Analysis of PM2.5 spatial association evolution in the Sichuan Basin and its driving factors

The swift urbanization in China during the past two decades heightened the environmental pressure on cities from anthropogenic production and consumption beyond the regional capacity. The Sichuan Basin, situated in southwest China, faced severe air pollution as its unique topography as a basin surrounded by mountains caused frequent temperature inversion and trapped air pollutants. In this paper, we investigated the evolution of spatiotemporal patterns of particulate matter with diameter not greater than 2.5 mu m (PM2.5) across eighteen cities in the Sichuan Basin. In addition, a spatial association network was characterized through social network analysis using China's air monitoring data and relevant socioeconomic data. The results were as follows: (1) A significant reduction in PM2.5 emissions in the Sichuan Basin occurred from 2015 to 2020, but challenges remain: the number of cities with a PM2.5 annual concentration above 35.00 mu g/m(3) decreased from seventeen to six, and the annual concentration declined from 62.92 mu g/m(3) to 40.83 mu g/m(3) in Chengdu and from 55.08 mu g/m(3) to 32.67 mu g/m(3) in Chongqing. PM2.5 concentrations were the highest in the winter, around 1.50-2.00 times the annual concentration. (2) The PM2.5 overall network displayed significant spatial association with periodic changes implying that the inter city association strengthened , then weakened, and then strengthened again from 2015 to 2019 despite a sharp drop in the PM2.5 concentration. Network density remained at 0.29 between 2015 and 2019 but increased to 0.30 in 2017, efficiency increased from 0.72 to 0.80 and connectedness from 0.78 to 1.00. A "core edge" pattern explicitly presented that Chengdu and Chongqing were located at the center with degree of 94.40 and 82.35, respectively, while other cities marginalized less than 35.30. (3) The driving factors of network structure were explored by quadratic assignment procedure, which showed that enlarging the difference in gross domestic product gap between secondary and tertiary industries and urban population and narrowing the difference among urban ratio, the number of civilian vehicle and distance between cities efficiently promotes the PM2.5 spatial association in the Sichuan Basin. The dominant factors impacting the PM2.5 spatial and temporal differences were the gross domestic product gap between secondary and tertiary industries, urban population and the number of civilian vehicle identified by quadratic assignment procedure. A synergetic effect among these factors played an important role as the cities with annual concentration PM2.5 < 35.00 mu g/m(3) corresponded to those with a combination of relatively small value for these driving factors.