Study On Impacts Of Green Walls And Thermal Effect On The Pollutant Dispersion And NO_x-O₃ Reaction Kinetics Within Street Canyons

With the development of urbanization,the problem of air pollution in urban areas has become more and more serious.As an important part of urban environmental ecological construction,Green Walls（GWs）can improve the environmental quality and microclimate of the city with low occupation of public space and low impact on urban ventilation,so they can be widely applied in high-density urban areas.In order to investigate the impacts of GWs and wall heating caused by solar radiation on the diffusion of pollutants and NOx-O₃ photochemical reaction kinetics within different types of street canyons（Aspect Ratio,AR=1.0 and 0.5）,which are common in urban industrial areas,numerical simulation and wind tunnel experiment are used for research.In this study,various leaf area densities（LAD）and four wall heating scenarios were considered.Results are as follows.（1）Within the street canyon with AR=1.0,compared with leeward wall and ground heating,windward and all walls heating can greatly change the flow structure.While the cooling effect of GWs can weaken the influence of the thermal effect on the flow structure,and the aerodynamical effect can decrease the velocity near the wall and at pedestrian level（z=1.5 m）.The thermal effect of the wall without GWs can increase the average temperature within the canyon by about 0.55 K（leeward wall heating）,0.97K（windward wall heating）,0.91 K（ground heating）,1.36 K（all walls heating）,and GWs with LAD=2.0 can effectively reduce this value by about 0.85 K（leeward wall heating）,1.09 K（windward wall heating）,0.79 K（ground heating）,and 1.02 K（all walls heating）.In addition,the impact of GWs on the distribution of various pollutants largely depends on the wall heating scenarios.Windward wall heating will greatly increase the average CO concentration at pedestrian level by about 43.80%（no GWs）,and the existence of GWs can further increase it by about 46.96%,higher than the other three wall heating scenarios.For the leeward wall and ground heating,the lower concentration of reactive pollutant NOx is the limiting factor for ozone depletion rate（d O₃）and chemical equilibrium.While for the windward wall and all walls heating,with higher reactive pollutant concentration,the lower background O₃ concentration is the limiting factor for chemical equilibrium.（2）Within the street canyon with AR=0.5,the flow structure is dominated by lateral airflow and the clockwise vortex near the windward wall due to the increased relative width of the canyon.Windward wall heating can weaken the clockwise vortex and strengthen the lateral flow.Leeward wall and ground heating can strengthen the clockwise vortex.All walls heating can dissipate the vortex structure.Under various wall heating conditions,GWs has little effect on the average wind velocity at the pedestrian level.Secondly,the thermal effect of the wall can increase the average temperature within the canyon by about 0.35 K（leeward wall heating）,0.60 K（windward wall heating）,0.90 K（ground heating）,1.43 K（all walls heating）,and GWs with LAD=2.0 can effectively reduce this value by about 0.38 K（leeward wall heating）,0.40 K（windward wall heating）,0.35 K（ground heating）,and 0.39 K（all walls heating）.In addition,within AR=0.5 street canyon,the concentration of various pollutants is higher,and wall heating and GWs have little effect on them.Only ground and all walls heating can slightly reduce the concentration of CO by about 9.50%and 8.80%（no GWs）.GWs only increases it under conditions of ground and all wall heating.The average d O₃ within the street canyon also increases due to the higher NOx concentration.With the increase of the LAD of GWs,the concentration of reactive pollutants increases.While the average temperature within the canyon decrease.So,the reaction rate of each reaction decreases together with the average d O₃.This study preliminarily revealed the effect of GWs on the flow structure,the diffusion and transformation of pollutants within the street canyon,which can provide the reference for urban planning,GWs planting and environmental assessment.