## 3 Results and Discussion
### Characteristics of the Four Pollution Episodes  
The haze episode definition in this study referred to a previous study by [PERSON], [PERSON] [37]. Briefly, a complete haze episode is defined as the mass concentration of PM\({}_{2.5}\) starting from less than 50 \(\upmu\)g\(\cdot\)m\({}^{-3}\), up to a peak of more than 100 \(\upmu\)g\(\cdot\)m\({}^{-3}\) and subsequently ending at less than 50 \(\upmu\)g\(\cdot\)m\({}^{-3}\) again. We obtained four episodes that were 20-24 December 2017 (PE1), 24 December 2017-3 January 2018 (PE2), 3-8 January 2018 (PE3), and 10-17 January 2018 (PE4), respectively. Notably, PE4 was not a complete pollution episode without the concentration of PM\({}_{2.5}\) decreasing to less than 50 \(\upmu\)g\(\cdot\)m\({}^{-3}\) due to failure sampling after January 17. The mean mass concentrations of PM\({}_{1}\), PM\({}_{2.5}\) and PM\({}_{10}\) in the four pollution episodes were in the range of 59.69 to 192.32 \(\upmu\)g\(\cdot\)m\({}^{-3}\), 79.35 to 232.37 \(\upmu\)g\(\cdot\)m\({}^{-3}\), 33.78 to 122.34 \(\upmu\)g\(\cdot\)m\({}^{-3}\), and 59.85 to 257.19 \(\upmu\)g\(\cdot\)m\({}^{-3}\), respectively (Figure 2). According to durations and the peak values of PM\({}_{2.5}\) (Table 1), the four PEs can be categorized into two short evolution pattern events (lasting for 5-6 days with relatively lower peak PM\({}_{2.5}\) concentration for PE1 and PE3) and two long evolution pattern events (lasting for approximately 10 days with higher peak PM\({}_{2.5}\) concentration for PE2 and PE4). The PM\({}_{2.5}\) to PM\({}_{10}\) ratio showed the same increase tendency to all four PEs as the elevation of PM\({}_{2.5}\) (from 35.2% to 60.0%), which suggested the predominant contribution of fine particles on the developing of haze pollution. However, compared to a previous study in Beijing [37], the duration of the long evolution pattern was longer (11 vs. 9), but the peak concentrations of PM\({}_{2.5}\) were lower than in Beijing (250 vs. 310 \(\upmu\)g\(\cdot\)m\({}^{-3}\)). This was most likely attributed to the different key chemical compositions and atmospheric chemistry processes between the two cities, especially in the extensity of secondary inorganic and organic formation. Figure 2: Concentration of particle mass in PM\({}_{1}\), PM\({}_{2.5}\) and PM\({}_{10}\). In order to obtain more information on the four PEs, variation in the growth rate of size-dependent PM in each episode was calculated (Table 1). All three size fractions increased and were comparable in PE1 (25.0, 17.3 and 14.5 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\) for PM\({}_{1}\), PM\({}_{1-2.5}\) and PM\({}_{2.5-10}\), respectively), while a predominant increase appeared for PM\({}_{1}\) (25.2 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)) in PE2. The air mass wiggled between the south of Henan province and the south of Hebei province on 22-23 December in PE1 (Figure S2a), which facilitated the accumulation of PMs transported in the study area. However, the relatively clear and humid air mass from the south on December 27 and stable meteorological conditions appeared on the following December 28 in PE2, which most likely pointed to a heterogenous reaction under high local primary emission (Figure S2b). On the contrary, a predominant increase rate was observed for PM\({}_{1}\) (11.6 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)) in PE3, which may be attributed to the aerosol merging effect in ice crystal. Differently, for PE4, the growth rate of PM\({}_{2.5}\) (26.0 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)), especially for PM\({}_{1-2.5}\) (23.9 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)) was significantly higher than that of PM\({}_{10}\) and PM\({}_{1}\). This was typically a feature of the secondary reaction and hygroscopic growth of PMs under stagnant conditions with high RH (58.2%) and low WS (0.82 \(\mathrm{m\cdot s^{-1}}\)) (Table S1). The detailed reasons for this will be discussed in Section 3.3. Besides all four PEs, a lasting three days of snowfall was observed on 2-4 January. Subsequently, the PM\({}_{2.5-10}\) mass decreased by 75%.