarios of roadways, pathways, GI, and residential buildings, and further analyze the particle distribution patterns under six roadside GI configurations. Simulation results suggest that tall vegetative barriers can lead to the accumulation of particles on the pathways and increase cyclists' and pedestrians’ exposure to local particulate pollution. The GI case of “tree canopy only” can be a potentially viable choice to reduce particle concentrations on the pathways due to the fact that its better ventilation conditions are more favorable for particle diffusion than other GI configurations. However, compared with the complex GI configurations, the scenario with no roadside GI exhibits more positive effects on decreasing particulate exposure on the pathways. These findings could provide practical insights into roadside GI design and important implications in near-road air quality improvements.