**1. Introduction**

Regional-scale, high-concentration aerosols are important causes of haze in central and eastern China [1,2]. Among a variety of pollutants, high concentrations of PM2.5 are still the most important aerosol pollutant, as well as the main cause of haze [3–5]. Elevated PM2.5 affects human health, the environment, and even climate [6,7]. The main causes that affect the production and consumption of PM2.5 include chemical reaction processes, meteorological factors, and emission sources [8–10]. When the emission source is stable, the temporal and spatial characteristics of air pollution mainly depend on meteorological

**Citation:** Jiang, Q.; Zhang, H.; Wang, F.; Wang, F. Research on the Growth Mechanism of PM2.5 in Central and Eastern China during Autumn and Winter from 2013–2020. *Atmosphere* **2022**, *13*, 134. https://doi.org/ 10.3390/atmos13010134

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 24 November 2021 Accepted: 10 January 2022 Published: 14 January 2022

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factors [11]. The meteorology can influence the particulate matter (PM) evolution through many ways, e.g., secondary formation, accumulation or dilution, liquid-phase and heterogeneous reactions to secondary aerosols, etc. [12,13]. Under adverse atmospheric diffusion conditions, generally characterized by weak wind speeds, high relative humidity (RH), and low planetary boundary layer height, PM2.5 can quickly accumulate to a very high concentration [8]. The study by Zhang et al. [14] shows that the unfavorable weather elements in the winter of Beijing-Tianjin-Hebei can cause the PM2.5 concentration to increase by about 40% to 100% compared with other seasons. In light of single meteorological elements, Sun et al. revealed that, at low RH levels (<50%), PM increases linearly as a function of RH, among which OAs present the largest mass increase rate at 11.4 mg m<sup>−</sup>3/10% RH during wintertime in Beijing. In addition, the secondary formation is also one of the most important factors for the occurrence and development of haze weather [15]. Quan et al. [16] found that the conversion from NOx and SO2 to nitrate and sulfate was likely accelerated, and that both significantly increased in haze events. The secondary formation also promotes the formation of organic aerosols under certain conditions, and the rapid formation of secondary OA (SOA) under strong photochemical reactions can lead to more serious air pollution [17,18].

As a result of large reductions in anthropogenic emissions, the air pollution has been significantly improved with the successful implementation of "Action Plan on Prevention and Control of Air Pollution" in 2013 in China [19]. However, severe haze episodes still happen in some areas of central and eastern China (including Beijing-Tianjin-Hebei [10], Yangtze River Delta [20], Sichuan-Chongqing area [21], Fenwei Plain [22], Central China [23], etc.) in autumn and winter. For example, the rapidly spread coronavirus disease limited people's outdoor activities and, hence, caused substantial reductions in anthropogenic emissions in 2020; however, there are still two persistent heavy pollution incidents from January 25 to 28 and February 8 to 14 in Beijing [24].

In heavy haze pollution, pollutants exhibit different growth rates at various stages of accumulation. Under different meteorological conditions, they may exhibit slow growth or rapid growth, and may increase by tens or even hundreds in one hour or several hours called "explosive growth" in the later stage of pollution. There is no qualitative conclusion about the cause of the rapid and even explosive growth of PM2.5. Zheng et al. [25] highlight that the trans-regional transportation of pollutants has led to a rapid increase in pollution. Wang et al. [26] believed that the secondary transformation and nucleation effect of aerosols played a more important role through simulation studies. The study by Zhong et al. [13] attributed the rapid increase in pollution more to the effect of meteorological factors, and their study concluded that more than 70% of the increase in PM2.5 can be attributed to the feedback effect after the persistent deterioration of the boundary layer meteorological conditions. Zhong et al. [27] conducted a study of 28 pollution episodes in Beijing from 2013 to 2017 and concluded that a threshold value for PM2.5 explosive growth is 100 μg m−<sup>3</sup> in Beijing. Above this threshold, the positive feedback from aerosols to near-ground radiative cooling to anomalous inversion is effectively triggered. However, faced with the decreasing PM2.5 concentration year by year, this threshold may change over time and show some geographical differences. Regional emission controls were effective in reducing the PM2.5 mass concentration. However, the changes in SOA and inorganic aerosol were comparably small and even had slight increases [24]. Therefore, exploring the chemical component contribution and particle size evolution of aerosol particles is of great significance for further understanding the growth characteristics and physicochemical mechanism of PM2.5. In this study, ground-based PM2.5 observation data and meteorological element data are used to discuss the thresholds and year-on-year changes of PM2.5 in several major polluted areas in China from 2013 to 2020 under three growth mechanisms (slow, rapid, and explosive). The correlation between meteorological elements and the accumulation rate of pollutants is also discussed. At the same time, using the PM chemical composition and particle size distribution data, the contribution of PM composition and size distribution at different rates of pollutant cumulative stage is analyzed.
