## 3 Results and Discussion
### OC and EC Fractions
#### 3.3.2 Effect of Photochemical Reaction and Hygroscopic Growth in PE3 and PE4  
Different to PE1 and PE2, the RH was relatively higher in PE3 and PE4. Carbonaceous components were predominantly in PM\({}_{1}\), accounting to 37% and 39% in the day with peak PM\({}_{1}\) mass concentration in PE3 and PE4, respectively. For PE3, it started after a snowfall. The temperature was below 0 \({}^{\circ}\)C  
Figure 4: The percentage contributions (to TC) of eight carbon fractions in PM\({}_{1}\), PM\({}_{2.5}\) and PM\({}_{10}\) when PM\({}_{2.5}\) exceeded 100 \(\mu\)g\(\cdot\)m\({}^{-3}\) during the pollution formation of the four PEs.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Sizes** & **PE1** & **PE2** & **PE3** & **PE4** \\ \hline \multirow{2}{*}{Average growth rate of OC (\(\mu\)g\(\cdot\)m\({}^{-3}\)-day\({}^{-1}\))} & PM\({}_{1}\) & 5.18 & 3.67 & 2.27 & 1.75 \\  & PM\({}_{2.5}\) & 8.02 & 3.47 & 1.54 & 3.99 \\  & PM\({}_{10}\) & 9.31 & 2.89 & 1.70 & 3.27 \\ \hline \multirow{2}{*}{Average growth rate of SOC (\(\mu\)g\(\cdot\)m\({}^{-3}\)-day\({}^{-1}\))} & PM\({}_{1}\) & 2.65 & 1.46 & 0.27 & 0.12 \\  & PM\({}_{2.5}\) & 3.47 & 1.93 & 0.60 & 2.27 \\  & PM\({}_{10}\) & 5.03 & 1.58 & 0.99 & 1.66 \\ \hline \multirow{2}{*}{Average growth rate of POC (\(\mu\)g\(\cdot\)m\({}^{-3}\)-day\({}^{-1}\))} & PM\({}_{1}\) & 2.53 & 2.22 & 1.19 & 1.63 \\  & PM\({}_{2.5}\) & 4.55 & 1.54 & 0.94 & 1.72 \\  & PM\({}_{10}\) & 4.64 & 1.30 & 0.71 & 1.61 \\ \hline \multirow{2}{*}{Average growth rate of PC (\(\mu\)g\(\cdot\)m\({}^{-3}\)-day\({}^{-1}\))} & PM\({}_{1}\) & 4.27 & 4.21 & 1.67 & 2.42 \\  & PM\({}_{2.5}\) & 7.36 & 9.85 & 1.29 & 4.67 \\ \cline{1-1}  & PM\({}_{10}\) & 7.27 & 7.27 & 0.99 & 4.27 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The growth rate of OC, SOC, POC and PC of PM\({}_{1}\), PM\({}_{2.5}\) and PM\({}_{10}\) in 4 PEs.  
and RH was 84%. In ice dominant condition, carbon components as ice cores could grow through ice collision, leading to aerosol aging [47]. This maybe the reason we overserved such high contribution of TC to PM\({}_{1}\) mass. However, the situation changed in PM\({}_{1-2.5}\), with only 2% contribution to PM increasement. Considering high RH and more radiation in clear air, a strong photochemical reaction or heterogenous reaction with SO\({}_{2}\) or NO\({}_{2}\) took place, which contributed to the fast increase of PM\({}_{2.5}\).  
In PE4, carbonaceous components contributed to 13% PM\({}_{2.5}\) increasement. Although it was higher than that of PE3, it was 46-63% lower than that of PE1 and PE2. Moreover, the SOC growth rates were predominantly in PM\({}_{2.5}\) with a value of 2.27 \(\mathrm{\mu}\)g\(\cdot\)m\({}^{-3}\)day\({}^{-1}\) (Table 3). Although an increase in SOC dominance occurred in PM\({}_{2.5}\), the low percentile of SOC to PM\({}_{2.5}\) did not seem to contribute to such an explosive growth in PM\({}_{2.5}\) mass. Other components should contribute more in PE4, such as secondary inorganic aerosol through heterogenous reaction and hygroscopic growth. A previous study revealed that the particle size rapidly increased when the relative humidity exceeded 50% [48]. The liquid water not only satisfied the demand for the hygroscopic growth of PM, but also facilitated heterogeneous reactions for forming more secondary aerosols, and subsequently accelerating hygroscopic growth, eventually accounting for 48% of the total secondary organic aerosols [49]. In PE4, the average wind speed was 0.82 m\(\cdot\)s\({}^{-1}\) and the average relative humidity was 58.2%. Long-term stagnant and high-humidity meteorology conditions promoted the non-carbonaceous components to grow into PM\({}_{1-2.5}\) through hygroscopic growth. In summary, these above clues strongly point to the influence of hygroscopic growth of PM and the heterogeneous secondary reaction from non-carbonaceous components on the formation of haze in suitable RH condition.