## 3 Results and Discussion
### Organic and Elemental Carbon  
Carbonaceous species are considered to be major components of PM\({}_{10}\). As [PERSON] and [PERSON] [67] report, EC can be emitted as a product of incomplete combustion (e.g., diesel), while OC is mainly produced from anthropogenic or biogenic sources, either directly or formed in the atmosphere. In the present study, differences between internal and external concentrations of OC were found to be relatively balanced. More specifically, OC and EC accounted for 51.92% of total PM\({}_{10}\) within classroom C3 and 57.92% on the GRS, respectively, consistent with the results of [PERSON] et al. [68]. Mean concentrations, along with standard deviations, of indoor and outdoor OC and EC are summarized in Table 3. In classroom C3, the average concentration of OC (16.53 \(\pm\) 7.70 \(\upmu\)g m\({}^{-3}\)), was slightly lower than the respective outdoor value (16.67 \(\pm\) 9.29 \(\upmu\)g m\({}^{-3}\)), while the mean value of EC on the GRS (2.78 \(\pm\) 1.62 \(\upmu\)g m\({}^{-3}\)) was almost 4.5 times higher than the respective indoor value (0.60 \(\pm\) 0.44 \(\upmu\)g m\({}^{-3}\)) due to local combustion activity. Figure 3 demonstrates that in 60% of the experimental days, OC concentrations in classroom C3 were elevated compared to outdoors (GRS). This result is related to resuspension of airborne coarse particles due to activities of pupils in the classroom (especially during the breaks), with possible internal emitting sources including small pieces of paper, skin debris and clothing fibers. These sources are also reported in the publication of [PERSON] et al. [69], referring to similar experiments in Portuguese schools. It should also be noted that closed windows seemed to be related with slightly higher concentrations of OC in classroom C3, as the mean internal value for the cold days was measured at 17.64 \(\upmu\)g m\({}^{-3}\). During the warmer days, the frequent opening of the windows led to a decreased mean value of OC (15.13 \(\upmu\)g m\({}^{-3}\)). Additionally, during the cold period, for some cases, such as the 2 nd of February, OC was found to be higher in the ambient environment than indoors, as expected. Elevated external concentrations of OC (such as the 31 st of May) during the warm period could be related to the decomposition of plants on the GRS along with increased traffic emissions from nearby busy streets. For all measurements, the mean I/O ratio for the OC concentrations was found to be 1.2, indicating the importance of indoor activities (Table 4). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Spearman’s Rho**} & \multicolumn{1}{c}{**PM\({}_{\mathbf{10}}\) (GRS)**} & \multicolumn{1}{c}{**T**} & \multicolumn{1}{c}{**RH**} & \multicolumn{1}{c}{**WS**} \\ \hline \multirow{3}{*}{PM\({}_{10}\) (GRS)} & Correlation Coefficient & 1.00 & & & \\  & Sig. (2-tailed) & - & & & \\  & N & 25.00 & & & \\ \hline \multirow{3}{*}{T} & Correlation Coefficient & 0.16 & 1.00 & & \\  & Sig. (2-tailed) & 0.45 & - & & \\  & N & 25.00 & 25.00 & & \\ \hline \multirow{3}{*}{RH} & Correlation Coefficient & 0.13 & \(-\)0.48 \({}^{*}\) & 1.00 & \\  & Sig. (2-tailed) & 0.53 & 0.01 & - & \\  & N & 25.00 & 25.00 & 25.00 & \\ \hline \multirow{3}{*}{WS} & Correlation Coefficient & \(-\)0.56 \({}^{**}\) & \(-\)0.33 & 0.05 & 1.00 \\  & Sig. (2-tailed) & 0.00 & 0.11 & 0.80 & - \\ \cline{1-1}  & N & 25.00 & 25.00 & 25.00 & 25.00 \\ \hline \end{tabular} \({}^{**}\) Correlation is significant at the 0.01 level (2-tailed); \({}^{*}\) correlation is significant at the 0.05 level (2-tailed).  
\end{table} 
Table 2: Spearman correlations of ambient (GRS) PM\({}_{10}\), air temperature (T), relative humidity (RH) and wind speed (WS) for all experimental days. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Carbonaceous** & **Classroom C3** & **GRS** \\
**Species** & **(\(\upmu\)g m\({}^{-3}\))** & **(\(\upmu\)g m\({}^{-3}\))** \\ \hline EC & 0.60 \(\pm\) 0.44 & 2.78 \(\pm\) 1.62 \\ OC & 16.53 \(\pm\) 7.70 & 16.67 \(\pm\) 9.29 \\ \hline \hline \end{tabular}
\end{table}