Influence of tree planting pattern coupled with wall thermal effect on pollution dispersion within urban street canyon

Among the factors influencing the flow field and dispersion characteristics of pollutants in street canyons, trees not only act as obstacles that impede canyon ventilation but also contribute to non-uniform temperature distributions on its walls through transpiration and shading effects. The coupling effects, however, have not been given adequate attention. This study employed numerical simulations validated by wind-tunnel experiments to investigate the coupling effects of tree planting patterns (crown density Cd, planting density Pd) and wall heating conditions on airflow and pollutant diffusion within urban street canyons. Results show that the variations in Cd or Cd of the tree planting configurations studied have no significant impact on flow structure and pollutant dispersion in the isothermal street canyons. When Pd is 0.25, all three monitoring planes exhibit relatively high net escape velocity (NEV*) and notably reduced overall pollutant concentrations, indicating that this planting configuration is optimal for reducing pollution levels under leeward wall heating. When the windward wall is heated, the pollutant concentration is reduced more significantly under Pd of 0.25 and 0.75 while NEV* on the windward side is about 48. The average NEV* is minimized to approximately 21 at a Cd of 80 % when crown density or planting density is increased for all wall heating scenarios. Moreover, the other configurations of tree planting had little impact on ventilation and dispersion of pollutants in street canyons. The findings can offer technical guidance for the optimal design of urban green facilities to improve local microclimate environment and air quality.