The influence of a viaduct on PM dispersion in a typical street: Field experiment and numerical simulations

To investigate the effects of a viaduct on particle diffusion below the viaduct, field measurements for particulate matter (PM) concentration along roadsides with and without viaducts were conducted in this study. Air flow and particle (PM2.5) distribution around a viaduct were simulated by Computational Fluid Dynamic (CFD) equipped with the Discrete Phase Model in a Lagrange coordinate system. Experimental results of this case study indicate that the average mass concentrations of PM10 (156.7 mu g m(-3)), PM2.5 (77.3 mu g m(-3)) and PM1 (56.5 mu g m(-3)) on an elevated highway exceeded those on the ground-level expressway by 15%, 10% and 12%, respectively. Simulation found that two main vortices as well as two or three secondary vortices are generated in the canyon when both a viaduct and a barrier are added, while there was only one main vortex in the street canyon without a viaduct. A viaduct causes the particulate matter to be distributed unevenly, and the concentration on the leeward side near the ground is the highest. The vortex center is elevated as the value of Delta T (ground-level air temperature) increases, but the concentrations are basically maintained. We concluded that a viaduct increases the concentration of particulate matter in the street canyons and greatly affects the airflow field. Both Delta T and a sound barrier on a viaduct reduce the PM concentrations, but the effects are limited.