## 3 Results and Discussion  
### PM\({}_{10}\) Mass Concentration  
The mean daily PM\({}_{10}\) concentrations recorded on the GRS and within classroom C3 (including class hours), are illustrated, respectively, in Figure 2. However, it should be noted that a limitation of this study was that measurements of ambient PM\({}_{10}\) were implemented only on the GRS and the classroom beneath it. No control classroom (with a conventional roof above) was selected. Concentrations of PM\({}_{10}\) ranged between 21.88 and 72.02 ug m\({}^{-3}\) outdoors and from 21.11 to 50.92 ug m\({}^{-3}\) indoors. It was obvious that PM\({}_{10}\), both indoors and outdoors, did not exceed the daily limit of 50 ug m\({}^{-3}\) proposed by the European Directive 2008/50/EU [59], with the exception of the 2 nd of February (cold period). Thus, one may notice that the ambient environment of the school was not severely burdened by coarse airborne particles. Furthermore, seasonal changes seemed to have an important role on the results. During the cold period, 57.14% of ambient measurements were found to be higher on the GRS, mainly because of airborne suspended particles that were emitted as combustion products from fireplaces or other heating systems of the local dwellings. This is also noticed in the publication of [PERSON] and [PERSON] [60]. On the contrary, 63.64% of PM\({}_{10}\) recordings amid the warm period were found to be higher in classroom C3. Resuspension of particles due to indoor activities seemed to prevail for most cases. Ambient particulate matter can be directly emitted from vehicles of nearby busy streets or transported by dust transfer episodes, as [PERSON] and [PERSON] [61], [PERSON] et al. [62] and [PERSON] et al. [63], report. The significant role of seasonality could also be noticed by the calculation of the indoor to outdoor ratio (I/O) of PM\({}_{10}\). For the warm period, the mean I/O value was found to be 1.12, which demonstrates that PM\({}_{10}\) concentrations within classroom C3 were slightly higher with respect to GRS. On the other hand, the average I/O ratio for the cold period was 1.04, depicting the elevated levels of external PM\({}_{10}\) mainly because of the operation of local heating systems at this time of the year. Additionally, no significant correlation between indoor and outdoor PM\({}_{10}\) was found, with [PERSON]'s rho equal to 0.09 (\(p=0.69\)), probably because of different emission sources and daily window openings. These results are in accordance with the publication of [PERSON] et al. [64], as they reported a higher PM\({}_{10}\) I/O ratio during summer than amid winter at a school of Wroclaw. Regarding the meteorological conditions during the experimental period, the dominant wind direction was of NNW and NW direction (52% of the samples). For 32% of the measurements, the wind blew from the SW direction, while NE and SE directions were also recorded for the rest of the cases. Wind speed remained at relatively low levels for all experimental days with daily average  
Figure 2: Mean daily concentrations of PM\({}_{10}\) indoors and outdoors during the experimental period. values ranging from 0.5 to 8.7 km/h. Mean daily values of temperature and relative humidity ranged from 6.2 to 23.4 \({}^{\circ}\)C and from 48.3 to 94.3%, respectively. Table 1 summarizes all the investigated meteorological parameters. It should be noted that there was no specific pattern associating wind direction with high or low concentrations of PM\({}_{10}\) for the external measurements on the GRS of the building. All data refer to dominant wind directions, as the wind can change several times during the day. The wind speed was found to be negatively correlated with ambient PM\({}_{10}\) concentrations, with Spearman's rho coefficient being \(-0.56\) (\(p=0.03\)). Daily concentrations of ambient PM\({}_{10}\) were found elevated (above the respective exposure limit) only on the 2 nd of February when the average wind speed was relatively low (0.5 km/h).