*3.3. Size Distribution*

Figure 6 shows a comparison of average size distributions before and after GP, SLG, RAG, and EXG in Beijing (27 November 2014 to 28 February 2015). As indicated in Figure 6, from SLG, RAG, to EXG, the peak number concentration gradually increasing. The peak number concentration spectrum distribution of GP is between SLG and RAG. After the growth started, the peak number concentration of the three growth rates all increased significantly. However, there is no significant difference in the peak number concentration particle size before and after the growth of the three growth ways. According to the definition of Hussein et al. (2004), different particle sizes could be divided into four modes: the nucleation mode (0.01~0.02 μm), the Aitken mode (0.02~0.1 μm), the accumulation mode (0.1~1 μm), and the coarse mode (1~10 μm) [38]. Limited by the measuring range of the instrument, the coarse mode is not studied in this article. In terms of the number of concentrations, before and after the three growth ways, the number concentration of the nucleation mode (NUM), the Aitken mode (AIM), and the accumulation mode (ACM) all show varying degrees of growth (Table 1). Among RAG and EXG, the increasing number concentration of AIM and ACM is more significant. Calculated from Table 1, during the SLG, the particles' concentration growth rate of the NUM, AIM, and ACM are 1691/cm−<sup>3</sup> h<sup>−</sup>1, 586/cm<sup>−</sup><sup>3</sup> h<sup>−</sup>1, and 325/cm<sup>−</sup><sup>3</sup> h<sup>−</sup>1, respectively. However, the growth rate of the number concentration of NUM decreases to 941/cm−<sup>3</sup> h−<sup>1</sup> and 668/cm−<sup>3</sup> h−<sup>1</sup> in the RAG and EXG, respectively. Simultaneously, the growth rate of AIM in the RAG and EXG

is 3.5 times and 7.7 times higher than that of SLG, respectively. For the ACM, in particular, the growth rate in the EXG can reach 4483/cm−<sup>3</sup> h−<sup>1</sup> in Beijing during autumn and winter.

**Figure 6.** Average size distribution of aerosol particle before and after slow growth (SLG), rapid growth (RAG), explosive growth (EXG), and all growth period (GP) in Beijing.

**Table 1.** Statistical values of particle number concentration in each mode before and after (1-h interval) slow, rapid, and explosive growth in Beijing.


Comparing with the three growth rates, the peak diameters gradually shift to larger sizes with the growth rate increasing. The particle diameters corresponding to the peak concentration of SLG is ~65 nm, and the peak particle diameters of RAG and EXG grow to ~94 nm and ~103 nm, respectively. Since the growth rate of PM2.5 is closely related to the degree of air pollution, the average growth rate under pollution background is, on average, higher than that of clean conditions, which is similar to the results of Guo et al. [39], who showed that the average particle diameters of aerosols in Beijing gradually increase from cleaning to pollution with an average daily mass growth of 50~110 μg m<sup>−</sup>3. The study by Xu et al. [40] showed that the increased particle size of OA mainly corresponds to SOA, while the particle size of POA hardly changes. At the same time, the hygroscopicity parameter of OA increased substantially with particle size and has played a further role in promoting the increase in pollution.

Figure 7 shows the number concentration spectrum distribution of 12 times PM2.5 episodes in 2014 and 2015 during autumn and winter. In the cumulative phase of the pollution process, slow, rapid, and explosive growth alternately occur. Before reaching the peak concentration in most heavy pollution episodes (especially the peak concentration of

PM2.5 exceeding 200 μg m<sup>−</sup>3), it is accompanied by an obvious explosive increase in PM2.5. Consistent with the above conclusions, when the EXG occurs, the number concentration of NUM does not increase significantly, and the particle size with increased concentration is mainly distributed in AIM and ACM. Since the growth rate of pollution is related to the mass concentration of PM2.5 (Section 3.2), the rapid and explosive growth usually occurs in severe air pollution. On the one hand, there is a noticeable absence of new particle formation as the pollution episode develops. On the other hand, with the stable atmospheric situation, small particles keep growing by collision and hygroscopic growth. While the NUM particles contribute negligibly to the particle mass concentration, the severe pollution episodes (high growth rate) are attributable to the presence of numerous large particles.

**Figure 7.** The number concentration spectrum distribution and temporal evolution of PM2.5 mass concentration (right axis) during PM2.5 episodes in 2014 (**top**) and 2015 (**bottom**) in Beijing. The colors of PM2.5 mass concentration represent slow, rapid, and explosive growth. The white line indicates the concentration drop phase.
