**4. Conclusions**

We select a total of 27 representative stations in seven areas (Northeast China, Beijing-Tianjin-Hebei and surrounding areas, Feiwei Plain, Yangtze River Delta, Sichuan-Chongqing, Central China and Pearl River Delta) in central and eastern China to analyze the year-onyear evolution trend of PM2.5 pollution in the autumn and winter from 2013 to 2020. The increased stage of pollutants is divided into SLG, RAG, and EXG, according to the different accumulation rate of pollution. Taking Beijing as an example, a comparative study is conducted on the distribution characteristics of aerosol particle size, the contribution of

chemical components, and the differences in meteorological conditions in three growth rates. The average value of PM2.5 in the autumn and winter of each regional representative station shows a decreasing trend as a whole, especially after 2017, where the decreasing trend was significant. The +ΔPM2.5 in the north of the Huai River is lower than that in the south of the Huai River, and the +ΔPM2.5 after 2017 also showed a significant decreasing trend compared with that before 2017. The average PM2.5 threshold before the EXG is 70.8 μg m<sup>−</sup>3, and the threshold that is extremely prone to EXG ranges from 156 μg m−<sup>3</sup> to 277 μg m−<sup>3</sup> in the stations north of the Huai River. For the area south of the Huai River, the threshold for PM2.5 EXG is relatively low, while a more stringent threshold also puts forward stricter requirements for atmospheric environmental governance. With the growth rate increase, the peak number concentration diameters gradually shift to a larger size in Beijing. The number concentration increasing mainly distributed in AIM and ACM during EXG. Among the various components of PM1, OAs, especially POA, have become one of the most critical components for the EXG of pollutants in Beijing. During the GP, the contribution of SPM to the accumulated pollutants is significantly higher than that of PPM. However, with the increase in growth rate, the proportion of SPM gradually decrease. In the EXG, the contribution of the PPM can reach up to 48%. Compared to SLG and RAG, EXG mainly occurs in the stable atmosphere of higher humidity, lower pressure, lower temperature, small winds, and low MLH.

**Author Contributions:** Conceptualization, Q.J. and H.Z.; methodology, F.W. (Fei Wang, feiwang@cma.gov.cn); formal analysis, F.W. (Fei Wang, wfnk@foxmail.com). All authors have read and agreed to the published version of the manuscript.

**Funding:** National Key Research Program of China (Grant No. 2019YFC0214602) and National Key Research and Development Plan Project (2016YFC0203301).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** All individuals included in this section have consented to the acknowledgement.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Appendix A**

**Figure A1.** Time series of (**a**) Mixed layer height (MLH); (**b**) relative humidity (RH) and temperature (Temp); (**c**) wind speed (WS) and wind direction (WD); (**d**) PM2.5 and pressure (Pres.) during January 2015 in Beijing.

**Figure A2.** Diurnal of PM2.5 and meteorological variables for the entire study.
