## 3 Results and Discussion
### PyC, Split Time and EC Concentration  
PyC can be formed due to various reasons. PyC is present mainly in the He-Ox phase, where, pre-oxygen (Pre-Ox) split time is defined. In this study, the Pre-Ox split time ranged from 330 s to 390 s. PyC may be exceptionally created in the He phase. There are several reasons why PyC is created in the He phase. Oxygen may leak in the He phase due to \begin{table}
\begin{tabular}{c c c c} \hline  & **Slope** & **y-int** & **R\({}^{2}\)** \\ \hline EC1 vs. EC & 0.047 \(\pm\) 0.006 & 0.160 \(\pm\) 0.010 & 0.496 \\ EC2 vs. EC & 0.302 \(\pm\) 0.039 & \(-\)0.009 \(\pm\) 0.060 & 0.519 \\ EC3 vs. EC & 0.860 \(\pm\) 0.077 & 0.022 \(\pm\) 0.119 & 0.688 \\ EC1+EC2 vs. EC & 0.349 \(\pm\) 0.038 & 1.151 \(\pm\) 0.059 & 0.601 \\ EC2+EC3 vs. EC & 1.162 \(\pm\) 0.112 & 0.013 \(\pm\) 0.173 & 0.657 \\ EC1+EC3 vs. EC & 0.906 \(\pm\) 0.077 & 0.182 \(\pm\) 0.118 & 0.715 \\ EC1+EC2+EC3 vs. EC & 1.209 \(\pm\) 0.111 & 0.174 \(\pm\) 0.171 & 0.679 \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of linear regression on the EC fractions and their combination versus EC concentrations. Figure 5: The relationship between EC fractions and EC concentration: (**a**) EC1 vs. EC, EC2 vs. EC and EC3 vs. EC; (**b**) EC1 + EC2 vs. EC; (**c**) EC1 + EC3 vs. EC; (**d**) EC2 + EC3 vs. EC; (**e**) EC1 + EC2 + EC3 vs. EC. EC. electrical issues or the malfunction of sensors installed in the OC/EC analyzer. Loose fitting of connectors and extremely small amount of oxygen in the helium gas are potential causes of Pre-Ox split time that may constantly provide oxygen to a sample cell during the He phase and consequently are regarded as instrument-specific issues [24]. Another reason may be that the presence of metal salts in the samples caused the Pre-Ox split time issue. It was reported that metal salts reduced the temperature required to make EC [19]. Thus, metal salts may enhance the charring of OC. Post-oxygen (Post-Ox) split time ranged from 440 s to 500 s in this study, where 2% of oxygen is included in the He base environment. No split time is defined between 390 s and 440 s because the He phase is changed into the He-Ox phase. It is interesting to note that negative PyC exists sometimes. Figure 6a shows the split times at various EC concentrations. When EC concentration was higher than 3 ug/m\({}^{3}\), only Post-Ox split times were present. When EC concentration was lower than 3 ug/m\({}^{3}\), however, Pre-Ox split times were present together with Post-Ox split times between 300 s to 500 s. In addition, Pre-Ox split times were clearly separated with Post-Ox split times due to mode change from He phase to He-Ox phase. In Figure 6b, PyC is plotted as a function of split time. As for Pre-Ox split times, PyC was nearly zero or negative. As for Post-Ox split times, however, PyC showed non-zero values at high split times. This trend was observed in the other study [25]. Figure 6c shows that negative PyC was observed at EC concentration lower than 3 ug/m\({}^{3}\). This is reasonable because negative PyC can be observed only at Pre-Ox split times and Pre-Ox split times are present only for EC concentration lower than 3 ug/m\({}^{3}\) in the present study as shown in Figure 6a. Split time of 360 s corresponds to Pre-Ox split time and only OC is involved in the analysis of carbonaceous aerosols. In contrast, 450 s and 495 s are involved in the analysis of EC1 and EC2, respectively. The amount of carbon oxidation in EC1 and EC2 states are small as inferred from Figure 2b. Therefore, EC1 and EC2 did not contribute to EC concentration much. However, it is obvious that the amount of carbon oxidation in EC3 is larger than those in EC1 and EC2 as can be seen in Figure 2b. It is noted that the oxidation quantity is dramatically increased at 700 \({}^{\circ}\)C as shown in Figure 2b. This is why EC3 mainly contributed to EC concentration. As for Post-Ox split time as shown in Figure 6b, the longer the split time, the larger amount of PyC was observed. As explained in Figure 6b, PyC was correlated with the split time.