The Extraordinary Trend of the Spatial Distribution of PM2.5 Concentration and Its Meteorological Causes in Sichuan Basin

Sichuan Basin is an area with some of the most serious PM2.5 pollution, and it is also a key area for joint prevention and control of air pollution in China. Therefore, it is necessary to clarify the temporal and spatial distribution characteristics of PM2.5 concentration in Sichuan Basin (SCB) and study the influence of meteorological conditions. In this study, the spatial disparity of PM2.5 concentration in SCB and its variation trend from 1 December 2015 to 30 November 2019 were analyzed. The results showed that the spatial disparity of SCB was decreasing and distinct variation trends of PM2.5 concentration were observed in different areas. The PM2.5 concentrations declined rapidly in the western and southern basin (most severely polluted areas), decreased at a slower rate in the central and eastern basin, but unexpectedly increased slightly in the northern and northeastern basin. From the perspective of relative spatial anomalies (RAs), the decreasing (increasing) trend of RAs of PM2.5 concentrations in the western and southern (northern and northeastern) parts of SCB were also prominent. The reduction in spatial disparity and the regionally extraordinary increasing trend could be partly explained by the variations in synoptic circulations. Specifically, the reasons for the decrease in wintertime spatial disparity and the increase in RAs in the northern basin were the reduction in synoptic pattern Type 2 (weak high-pressure system and uniform pressure fields) and Type 3 (high-pressure system to the north) and the growth of Type 6 (weak low-pressure system with high-pressure system to the north). In spring, the reasons were the reduction in Type 1 (weak low-pressure system) and Type 5 (weak low-pressure system to the southwest) and the growth of Type 2. The reduction in Type 2 and the growth in Type 4 (weak high-pressure system to the east) could explain the variation in PM2.5 distribution in autumn. This study showed the importance of implementing more precise and effective emission control measures, especially in relatively cleaner areas, in which the impacts of meteorological conditions might cause fluctuation (even rebounding) in the PM2.5 concentration.