The Evolution Of Spatial Structure In City Regions And Its Influence On PM_2.5

With the rapid advancement of urbanization and the continuous growth of urban scale,a series of development problems like environmental pollution began to attract considerable attention.Since the large-scale haze pollution incident occurred in most parts of China in early 2013,PM_2.5 Pollution has been attached great importance.With the effective implementation of various pollution action plan,the overall air pollution problem across the country improves,but the severe or moderate pollution weather in some cities have repeated from time to time.It means that the situation of anti-smog is still severe.Polycentric spatial structure is an important idea of regional environmental governance.However,due to the differences in socio-economic background and policy implementation,the real effects of spatial structure optimization varied greatly.Some achieved goals,while others were far from reaching expectations.Therefore,it is meaningful to learn the current situation and the evolution of spatial structure in city regions in China,as well as explore the influence of polycentric spatial structure on PM_2.5 impact of pollution.Taking 369 cities or regions in China as study area,the paper screens the population centers of each city or region from 2001 to 2018,calculates the polycentricity,and analyzes the current situation of the overall spatial structure of cities in China and the evolution mode of spatial structure.Build the fixed effect model and the threshold model to explore the influence of spatial structure on PM_2.5 pollution in China and different regions.The main results are as follows:（1）From 2001 to 2018,the spatial structure in city regions of the country tended to be polycentric,and the total number,area,and population of city centers all increased by a large margin.The rate of multi center cities has increased significantly.The speed of urban polycentric evolution slowed down obviously after 2013,and the polycentric structure of Chinese cities was basically formed around 2013.（2）Taking polycentricity as the measurement index of spatial structure in city regions,the evolution patterns of 318 cities or regions with polycentric changes from2001 to 2018 are classified into seven categories:non-center to single center,single center to multi-center,polycentric stable growth,uncentered transformation to polycentric,polycentric decline year by year,polycentric fluctuation and other categories.The results show that the increase of polycentricity is the mainstream evolution model.（3）Taking 240 polycentric cities in China as the research object,the fixed effect model of polycentricity and PM_2.5 is constructed.The results show that polycentricity plays a significant positive role in promoting PM_2.5 pollution.From the sub-regional point of view,polycentricity and PM_2.5 pollution in the eastern and central regions also show the same positive correlation trend,and the positive effect in the central region is greater;in the western region,the relationship between spatial structure in city regions and PM_2.5 pollution is not significant.In addition,there is also a significant positive correlation between population dispersion and PM_2.5 pollution in the eastern region.（4）The size of population has a single threshold effect on PM_2.5 pollution.In the cities with small population size,the positive promoting effect of polycentricity on PM_2.5 pollution is more obvious;when the population size is larger,the positive promoting effect of polycentricity is smaller.