The right tree for the right street canyons: An approach of tree species selection for mitigating air pollution

Abstract

Trees planted in street canyons affect the dispersion and deposition of air pollutants primarily through their aerodynamic and deposition effects to change the air quality. However, the potential of trees to improve air quality is uncertain as it depends on a combination of tree forms (species), street canyon geometry and local meteorological conditions. We developed a method to select tree forms with appropriate morphology in specific street canyons based on Computational Fluid Dynamics (CFD) simulation, aiming to promote the dispersion and concentration reduction of pollutants at pedestrian height. In this method, we combined the leaf area density (LAD), crown diameter (CD), tree height (H), and trunk height (TH) as representative metrics for various tree forms. The effects of street canyon aspect ratio, wind direction, and wind speed on the potential of trees to regulate pollutant concentrations were comprehensively considered. In a case study in Wuhan, China, 15 tree forms and corresponding 36 tree species adapted to different street canyons were identified using the validated ENVI-met model. Our research offers practical recommendations and effective tools for professionals and policymakers to select tree species that can enhance the air quality in street canyons.