Quantifying the impact of urban trees on passive pollutant dispersion using a coupled large-eddy simulation–Lagrangian stochastic model

Abstract

The use of shade trees is among favorable urban planning strategies to improve outdoor thermal comfort and promote energy efficiency. Meanwhile, the presence of trees alters flow patterns and turbulent transport and influences the dispersion of air pollutants. In this study, we investigated the effects of urban trees on air pollutant dispersion in urban canopy layers using a coupled large-eddy simulation–Lagrangian stochastic modeling framework. The dispersions of two-way traffic emissions were simulated using a set of twenty four scenarios with varying canyon and tree geometries. Results show that tall trees lead to the strongest modification of the canyon flow and pollutant concentration, except in narrow canyons. Trees can exacerbate canyon pollution level in certain built environment owing to the presence of isolated canyon vortices. Trees with high leaf area density are beneficial to reducing concentration in broad street canyons, while trapping of pollutants is manifest in narrow canyons. The participatory role of trees, in conjunction with the effect of urban morphology, is crucial and needs to be meticulously evaluated in urban planning for promoting environmental quality.