Research On The Construction Of Evaluation Index System Of Urban Green Space Landscape Pattern Under The Constraint Of PM_2.5

With the acceleration of urbanization and the internal demand of urban development,the types of urban land use are undergoing complex and profound changes,and the urban ecological environment is also facing great challenges.Unreasonable urban development has led to a large number of urban pollution problems,especially haze pollution,which has become the norm of pollution in most major cities.In particular,the increase in population density will lead to the shortage of urban land and the substantial reduction of urban green space,thus further aggravating the haze phenomenon.Therefore,rational use of limited urban green space to maximize its ecological benefits is not only the inevitable trend of urban development in China,but also the internal demand of realizing ecological cities.In order to effectively plan the urban green space and realize the harmonious development of the city and nature,it is necessary to study the mitigation degree of PM_2.5 pollution by the current urban green space landscape pattern and evaluate it through scientific methods,so as to provide some suggestions for urban green space planning in the future.By absorbing and drawing on a large number of previous studies,this paper systematically elaborated the correlation between urban green space landscape pattern and PM_2.5 concentration,as well as the screening and construction methods of green space evaluation index system.Based on this,first of all,the main urban areas of Chongqing from 2015 to 2017 were taken as the research objects,and the evaluation indexes to be screened were preliminarily determined through the literature frequency statistics.Secondly,through basic research and official download(includes the website of Chongqing bureau of ecology and environment,the website of real-time monitoring of PM_2.5 and air quality index,Chongqing municipal bureau of Beibei district and the website of the United States Geological Survey),raw data such as PM_2.5concentration,green space quantity index and remote sensing image of the research area were obtained,and R language,Excel,ENVI,Arc GIS and Fragstats software were used for further processing to obtain data information satisfying the analysis conditions.Thirdly,SPSS software was used to conduct Pearson correlation analysis,and finally determined the indicators with strong correlation with PM_2.5 concentration,such as green space area of the park,class area and contagion,and completed the indicator screening of green space evaluation system.And then,the green space evaluation index system was constructed from two aspects,which were green space quantity index and green space structure index,and the entropy method was used to calculate and analyze the index weights.Finally,the TOPSIS evaluation method was used to comprehensively evaluate the green space landscape pattern of the main urban areas in Chongqing,so as to evaluate the landscape pattern of green space which effectively alleviated PM_2.5pollution in the main urban areas.The following results could be obtained through a series of research analysis:（1）In the index screening,the green space quantity indexes that were screened were the green space area of the park,green space coverage area and green space area of the garden,while the green space structure indexes that were screened were class area,percentage of landscape,largest patch index,patch area distribution,total edge,edge density,shape index distribution,fractal index distribution,contiguity index distribution,landscape shape index,number of patches,patch density,interspersion&juxtaposition index,connectance,splitting index,aggregation index,contagion,Shannon’s diversity index,Shannon’s evenness index and Simpson’s evenness index.In the selection of green space quantity indexes,the result showed that the increase of urban green space area could alleviate PM_2.5pollution,while the result of green space structure indexes showed that if the degree of fragmentation in urban green patches was higher,the green space connectance and the aggregation was lower,the green space could be more conducive to alleviate the PM_2.5 pollution.The two indicators appeared to contradict each other.After analysis,this paper tended to adopt the mainstream conclusion.That was the larger the green space area,the lower the degree of fragmentation,the higher the green space connectance,the more conducive to the reduction of PM_2.5concentration.（2）In the weight analysis,the weight of green space landscape structure was relatively large in the whole evaluation system,indicating that green space structure index occupied a relatively important position.Moreover,in the index layer of green space structure,the index of class area and the index of contagion had greater weight,indicating that green space area and spatial aggregation degree should be focused on and constructed in green space planning.（3）In the horizontal comparison,Banan district,Yubei district and Beibei district had the highest comprehensive evaluation value,and these three areas had the largest green space areas and good green space connectivity,so they played a good role in reducing PM_2.5 pollution.Therefore,it could be considered that the spatial structure model of green space landscape in Banan district,Yubei District and Beibei district had a more obvious effect on reducing PM_2.5 pollution and it was more friendly to the ecological environment.（4）In the vertical comparison,the year-on-year decline rate of the comprehensive evaluation value of Yuzhong district was the lowest among the main urban areas,which its value was-4.22%.Because the green space area of Yuzhong district had been rising from 2015 to 2017,so the decline was the smallest.This result showed that the increase of urban green space area was conducive to maintaining the high evaluation value of urban green space.The results also further indicated that the reduction of PM_2.5 pollution was closely related to the urban green space area.The larger the green space area,the more significant the reduction of PM_2.5pollution is.The discussion in this paper is as follows:The contradiction analysis of the correlation results of green space structure index might be caused by the following reasons.First of all,the volume of three-dimensional greening had not been taken into consideration,which made the loss of green area.Secondly,the calculation method of green space area was too simple.Only the vertical projection area was considered,while the amount of green plants in the three-dimensional space was not.Thirdly,it existed the limitation in administrative division.Due to the large scope of administrative divisions in each main urban area,only a few air quality monitoring stations could not objectively describe the change of PM_2.5 concentration,thus affecting the final correlation results.