Effects of Vertical Forests on Air Quality in Step-up Street Canyons

Abstract

Vertical forests have been constructed as environmentally friendly solutions for air quality and urban heat problems. With an expectation to provide insights into the value of vertical forests, we examined the impact of these structures on flows and distribution of fine particles in step-up street canyons using a computational fluid dynamics (CFD) model. The CFD model was rigorously validated against wind-tunnel experiments and accurately simulated particle deposition efficiency by tree leaves under varying wind speed and leaf area density (LAD) conditions, as well as the flow structures in step-up street canyons. We analyzed the characteristics of flows and fine particle distributions in step-up street canyons for different LADs and tree deposition velocities. Despite a substantial reduction in fine particles through dry deposition, the fine particles emitted from two line-type sources within the street canyon increased by 20-25% in the canyon's lower layer due to a 14-20% reduction in wind speed. This study sheds light on the complex flow structures and pollutant distributions surrounding vertical forests and highlights the need for a comprehensive evaluation and optimal design of these structures.