**1. Introduction**

Haze pollution has occurred frequently in the past few decades due to the rapidly developing economy and accelerating urbanization in China. As the major cause of haze, PM2.5 (particles with aerodynamic equivalent diameter less than 2.5 μm) was one of the most concerned air pollutants because of its harm to human health and impact on global climate [1–3]. For example, it was found that long-term exposure to high PM2.5 concentration could lead to cardiovascular diseases, respiratory diseases and even premature deaths [4,5]. PM2.5 could change the climate on both global and local scales through directly scattering incident solar radiation or indirectly influencing clouds and precipitation [6–8]. This made the collocation between PM2.5, population and ecosystem of considerable interest. Therefore, the temporal and spatial characteristics of PM2.5 concentration and its causes were the most discussed topics in previous studies.

**Citation:** Xiang, X.; Shi, G.; Wu, X.; Yang, F. The Extraordinary Trend of the Spatial Distribution of PM2.5 Concentration and Its Meteorological Causes in Sichuan Basin. *Atmosphere* **2022**, *13*, 853. https://doi.org/ 10.3390/atmos13060853

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 10 April 2022 Accepted: 23 May 2022 Published: 24 May 2022

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<sup>2</sup> National Engineering Research Center on Flue Gas Desulfurization, Chengdu 610065, China

Excessive anthropogenic pollutant emissions and adverse meteorological conditions were the determinant factors causing frequent and severe PM2.5 pollution events [9–12]. A series of controlling measures were implemented aiming to reduce the anthropogenic emissions of air pollutants, which led to a significant decrease in PM2.5 concentration across China [13,14]. However, the impact of meteorological conditions on PM2.5 concentration was complicated. Generally, wind is one of the most important meteorological factors. High wind speeds facilitate the diffusion of pollutants and the transport of pollutants is related to both the direction and speed of prevailing wind [15,16]. The vertical diffusion capability, characterized by temperature inversion or planetary boundary layer height (PBLH), limits the space for pollutant mixing and, hence, affects the accumulation process [17–20]. Relative humidity (RH) is also important for the secondary formation and hygroscopic growth of atmospheric particles [21,22]. On the synoptic scale, the relevant meteorological parameters are affected by atmospheric circulation. Therefore, the classification of atmospheric circulation is of great significance to examine the relationship between meteorology and air pollution [23–25].

Sichuan Basin (SCB), located in Southwest China and surrounded by plateaus to the west and south and high mountains to the north and east, is one of the most polluted areas in China [12,26]. The complex topography led to special meteorological conditions with extremely calm winds and stagnation in the basin area [27]. The average occurrence frequency of air stagnation in winter, from 2013 to 2016, exceeded 76% in SCB [28]. The RH in SCB was high, which was conducive to the hygroscopic growth of particles [29,30]. The circulation in SCB was also important, as previously revealed, as southerly warm flows favored the PM2.5 pollution and northerly cold flows were conducive to the dissipation of PM2.5 [17,30]. Moreover, the complicated topography modulated the distribution of PM2.5 from both the horizontal and vertical perspective. Ning et al. found that there was a nonlinear relationship between urban PM concentration and altitude in SCB [11]. Shu et al. found that there was a higher PM2.5 layer at a height of 1.5~3 km in the basin, and the PM2.5 concentration between this layer and the ground was relatively low [31].

Although many studies on the distribution characteristics of PM2.5 concentration and their relationship with meteorological conditions were conducted in SCB [28,30,32], few focused on the trend of spatial distribution of PM2.5 concentration. In this study, we examined the variations in PM2.5 distribution from 2016 to 2019 and found extraordinary trends in the northeastern basin. The possible meteorological causes of these regional characteristics were explored from the perspective of synoptic classification. The results could provide potential reference for joint prevention and control measures of PM2.5 pollution in SCB.
