## 3 Results and Discussion
### Chemical Composition of Indoor Particulate Matters  
This study focused on chemicals derived from geological and anthropogenic origins in which significant amounts were detected. In particular, the materials known to be related to automobiles such as vehicle exhaust, motor oil combustion, brake and tire wear, and abrasion of mechanical parts of vehicles, were carefully observed. While indoor PM\({}_{2.5}\) contained 59.4% of substances originating mainly from the earth crust (Al, Ca, Na, K, Fe, Si), anthropogenic substances (As, Ba, Cr, Cu, Pb, Ti, Zn) were analyzed with 2.7%, as summarized in Table 2. Substances such as sulfur (S) and chlorine (Cl) probably occurring from human activities as well as natural sources accounted for 33.7%. Sulfur, which is the most abundant element, was found maximum 1.417 \(\mathrm{\SIUnitSymbolMicro g/m^{3}}\) and 3.805 \(\mathrm{\SIUnitSymbolMicro g/m^{3}}\) in PM\({}_{2.5}\) of indoor and outdoor, respectively. Sulfur in urban atmosphere occurs primarily  
Figure 2: Mass concentration of particulate matter in test classrooms and outdoors. during combustion, but often also comes from heavy vehicles, long-distance pollutants and open burning. Therefore, the amount of sulfur is estimated to be closely related to the concentration of EC. Calcium (Ca) and chlorine (Cl) were found to be higher indoors than outdoors, indicating that, in addition to the infiltration from the outside, some emission sources are probably present in the classroom such as cleaning agents, paint and plastic debris [12]. Atmospheric Cl results usually from sea salt and decomposition of waste plastics. Metals of iron (Fe), lead (Pb), manganese (Mn) and zinc (Zn) were on average lower in the indoor PM\({}_{2.5}\), but the maximum concentration in some classrooms was as high as outdoor particulate matters. In particular, iron components arisen from various emission sources was highly found, up to 0.545 \(\mathrm{\SIUnitSymbolMicro m}^{3}\) in indoor PM\({}_{2.5}\). Table 3 shows the list of ion components discovered in PM\({}_{2.5}\). Ions such as NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), Cl\({}^{-}\), NH\({}_{4}\)\({}^{+}\) and K\({}^{+}\), formed mainly from anthropogenic sources and chemical reactions [17], were 5.03 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) in indoor PM\({}_{2.5}\) and 13.25 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) in outdoor. Whereas, ions from natural sources and physical processes such as Na\({}^{+}\), Mg\({}^{2+}\) and Ca\({}^{2+}\) were 0.98 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) in indoor PM\({}_{2.5}\) and 0.99 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) in outdoor. Despite similar amount of indoor natural ions to outdoor ions, the absolute amount of artificial ions was found more in outdoor PM\({}_{2.5}\). In particular, ions of nitrate and sulfate proved the automobiles as a major pollutant source. A typical urban cation, ammonium ion, also was abundant in outside. Significant amounts of ammonia are released from car exhausts due to excessive feed for the selective catalytic reduction (SCR) stage. The relative ratio of chlorine contained in indoor PM\({}_{2.5}\) (3.8%) to outdoor (1.7%) was more than two as discussed above, implying presence of indoor emission sources such as polymer debris, stationeries and cleaning agents.