## 3 Results and Discussion
### Carbon Content  
Figure 3 shows carbon concentrations in indoor PM\({}_{2.5}\) for each school. The total carbon fraction (OC and EC) accounted for average 45.4 \(\pm\) 10.9% of the PM\({}_{2.5}\) mass indoors, which was much higher than outdoor (19.3 \(\pm\) 7.6%). The carbon component found predominantly both in indoor and outdoor PM\({}_{2.5}\) was OC. Average OC concentration inside the classrooms for all the test schools was 9.47 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), contributing 41.5% of PM\({}_{2.5}\). In particular, OC adsorbed on fine particles contains very harmful components including polyaromatic carbons (PAHs) [18]. Thus, more precise studies on OC through living lab tests are further required to verify the profound chemical composition. While EC is generally thought to not undergo chemical transformation once emitted, organic carbon aerosols can be produced indoors through chemical transformation in addition to a variety of primary sources. While EC exists mostly in PM\({}_{2.5}\); 0.91 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) for PM\({}_{2.5}\) and 0.93 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) for PM\({}_{10}\), OC is distributed from fine to large particles; 9.47 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) for PM\({}_{2.5}\) and 12.76 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) for PM\({}_{10}\). Thus, student activity in the classroom is one of  
Figure 3: Carbon concentration of indoor PM\({}_{2.5}\) for each school. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Species**} & \multicolumn{4}{c}{**Indoor (No. of Sample = 25)**} & \multicolumn{4}{c}{**Outdoor (No. of Sample = 8)**} \\ \cline{2-9}  & **Mean** & **SD** & **Min** & **Max** & **Mean** & **SD** & **Min** & **Max** \\ \hline F\({}^{-}\) & 0.13 & 0.07 & 0.04 & 0.10 & 0.04 & 0.01 & 0.03 & 0.06 \\ Cl\({}^{-}\) & 0.24 & 0.21 & 0.09 & 1.12 & 0.25 & 0.25 & 0.07 & 0.86 \\ NO\({}_{3}\)\({}^{-}\) & 2.20 & 1.44 & 0.39 & 6.47 & 7.31 & 8.77 & 2.26 & 30.17 \\ PO\({}_{3}\)\({}^{-}\) & 0.09 & 0.08 & 0.01 & 0.27 & 0.08 & 0.09 & 0.00 & 0.25 \\ SO\({}_{4}\)\({}^{-}\) & 1.84 & 1.02 & 0.62 & 5.84 & 3.21 & 2.89 & 1.69 & 10.84 \\ Ca\({}^{2+}\) & 0.53 & 0.25 & 0.24 & 1.20 & 0.52 & 0.18 & 0.30 & 0.90 \\ K\({}^{+}\) & 0.14 & 0.06 & 0.05 & 0.27 & 0.18 & 0.09 & 0.10 & 0.40 \\ Mg\({}^{2+}\) & 0.06 & 0.05 & 0.01 & 0.24 & 0.06 & 0.02 & 0.04 & 0.12 \\ Na\({}^{+}\) & 0.39 & 0.38 & 0.08 & 2.08 & 0.41 & 0.34 & 0.17 & 1.29 \\ NH\({}_{4}\)\({}^{+}\) & 0.61 & 0.53 & 0.03 & 2.12 & 2.30 & 2.93 & 0.50 & 9.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of ion components in PM\({}_{2.5}\). the key factors influencing indoor OC levels. At the same time, various mechanisms of formation and decay cause large differences between classrooms even in the same school. For example, school E-2 showed 8.13 ug/m\({}^{3}\) at classroom 1-1 and 12.84 ug/m\({}^{3}\) at classroom 2-5, indicating 39.1 \(w\)/\(w\)% and 53.3 \(w\)/\(w\)%, respectively. The PM\({}_{2.5}\) sampled at school E-4 contained relatively high amounts of carbon (49.3-61.1%, averaging 56.6%). Sampling at school, E-4, was conducted in the first week of December, early winter, and seasonal variation might also influence the chemical composition. Above all, the school, E-4, is near the intersection of two main roads and one of the busiest stations in Seoul crossing the two lines. These seasonal and locational factors greatly dominate the carbon concentration in the classroom fine particulate matters. On the other hand, the absolute amount of OC was approximately 3.5 to 10 times higher than EC in both indoor and outdoor PM\({}_{2.5}\) as seen in other references [19; 20; 21]. The average OC/EC ratio for 25 classrooms in this study was 11.9, as already estimated from Figure 2. This ratio was obviously higher than in the outdoor (5.3). This relative value was high in E-1 showing 15.1, and least in E-2 (9.7).