Study On The Influence Of Spatial Form Of Urban Street Canyons On The Dispersion Of Traffic-related Particulate Matter

With the continuous improvement of urbanization rate,more and more artificial urban structures have replaced the original natural land cover,and high-rise buildings have replaced low-rise buildings to form street canyons.The characteristics of dense traffic and poor ventilation in street canyons have a direct impact on the diffusion of pollutants in street canyons,and the air quality of the microenvironment in urban street canyons is gradually deteriorating.Traffic air pollution in modern cities is often attributed to continued urbanization and the rapid increase in global motor vehicle use.Improving the layout of urban buildings,enhancing the ventilation capacity of street canyons,and enhancing the transmission and diffusion capacity of pollutants are effective measures to control vehicle exhaust pollution in urban street canyons.Taking a typical street canyon as the research object,this article explored the distribution of traffic particulate matter PM(PM₁₀,PM_2.5,PM₁)in the street canyon,and explored the distribution of street canyon flow field and pollutant diffusion laws under various environmental impact factors,providing guidance for improving the urban street canyon microenvironment air quality.This study obtained the spatiotemporal distribution characteristics of particulate matter in street canyons and the correlation between particulate matter concentration and various environmental factors through field monitoring methods;Based on the ENVI-met model,a street canyon scene with different building layouts was established,and numerical simulation methods were used to analyze the differences in PM_2.5 concentration distribution in the street canyon under different aspect ratios,aspect ratios,and building height ratios.The analysis results of spatiotemporal distribution characteristics indicate that the weekly trend of time variation of traffic particulates presents a"weekend effect".The concentration and time variation trends of the three types of particulate matter PM₁₀,PM_2.5,and PM₁ are the same,showing a"double peak distribution"characteristic.The concentration of roadside particulates is significantly higher than the background concentration of fixed monitoring stations.The monitored concentration of pollutants at road intersections is higher than that at the intersection connecting sections,especially at the intersection entrance.Through analyzing the quantity and concentration distribution of particles under different particle sizes,it was found that more than50%of particles concentrated below the particle size of 300 nm.Pearson correlation analysis results show that meteorological factors significantly affect the concentrations of PM_2.5 and PM₁,and traffic volume significantly affects the concentrations of the three pollutants.The numerical simulation results show that the concentration of PM_2.5 in street canyons with different spatial patterns first increases and then decreases from the leeward side to the windward side.The concentration of pollutants at any height on the leeward side is always higher than the concentration at other points on the horizontal plane,forming a"wall climbing effect".There are significant differences in the flow field in the street valleys with different aspect ratios.As the aspect ratio increases,airflow vortices will form in the street valleys to interfere with the diffusion of pollutants,and the concentration of PM_2.5 will increase.With the gradual increase of the length to height ratio of the street valley,the concentration of PM_2.5gradually increases.The PM_2.5 concentration in the"down step"type street canyon is lower than that in the"up step"type street canyon,indicating that the buildings on the leeward side are higher than those on the windward side,which is conducive to the diffusion of pollutants in the street canyon.This study can provide empirical evidence for exploring the distribution and diffusion mechanism of pollutants in urban traffic street canyons,and mitigating urban air pollution.It will also provide scientific basis for future planning and architectural design of urban road builders and urban planners in mitigating urban air pollution.