New insights on precise regulation of pollutant distribution inside a street canyon by different vegetation planting patterns.

Mixed-vegetation planting patterns are commonly seen in urban areas for specific reasons like aesthetic, cooling, and particle deposition effects of the vegetation. However, they may have a negative impact on human health by worsening the air quality inside the street canyon due to the decreased air exchange rate. From the view of precise control of pollutant concentration in the sensitive areas of people's concern in the existed street canyons, thirty-four cases with different vegetation planting patterns and pressure loss coefficients (λ) are studied numerically to investigate the effects of vegetation on airflow and pollutant dispersion inside the canyon. The cases of treeless and 2 rows of tree planting patterns in wind-tunnel measurements were selected for the model validation. The results demonstrate that compared to the treeless case, the greenbelts can greatly change the airflow features and reduce the pollutant concentration at the leeward side, while the only-tree planting patterns have little impact on the flow and deteriorate dispersion within the street canyon. Moreover, rows of greenbelts planted under the corresponding trees can reduce the average pollutant concentrations on the leeward wall and the footpath of the street canyon by up to 22.6% and 33.2%, respectively. Besides, the pattern of 1 row of trees with 1 row of greenbelts planted in the street canyon center should be suggested as the optimal mixed vegetation configuration in this study. That is because compared to the treeless case the pollutant concentration on leeward wall, windward wall, leeward footpath, and windward footpath can be reduced by 14.2%, 10.0%, 24.6%, and 37%, respectively. It is helpful to the city planners to consider whether the disadvantages of planting vegetation inside the street canyon would overwhelm the advantages.