## 3 Results and Discussion
### Various EC Fractions  
Figure 4 shows EC concentrations together with each EC fraction from EC1 to EC6. The black bars represent EC concentrations. EC1 to EC6 fractions are shown as red, blue, green, sky blue, white and yellow bars, respectively. When the sum from EC1 to EC6 is larger than EC, it is regarded as being overestimated than the actual amount of EC collected by the analysis process. The difference between EC and the sum from EC1 to EC6 is calculated to be PyC as explained earlier. As a matter of fact, PyC was observed to be sometimes prominent as can be seen in Figure 4. PyC values sometimes reached up to 50% of the sum from EC1 to EC6 fractions. However, the high PyC values were not unreasonable because the EC concentration was similar to eBC concentration measured when PyC was high. In this study, the dominant fractions of the EC turned out to be EC1, EC2 and EC3, which implies that carbon was oxidized mostly at relatively low temperature. Ambient organic aerosols may exist as highly viscous semi-solids or amorphous glassy solids [23]. High viscous state can result in the decrease in diffusion as can be inferred from the Stokes-Einstein equation, so that diffusional mixing can subdue. The suppressed diffusion can prevent the oxygen from contacting with the EC on filter. This is valid in ambient temperature range. Once the ambient organic aerosols are introduced into the analyzer, the temperature surrounding the aerosols increases to 310 \({}^{\circ}\)C at OC1 step in He phase, and further increases up to 870 \({}^{\circ}\)C at EC6 step in He-Ox phase. In these temperature ranges, most of the ambient organic aerosols are prone to be vaporized or volatilized. The viscosity of gaseous organic compounds increases as temperature increases. Consequently, the high viscous state may cause the inhibition of the oxygen from touching the EC at some high temperatures, for example, 850 \({}^{\circ}\)C in EC5 step, and 775 \({}^{\circ}\)C in EC4 step. This is why EC4 and EC5 fractions turned out to be minor fractions in this study. EC6 fractions outstood intermittently during the measurement period. The EC6 fraction is calculated  
Figure 3: Time series of comparison plot between EC and eBC concentrations. from the area of FID signal, where the oven temperature is maintained at 870 \({}^{\circ}\)C. This means that some portion of carbon fraction was reluctant to react with oxygen at some temperatures, but rather was oxidized at 870 \({}^{\circ}\)C. EC6 was excluded in the comparison study. Thus, we considered only EC1, EC2 and EC3 for comparison as follows. Figure 5 shows the correlation of EC fractions and their combinations with the EC mass concentration. It was possible to determine a dominant EC fraction in EC mass concentration. As shown in Figure 5a, EC1, EC2 and EC3 concentrations versus EC mass concentration together with linear regression fitting curves demonstrated the contribution of individual EC fraction to EC. Figure 5b presents EC1 + EC2 concentrations versus EC mass concentration, which show the effect of the carbon oxidation at relatively low temperature. Figure 5c shows EC1 + EC3 concentrations versus EC mass concentration. Figure 5d shows EC2 + EC3 concentrations versus EC mass concentration. Figure 5e shows EC1 + EC2 + EC3 concentrations versus EC mass concentration. The slope, \(y\)-intercept, their standard deviations and R\({}^{2}\) of each graph are shown in Table 2. The slope of linear fitting curve of EC1 versus EC was the lowest, which implies that EC1 contributed to EC less than EC2 and EC3 did. Among EC1, EC2 and EC3, the slope of linear fitting curve of EC3 versus EC was the highest, too. Therefore, EC3 turned out to be the dominant fraction for the quantification of EC in the present study. As can be seen in Figure 5b-d and Table 2, EC1 + EC3 versus EC exhibited the highest correlation, while EC1 + EC2 versus EC showed the lowest correlation.