Investigation of O3–NOx–VOCs chemistry and pollutant dispersion in street canyons with various aspect ratios by CFD simulations

Abstract

Based on the OpenFOAM environment, a new CFD code named as APFoam coupling turbulence with the complex photochemical mechanism of NOx-O3-VOCs reactions has been developed. The standard k-ε model is selected for turbulent airflow simulation and the SAPRC07 is for complex NOx-O3-VOCs photochemical reactions. Two-dimensional(2-D) full-scale street canyons with two aspect ratios(AR = H/W = 1 and 5, W = 24 m) are studied. By considering both physical effects and chemical reactions, the influences of three key factors on the photochemical pollutant dispersion and human health risk analysis are investigated, including the background wind speeds(Uref = 1.5 m/s, 3 m/s), pollutant concentrations from ambient air(O3-NOx-VOCs) and the emission(NOx-VOCs) source settings at the pedestrian-level. Besides, the effective measures to control O3 concentration is discussed.This study mainly concludes below. 1) The variations of NOx concentration increases about 20 times and O3 concentration decreases about 30% with the increase of AR. The chemical effect on photochemical pollutants is more important for the deep canyon(AR = 5) than that for the typical canyon(AR = 1). 2) The indoor and pedestrian-level exposure risk of NO2 is much larger than that of O3. The deep street canyon(AR = H/W = 5) has a higher exposure risk of NO2 than the typical canyon(AR = H/W = 1). 3) The NOx concentration increases about twice and O3 concentration decreases 6.5–13.5% with the decrease of Uref. 4) O3 concentration decreases after removing the background NOx and VOCs; it increases if there are no traffic emissions. 5) Simple traffic control measures can effectively reduce NOx but the reduction of O3 still requires additional VOCs control measures in a street canyons.