Research On The Urban Neighborhood Built Environment Based On PM_2.5 Analysis

In China and many developing countries cities,air pollution with PM_2.5 as the main pollutant is a common problem.Research shows that there are great differences in PM_2.5levels among urban neighborhoods.Neighborhood built environment is closely related to these differences.As the texture unit of a city,urban neighborhoods are also the objects that can be effectively regulated and controlled by planning,design,and management.Deep analysis of the relationship between the built environment and the difference of PM_2.5pollution has good practical significance and application value in planning and control.However,there are relatively few studies on the built environment and PM_2.5 of the common neighborhoods which constitute the texture of the city,and there is a lack of attention to the differences of influences of the built environment under different pollution levels and the dynamic changes of PM_2.5.Therefore,this study focuses on common neighborhoods and aims at the influence and mechanism of built environments on PM_2.5,and the strategies for improving built environments.Considering the typical hot summer and cold winter areas in China,the seriousness of PM_2.5 pollution,and the number of urban sites,five provincial capitals in the same climate zone in the middle and lower reaches of the Yangtze River were taken as a background area,and the spatial units of 1000 m×1000 m formed by relatively uniform urban monitoring stations in the city were taken as neighborhoods.PM_2.5 data came from urban areas,and abundant data can be obtained through long-term fixed monitoring.Based on hourly data,the absolute concentration and relative index of PM_2.5 were analyzed.The green and gray spatial vector database of neighborhoods was interpreted by the 0.26 m resolution Google Earth image.The main chapters of this paper are as follows:Chapter two is a summary of related theories and research.Chapter three mainly introduces the spatial distribution characteristics and differences of PM_2.5 in neighborhoods,which is the basic premise of the study.Chapter four introduces the impact of neighborhood green space on PM_2.5.The scale of green space was measured by green space cover ratio and its correlation with PM_2.5concentration,influence rules and spatial scale effects were analyzed.The spatial form of green space was measured by MSPA,and the influence rules,influence intensities and relative contributions to PM_2.5 were analyzed.Chapter five mainly introduces the impact of the scale and form of gray space on PM_2.5.The scale of gray space was measured by hard surface cover ratio and its correlation with PM_2.5 concentration,influence rules and spatial scale effects were analyzed.The spatial form of gray space was measured by building layout and road form.The influence rules,influence intensities and relative contributions to PM_2.5 were analyzed.Chapter six mainly introduces the evaluation and prediction of PM_2.5.Based on the spatial indicators significantly influencing on PM_2.5,the prediction model of PM_2.5 was constructed combined with principal component analysis.Chapter seven puts forward the control strategy of improving gray and green space based on the current research.Results showed that,1)PM_2.5 levels fluctuated up and down at the city’s overall level.With the increase in the pollution level,the floating range of PM_2.5 level tended to decrease with the maximum value of 79%～123%（in Nanjing）.The absolute and relative difference of PM_2.5 level tended to increase.There were few neighborhood samples with significant differences in the PM_2.5 relative indexes.Absolute difference was generally higher in heavy pollution than other pollution levels and relative difference basically increased with the increase in pollution level.2)Green space cover ratio was significantly negatively correlated to PM_2.5 concentration,and had a non-linear impact on PM_2.5 concentration.The most significant influence appeared at the scale of 800 m×800 m and 1000 m×1000 m.Tree coverage had a stronger correlation with PM_2.5 concentration than did grass coverage.Seven types of MSPA elements had significant effects on the increase and decrease of PM_2.5.Core and bridge were helpful to promote the decrease of PM_2.5 concentration and inhibit the growth of PM_2.5 concentration,while edge,islet,and perforation played opposite roles.Loop and branch had unstable effects on PM_2.5.3)Hard surface cover ratio was significantly positive correlated to PM_2.5 concentration and showed a non-linear effect.The correlation decreased with the decrease in the neighborhood scale.Building density,floor area ratio,and road density promoted the increase of PM_2.5 concentration.Mean building height and building evenness index played opposite roles.Sky view factor had two-sided effects on PM_2.5.The influence of standard deviation of building height on PM_2.5 showed pollution differences.4)The Adj＿R² of prediction model of PM_2.5 relative indexes concentrated between 0.6 and 0.8,indicating that the models are higher goodness of fit and they can predict the increase and decrease of PM_2.5 well.According to the above quantitative analysis,the following strategies were put forward:1)On the basis of increasing green space cover ratio,increasing the core proportion by adding or connecting adjacent cores,while avoiding the artificial interference within the core is helpful to construct a planar green space and form a large background of micro-green network.Forming the core,bridge through the expansion of islet is helpful to decrease the fragmentation of small-scale green space,thereby forming point green space.Building a linear corridor of green space and enhancing the overall connectivity of green space by enhancing the connection between planar green spaces,increasing the proportion of bridges,and moderately controlling the proportion of loops and branches.2)On the basis of reducing the hard surface cover ratio,lower building density should be controlled,especially for buildings with 9 floors and less.Building density with 4-9 floors should be in the range of 10%～15%.Strength of block construction should be controlled with floor area ratio less than 1.5.Average building height of block should be in the range of 20 m～30 m.The difference of height should be arranged in a staggered way.The difference of building volume should be enlarged as far as possible in the layout of building space,and the openness of block should be increased.The sky view factor should be in the range of0.5～0.55.The road density should be controlled at 20 km/km²,and the proportion of sub-main roads,branch roads,pedestrian,and bicycle roads should be increased.