Mitigating PM_2.5 exposure with vegetation barrier and building designs in urban open-road environments based on numerical simulations

Vegetation barriers (VB) are commonly regarded as an effective strategy in urban planning for mitigating traffic-induced airborne pollutants and associated exposure of road-users and nearby urban dwellers. Locally verified evidence is needed to better understand the nexus between VB designs and PM_2.5 dispersion and provide practical insights for the creation of a healthy urban environment. This study constructs a 3D numerical model using in-situ measurements via field campaign, and then simulates PM_2.5 dispersion in an open-road environment in Shanghai, China, characterized by varying VB designs (short vs. tall bushes in different location and coverage) and building structure (present vs. absent). The simulation results reveal the effectiveness of VB in PM_2.5 mitigation in the sidewalk canyon, regardless the presence of building structure. Bushes at the windward side, regardless their size, tend to be more effective than arbors. The building structure flanked the sidewalk also contribute to PM_2.5 mitigation in the sidewalk canyon. Various scenarios using short bushes consistently reduce PM2.5 concentrations along building façade, which might be attributed to the formation of vortex circulations in the sidewalk canyon. In contrast, a greater tall-bush coverage (2/3 and above, regardless of their location) tends to exacerbate PM_2.5 pollution along building façade. Thus, constructing VB with a large proportion of short bushes could be a practical solution to minimizing the exposure of both sidewalk users and building inhabitants to traffic-induced PM_2.5 in open-road environments.