## 3 Results and Discussion
### Water-Soluble Anions  
All PM\({}_{10}\) filter samples were analyzed for six different anionic components (chloride, nitrite, nitrate, bromide, phosphate and sulfate). Only chloride (Cl\({}^{-}\)), nitrate (NO\({}_{3}\)\({}^{-}\)) and sulfate (SO\({}_{4}\)\({}^{2-}\)) were traceable. The contribution of all measured anions accounted for 6.95% of the total PM\({}_{10}\) within classroom C3 and 11.06% on the GRS, respectively, which is similar to those detected in the study of [PERSON] et al. (2018) in Bogota, Colombia. The significant contribution of the ambient emission sources (such as vehicles and heating systems from dwellings) to the indoor environment and, additionally, the presence of secondary inorganic particulates formed by the conversion of gaseous precursors were depicted. The most abundant anions were found to be SO\({}_{4}\)\({}^{2-}\) and NO\({}_{3}\)\({}^{-}\) followed by Cl\({}^{-}\). This is in accordance with the study of [PERSON] et al. (2018). Average concentrations along with standard deviations of indoor and outdoor anions per measuring period are summarized in Table 5. In the same Table, the results of other similar experiments are also presented. It is obvious that anionic concentrations of the examined building were significantly decreased compared to measurements in different schools of the greater Athens area (2018) and in a classroom close to a roadway within the city of Chennai, India (2018). However, the results are relatively comparable (especially for SO\({}_{4}\)\({}^{2-}\)) with those of a Portuguese school in Aveiro (2018) during the warm period. In general, concentrations of Cl\({}^{-}\) were relatively low both in the classroom C3 and the GRS during all experimental days. More precisely, during the cold period, mean indoor and outdoor concentrations of Cl\({}^{-}\) were found to be 0.40 \(\pm\) 0.25 ug m\({}^{-3}\) and 0.57 \(\pm\) 0.68 ug m\({}^{-3}\), whereas amid the warm period they reached 0.28 \(\pm\) 0.11 ug m\({}^{-3}\) and 0.08 \(\pm\) 0.10 ug m\({}^{-3}\), respectively. It is clear that for the cold days of the year, average concentration of Cl\({}^{-}\) on the GRS seemed to be slightly higher than that for the indoors. The presence of comparatively higher Cl\({}^{-}\) in the ambient environment is highly associated with transportation of marine aerosols from the sea, as [PERSON] et al. (2018) and [PERSON] et al. (2018) report. However, during warm days, an opposite effect was observed, with mean internal concentration of Cl\({}^{-}\) being 3.5 times higher than outdoors. Within the classroom, concentrations of Cl\({}^{-}\) were influenced both by particle resuspension from human activities and the daily use of cleaning products after the end of lessons. The key role of detergents (such as residual chlorine particles) on Cl\({}^{-}\) was also highlighted in the studies of [PERSON] et al. (2018), [PERSON] et al. (2018) and \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Location** & **OC/EC Indoors** & **OC/EC Outdoor** & \
\begin{tabular}{c} **OC** \\ **I/O** \\ \end{tabular} &
\begin{tabular}{c} **EC** \\ **I/O** \\ \end{tabular} \\ \hline Athens, Greece (This study) & 34.6 & 6.4 & 1.2 & 0.3 \\ Mira Loma, CA, USA (2018) & 7.4 & 5.0 & 1.4 & 0.8 \\ Osaka, Japan (2018) & 1.1 & 0.9 & 1.1 & 0.8 \\ Boston, MA, USA (2018) & 9.1 & 3.2 & 2.5 & 0.9 \\ Baltimore, MD, USA (2018) & 24.3 & 10.8 & 1.8 & 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Average ratios of organic carbon (OC) and elemental carbon (EC) concentrations in different sampling sites. [PERSON] et al. (2020). Thus, internal Cl\({}^{-}\) seemed to originate mainly from indoor sources rather than be influenced from external penetration. This was also supported from [PERSON]'s rho concerning indoor and outdoor Cl\({}^{-}\), which was calculated was 0.20 (\(p\) = 0.33) and demonstrated no significant correlation. Regarding NO\({}_{3}\)\({}^{-}\), a different behavior compared to Cl\({}^{-}\) was depicted. Concentrations were found to be relatively decreased for both periods.