Numerical Investigation of the Impact of Urban Trees on O3–NOx–VOCs Chemistry and Pollutant Dispersion in a Typical Street Canyon

Urban greening is one of major factors that influences flow, turbulence and air quality in street canyons. This paper aims to investigate the impact of urban trees on O3-NOx-VOCs (ozone -nitrogen oxides - volatile organic compounds) chemistry and pollutant dispersion in street canyons by Computational Fluid Dynamics (CFD). The Atmospheric Photolysis calculation framework (i.e., APFoam), which includes complex O3-NOx-VOCs chemistry into CFD, is employed to carry out the numerical simulations. The validation of the APFoam modelling results has been completed prior to further modelling works, including turbulent airflow, pollutant dispersion, and photochemical reactions. The influence of aerodynamic effects, biogenic VOC (BVOCs) emission and dry deposition of urban trees on air quality in a typical two-dimensional (2D) street canyon with aspect ratio H/W = 1 (where H is the building height and W the street width) are thoroughly examined. Moreover, the source contribution on ozone (O3) creation and the human health risk are also analyzed. Results show that, inside the street canyon, aerodynamic effects of trees have a greater impact on photochemical pollutant concentrations than BVOCs emission and dry deposition, the latter showing the smallest impact. In particular, the aerodynamic effects cause a wind reduction by 35%-45% at pedestrian level and subsequently an increase of nitrogen monoxide (NO) and nitrogen dioxide (NO2) concentrations by 95% and 66% near the ground, respectively, and an O3 concentration decrease by 35%. Further, the BVOCs emitted from trees, the vehicle VOCs and the background VOCs contribute 15%, 67%, and 9% to O3 production, respectively. These findings further suggest that the APFoam is an effective and promising tool which allows us to investigate the influencing mechanisms of trees on photochemical pollutant dispersion and urban air quality for the purpose of developing sustainable urban policy.