Impacts of viaduct and geometry configurations on the distribution of traffic-related particulate matter in urban street canyon

Viaduct is a ubiquitous transportation infrastructure in the congested megacities worldwide to improve the accessibil-ity and capacity of urban transportation network. However, there is a lack of understanding of the impacts of the in-terplay between viaduct-ground emissions and viaduct-canyon configurations on the particle distribution in urban street canyon. To fill the research gap, we conducted vertical measurements of particle number concentrations (PNCs) at different heights of street canyon along a viaduct to reveal effect of viaduct on the vertical distribution of PNCs in street canyon. Observation results indicated that the vertical profiles of PNCs exhibited bimodal distribution patterns, which were more significant for coarse particles than fine particles. The one peak appeared at ground level and the other at the viaduct height, indicating the impacts of double emission sources (i.e., the emissions on the ground and viaduct) and the hindrance of viaduct to particle diffusion. We further modelled the role of viaduct in street canyon through Computational Fluid Dynamics (CFD) simulations to reveal the vertical distribution of particles under different viaduct-canyon configurations and discern the contributions of viaduct and ground emissions to the particle distribution. Simulation results showed that viaduct changed airflow field and turbulence structure and elevated particle concentrations in street canyon while the optimized viaduct-canyon configurations including higher viaduct height (12 > 10 > 8 m), smaller aspect ratio (0.5 > 0.67 > 1), and shorter centerline distance (0 > 1 > 2 m) between canyon and viaduct could bring better dispersion conditions and lower particle concentrations. Additionally, ground emissions contributed more to the vertical distribution of particles on the leeward side of street canyon than viaduct emissions while the windward side displayed the opposite characteristics to the leeward side. These findings revealed the general patterns of particle diffusion in viaduct-canyon configurations and provided implications into viaduct design and traffic management to alleviate local particulate pollution.

Automated summary

This study investigated the impact of viaduct-ground emissions and viaduct-canyon configurations on particle distribution in urban street canyons. The results revealed that particle number concentrations exhibited bimodal distribution patterns, with one peak at ground level and another at the viaduct height. The study also found that optimized viaduct-canyon configurations and ground emissions played significant roles in particle distribution in street canyons. These findings have implications for viaduct design and traffic management to reduce particulate pollution.