Numerical Investigation of Urban Trees on O3–Nox–Vocs Chemistry and Pollutant Dispersion in Typical Street Canyons

Abstract

Urban tree planting is one of major factors that influences the flow turbulence and air quality inside street canyons. This paper aims to investigate the influence of urban trees on O3–NOx–VOCs chemistry and pollutant dispersion in street canyons by computational fluid dynamic (CFD). The Atmospheric Photolysis calculation framework (i.e., APFoam) coupling complex O3–NOx–VOCs chemistry into CFD. is employed here to carry out all numerical simulations. The validations between the APFoam modelling results and experimental data are conducted prior to further modelling works, including turbulent airflow field, pollutant dispersion, and photochemical reaction. The influences of aerodynamic or tree dragging processes, BVOCs emission and dry deposition of urban trees on air quality in a typical 2D street canyon(H/W=1) are thoroughly examined. Moreover, the source contribution of O3 creation and the human health risk are also analyzed. Results show that the tree aerodynamic has greater impact on photochemical pollutant concentrations inside the street canyon than the BVOCs emission and dry deposition of trees. This is followed by the BVOCs emission effect, and the dry deposition of trees has the least impact on photochemical pollutants. In particular, the tree aerodynamic effect causes wind reduction by 35%-45% at pedestrian level and subsequently increases the NO and NO2 concentration by 95% and 66% near ground region, whereas decrease the O3 concentration by 35%. Further, for the O3 production, the BVOCs emitted from trees, the vehicle emitted VOCs and the background VOCs contributes 15%, 67%, and 9% based on source analysis, respectively. These findings help us better understanding the influencing mechanisms of tree planting on photochemical pollutant dispersion and urban air quality, moreover APFoam is verified as an effective and promising tool to investigate  more complicated photochemical pollutant dispersion with urban tree plantings for the purpose of making sustainable urban policy

Keywords: NOx-O3-VOCs chemistry, pollutant dispersion, Urban tree planting, Two-dimensional (2-D) street canyon, Computational fluid dynamics simulation (CFD), APFoam