Study on the influence of wall thermal effect coupled with tree trunk height on pollution diffusion in urban street canyons

In semi-enclosed street canyons, traffic emissions and secondary pollutants tend to accumulate in poorly ventilated areas, posing a serious threat to public health. Among the various factors influencing flow dynamics and pollutant dispersion within street canyons, insufficient attention has been given to the thermal buoyancy effect caused by solar radiation on building walls and the impact of different tree trunk heights on aerodynamics. The effects of different tree trunk heights (coupled tree shading effect) and wall heating conditions on air flow and pollutant diffusion in urban street canyons were studied. Five different tree trunk heights (h=0.18H, 0.40H, 0.62H, 0.84H, 1.06H) combined with four wall heating configurations were considered for analysis purposes. The numerical results demonstrate that variations in tree trunk height and the thermal effects of walls significantly impact air flow and pollutant diffusion within urban street canyons. When the trunk height is lower than that of the building, wall heating generates thermal buoyancy which can reduce pollutant concentration in the street canyon and enhance ventilation performance. However, when the trunk height exceeds that of the building, thermal buoyancy generated by windward wall heating hinders pollutant diffusion. Using full-wall heating can achieve a lower accumulation of pollutants. The research results can provide technical guidance for the optimal design of urban green facilities and the precise control of local microclimate environment and air quality.