Analysis On The Influence Of The Built Environment On Urban PM_2.5 Concentration At Multiple Scales And Its Governance

China's rapid urbanization had created a rich material base and also led to serious air pollution,especially PM_2.5 pollution.Long-term exposure to high levels of PM_2.5 is a major threat to people's health.Exploring the impact of built environment on PM_2.5concentration based on the spatial scale of cities,blocks and buildings was of great significance for developing strategies to improve the urban built environment at both macro and micro scales,reducing residents'exposure to PM_2.5,and improving residents'health and happiness.Firstly,a theoretical framework for the correlation between multi-scale built environment and PM_2.5 concentration was established.Secondly,the effects of built environment on PM_2.5 concentration were investigated at the urban scale using a spatial regression model based on 288 prefecture-level cities in China.Thereafter,the effects of built environment on PM_2.5 concentration at block scale and building space scale were investigated by using structural equation modeling(SEM)and computer simulation methods in 185 blocks in the main urban area of Chongqing.Finally,the urban built environment optimization and governance measures were proposed from the macro and micro scales respectively using particle swarm optimization algorithm and computer simulation method based on the research conclusions.The results for the city scale showed that,city size,compactness and population density had significant positive effects on PM_2.5concentration in both southern and northern cities of China.In general,small scale,moderately dispersed and polycentric urban built environment was beneficial to reduce PM_2.5 concentration.The results for the block scale showed that:(1)the diffusion of PM_2.5 was the most important latent variable affecting the concentration of PM_2.5,followed by the production of PM_2.5,and finally the reduction of PM_2.5.(2)The factors affecting PM_2.5production had a positive impact on PM_2.5 concentration,on the contrary,the factors affecting PM_2.5 diffusion and mitigation had a negative impact on PM_2.5 concentration.(3)The built-up environmental factors that had a significant positive impact on PM_2.5concentration mainly include floor area ratio,average building height,tree aggregation,building density,orientation,population density,road network density and the proportion of industrial land outside the block;the built-up environmental factors that had a significant negative impact on PM_2.5concentration mainly include building height difference,building spacing and greening rate.(4)The standardized influence coefficients of building height,density,building spacing,orientation,greening rate and tree aggregation on PM_2.5 concentration in the blocks were relatively large,which should be paid attention to in the improvement of built-up environment.The results for the building space scale showed that:(1)the short length of windward side of building and the wide space between buildings was conducive to improving ventilation conditions and reducing PM_2.5 concentration.(2)The crown had a blocking effect on the airflow and could reduce the airflow velocity,and this negative effect was not related to the position of the trees.(3)The influence of trees on the airflow in a small range around it was similar in different locations in different block layouts,but the impact on the airflow velocity in a larger range around the tree was related to the location of the trees.(5)There was no significant positive or negative correlation between the number of planted trees and PM2.5 concentration.(6)Planting trees in a narrow building space may lead to the accumulation of particulate pollutants.Whereas,planting sparse trees in a wide building space could be conducive to promoting air convection and reducing the concentration of PM_2.5.(7)In general,PM_2.5accumulation is more likely to occur at the place where the wind velocity was small and the number of vortices was large.(8)The lowest average PM_2.5 concentration and the smallest area with the ratio of PM_2.5 concentrations exceeding the standard both occurred in lattice buildings with trees around the buildings and within the building spaces.(9)The largest average PM_2.5 concentration and the largest area with the ratio of PM_2.5 concentrations exceeding 75?g/m³ both occurred in enclosed type buildings with trees around the buildings and within the building spaces.In the planning and management of built environment at city scale,small scale,moderately dispersed and polycentric built environment should be encouraged.In the planning and design of the block,it is suggested to reduce the floor area ratio,building average height,tree aggregation and population density,or increase the difference of building height to reduce PM_2.5 concentration.In addition to the adjustment of one-way indicators,it is necessary to pay attention to the optimal combination of all built-up environmental indicators in order to achieve the shortest time of PM_2.5 concentration exceeding the standard.In terms of the layout of the building space in the block,the layout of the building-tree should be selected and arranged according to the topographic features and actual needs of different regions.The conclusions of this study had important reference value for multi-scale built environment improvement strategies aimed at reducing PM_2.5 concentration and PM_2.5exposure.