*3.1. Average Distribution of PM*2.5 *in Autumn and Winter of China from 2013 to 2020*

Figure 1 shows the average distribution of PM2.5 mass concentration in the autumn and winter (October of the current year to February of the following year) of 2013–2020 based on satellite inversion in the central and eastern of China. On average, Northeast China, Beijing-Tianjin-Hebei, and surrounding areas (Fenwei, Yangtze River Delta, Sichuan-Chongqing, Central China, and Pearl River Delta (seven regions)) are the major areas where PM2.5 concentration is relatively high in China. The following research on the growth law of PM2.5 is also mainly carried out for the above-mentioned regions and representative cities.

We carried out annual statistics on the average concentration of PM2.5 in autumn and winter (Hereinafter refers to as average PM2.5 concentration) of selected representative stations since 2013. As shown in Figure 2, from the perspective of the evolution of the time series, since 2013, the PM2.5 of the seven regional representative stations shows an overall decreasing trend. For the three regional representative stations of Northeast China, Beijing-Tianjin-Hebei and surrounding areas and Fenwei (north of the Huai River in China), the pollution of PM2.5 in autumn and winter is significantly lower after 2016 than before. From 2013 to 2016, 30% of the representative stations in the above three regions had an average PM2.5 mass concentration more than 115 μg m−<sup>3</sup> (115 μg m−<sup>3</sup> is the limit concentration of average daily moderate pollution stipulated by the Ministry of Environmental Protection of the People's Republic of China), and 87% of statistical values exceed 75 μg m−<sup>3</sup> (the second grade of NAAQS released in 2012 by the Ministry of Environmental Protection of the People's Republic of China, http://kjs.mep.gov.cn/hjbhbz/bzwb/dqhjbh/dqhjzlbz/ 201203/t20120302224165.htm (accessed on 7 September 2021)). The highest average value of PM2.5 exceeds 200 μg m−<sup>3</sup> (Shijiazhuang in 2013). However, in the autumn and winter from 2017 to 2020, the average PM2.5 concentration is lower than 115 μg m<sup>−</sup>3, and only 30%

of sites have an average PM2.5 more than 75 μg m−<sup>3</sup> north of the Huai River. Especially by 2020, the statistics of all sites in the three regions are lower than 75 μg m<sup>−</sup>3.

**Figure 2.** Statistics of PM2.5 mean values of 7 regional representative stations in central and eastern China from 2013 to 2020 in autumn and winter.

The other four areas with a lower latitude south of the Huai River in China (Yangtze River Delta, Sichuan-Chongqing, Central China, and Pearl River Delta) have significantly less fine particulate pollution than the three areas north of the Huai River (Northeast, Beijing-Tianjin-Hebei and surrounding areas, and Fenwei). Among the valid data, except for HF and WH in 2013, which average PM2.5 concentration exceeded 115 μg m<sup>−</sup>3, other statistical values are all lower than 115 μg m<sup>−</sup>3. For the Pearl River Delta region, after 2015, the average PM2.5 value was lower than 35 μg m−<sup>3</sup> (24-h NAAQS of US EPA). Among sites south of the Huai River, the proportion of average PM2.5 concentration exceeds 75 μg m<sup>−</sup>3, which accounts for 26% from 2013 to 2017. However, from 2018 to 2020, the average of PM2.5 is all below 75 μg m<sup>−</sup>3. A number of studies showed that the main reason for the gradual decrease in the concentration of PM2.5 in central and eastern China is the decrease in the concentration of gaseous precursors under the joint emission controls and, hence, the suppression of secondary growth and formation [35,36]. However, many studies have also pointed out the importance of meteorological conditions, which can help to explain the reduction in PM2.5 concentration of over 50% [14]. The combined effect of meteorological factors and anthropogenic emissions makes the cause of air quality improvement still uncertain [37]. The main reason that the PM2.5 concentration in the north of Huaihe River is higher than that in the south of the Huaihe River is the comprehensive effect of industrial layout, industrial structure, meteorological factors (precipitation, atmospheric diffusion conditions), and topography, etc.
