Impact of various vegetation configurations on traffic fine particle pollutants in a street canyon for different wind regimes

Abstract

Vegetation establishment in urban areas is a potential solution to combat elevated particulate matter (PM) pollution, create cleaner environment for residents and enhance the sustainability of cities. However, vegetation effect at the points of interest in street-canyon on traffic pollutant from multiple interconnected factors (e.g., plant species, vegetation configurations, aerodynamic effect, deposition effect and complex wind regimes) is still not well studied. Therefore, taking roadside vegetation and street canyon as research objects, we evaluated vegetation effect (VE) for vegetation configurations (VCs) with several tree species on the dispersion, deposition, and distribution of traffic generated PM pollutant under different wind regimes. Results showed that (1) the transportation and distribution of traffic PM pollutant were different from wind regimes; (2) total VEs varied from -88.3% to 25.5%, depending on different VCs and wind regimes; perpendicular wind had the best VEs, while oblique wind had the worst VEs among the three wind directions; VEs of cypress were better than pine and poplar; VEs of one side planting were better than two sides planting. (3) the optimal VCs were found by each wind direction; two sides planting by shrub was suitable for parallel and oblique winds; for the perpendicular wind, the optimal VC was that two sides planting by cypress-shrub and increased canopy volume in the street center; and (4) VE were significantly correlated (P < 0.05) with vegetation parameters at lower wind speed, however, no correlations were found at higher wind speed under parallel wind; leeward wall VEs were significantly correlated with aerodynamic parameter (P < 0.001) while windward wall VEs and pedestrian-level VEs with deposition parameter (P < 0.05) under perpendicular wind; VEs were significantly decreased (P < 0.001) with aerodynamic parameter under oblique wind. The study highlights the impact of urban vegetation on air environment and provides insights for vegetation establishment from the viewpoint of improving air quality.

Keywords: Air quality; Street-canyon; Traffic pollutant; Vegetation configurations; Wind conditions.