*3.4. Chemical Composition*

To explore the contribution of aerosol components in SLG, RAG, and EXG during autumn and winter in Beijing (October 2012 to February 2013), Figure 8a shows the average contribution of PM1 species (PM1 = OA + SO4 <sup>2</sup><sup>−</sup> + NO3 <sup>−</sup> + NH4 <sup>+</sup> + Chl + BC) to the increasing mass concentration in GP. Among the growing concentration of PM1, the contribution of OA exceeds 50% on average (discuss only in the increased concentration, same below). With the increase in +ΔPM2.5, in particular, the proportion of OA gradually increases. The contribution of OA to the EXG reaches 57.3%, which is significantly higher than the RAG (52.1%) and SLG (54.3%). Among the inorganic components (SO4 <sup>2</sup><sup>−</sup> + NO3 <sup>−</sup> + NH4 <sup>+</sup> + Chl), in addition to the EXG, NO3 <sup>−</sup> contributes the most in SLG and RAG, followed by NH4 + and SO4 <sup>2</sup>−. However, when the growth rate increases, the contribution of NO3 <sup>−</sup> gradually decreases, while the contribution of SO4 <sup>2</sup><sup>−</sup> is the opposite. In the EXG, the contribution of SO4 <sup>2</sup><sup>−</sup> is slightly higher than that of NO3 <sup>−</sup>, becoming the largest contributor to the inorganic components. Sun et al. [41] concluded that with the increase in relative humidity in autumn and winter, SO4 <sup>2</sup><sup>−</sup> increased rapidly through liquid phase chemical reactions, while NO3 <sup>−</sup> mainly existed in the form of particulate matter, gas-particle transformation was inhibited. Photochemical reactions during the day become the main production mechanism of NO3 <sup>−</sup>. It can be seen from Figure 9 that the relative humidity (RH) before the EXG is significantly higher than that of RAG and SLG. Higher RH accelerated the growth rate of

SO4 <sup>2</sup>−, resulting in a significant increase in the proportion of SO4 <sup>2</sup><sup>−</sup> in the accumulated pollutants.

**Figure 8.** Average mass fractions of PM species ((**a**): Org, SO4 <sup>2</sup>−, NO3 <sup>−</sup>, NH4 +, Chl, and BC) ((**b**): SPM and PPM) and OA factors ((**c**): HOA and OOA) in increasing concentration during GP, SLG, RAG, and EXG and the proportion of the three growth methods in the total growth (**d**) in Beijing.

**Figure 9.** Statistic graphs of temperature (Temp), pressure (Pres), wind speed (WS), humidity (RH), and mixed layer height (MLH) before the explosive, fast, and slow growth in Beijing.

PMF analysis of ACSM mass spectra of OA identified two components, i.e., hydrocarbonlike OA (HOA) and oxygenated OA (OOA), as compared to the simultaneous observation of gas components (O3, SO2, NOx, and CO, etc.), and various organic source spectra. In this study, HOA is closely related to BC (a tracer for combustion emissions, *r*<sup>2</sup> = ~0.63) and NOx (*r*<sup>2</sup> = ~0.57), indicating the important contribution of vehicle sources. OOA has a high correlation with SO4 <sup>2</sup><sup>−</sup> (*r*<sup>2</sup> = ~0.68) and NO3 <sup>−</sup> (*r*<sup>2</sup> = ~0.77), which are both secondary inorganic species, indicating that OOA is driven by regional production mostly. In the RAG and EXG, POA (in this study = HOA) contributes more than 60% to the growth concentration of OA on average, which is significantly higher than the average proportion of POA in all growth periods (~55.8%) (Figure 8c). The high proportion of HOA is also reflected the important contribution of traffic sources to the accumulation of pollutants in the RAG and EXG. Contrary to the change in humidity, the average temperature before the EXG is the lowest among the three growth rates, followed by RAG (Figure 10b). Studies have shown that the decrease in temperature is conducive to the increase in POA mass concentration [42], which also partly explains the higher proportion of POA in the accumulated OA during the EXG. Therefore, among the various components of PM1, OA, especially POA, have become one of the most critical components for the EXG in Beijing during autumn and winter. In the GP, SOA (in this study = OOA) contributed 47.1% of OA in SLG. Furthermore, in the RAG and EXG, the contribution of SOA decreased to 39% and 34% on average. Figure 10 shows the mass spectra of average OA before and after GP and three growth ways. As illustrated, the intensities of hydrocarbon ion series of *m*/*z* 55 (mainly C3H3O+, C4H7 +), *m*/*z* 57 (mainly C3H5O+, C4H9 +), and *m*/*z* 43, characterized by a mass spectral pattern of HOA, show a significant increase after three growth ways. Among them, the growth intensity of EXG is higher than that of SLG and RAG. OOA is characterized by the prominent peak of *m*/*z* 44 (CO2 +). Before and after growth, the intensities of *m*/*z*

44 decreased significantly. In the EXG, the intensity of *m*/*z* 44 decreases from 11.3% to 10.2% of the total OA signals. Although the proportion of SOA is significantly lower than that of POA in GP, the proportion of SOA in GP is significantly higher than the average observed throughout the autumn and winter (31%). The research of Xu et al. [40] also showed that SOA plays an enhanced role during more severely polluted days. Therefore, in the process of pollution accumulation, SOA also plays a vital role in the increase in OA.

**Figure 10.** Mass spectra of average OA before and after GP (**a**), SLG (**b**), RAG (**c**), and EXG (**d**).

Secondary aerosol (SPM = SO4 <sup>2</sup><sup>−</sup> + NO3 <sup>−</sup> + NH4 <sup>+</sup> + OOA) is the most important component of PM1, accounting for ~71% of PM1 in autumn and winter in Beijing on average, and has a significant impact on atmospheric extinction [43]. During the GP, the contribution of SPM to the accumulated pollutants is still significantly higher than that of PPM (Chl + BC + HOA). However, from slow to rapid to explosive growth, the proportion of SPM in the increased PM gradually decreases (63% to 59% to 52%) (Figure 8b). During the EXG, the contribution of the PPM can reach to 48% in the increasing concentration of pollutants. PPM mainly comes from local biomass combustion, traffic and catering emissions, and its contribution to pollution accumulation cannot be underestimated. Figure 8d shows that, during the observation period, the proportions of SLG, RAG, and EXG are 71%, 21%, and 8%, respectively, in Beijing during observation. Compared to SLG and RAG, EXG mainly occurs in the quiet and stable atmosphere of higher humidity, lower pressure, lower temperature, small winds, and low MLH (Figure 10).
