Effects of tree characteristics and arcade design on the traffic pollutant dispersion inside the asymmetric street canyon

Abstract

Arcade design and tree planting both provide shading for pedestrians and improve the microenvironment of street canyons in urban areas, but their combined effect on air quality remains underexplored. To address this gap, wind tunnel experiments and numerical simulations were conducted in this study to analyze airflow and pollutant dispersion in a step-up street canyon, focusing on variables such as tree canopy shapes, pressure loss coefficients, and planting gaps. The results demonstrate that selecting tree with low-pressure loss coefficients and increasing canopy spacing significantly improve ventilation efficiency. Notably, triangular canopies outperform rectangular canopies in promoting airflow and pollutant dispersion, a trend observed in both step-up and step-down canyons with similar building height differentials. For step-up canyons, air exchange rates (ACH) are lowest in configurations without arcades, with leeward-side arcades outperforming windward-side arcades. Leeward-side arcades reduce pedestrian-level pollutant concentrations by 6.1% compared to non-arcade canyons. This study concludes that combining leeward-side arcades with triangular canopies is the most effective strategy for enhancing ventilation and reducing pollutant concentrations in step-up street canyons. This approach is particularly beneficial in hot, sun-exposed regions, providing both shading and improved air quality. The findings offer actionable recommendations for optimizing street canyon environments through integrated design strategies.