## 3 Results and Discussion
### Source Apportionment Results  
Application of the PMF model for both indoor and outdoor measurements (50 samples in total) resulted in six factors (sources) contributing to the measured PM\({}_{10}\). Figure 5 presents the contribution of each chemical element to the respective factor, while Figure 6 illustrates the contribution of each factor to the total mass of the measured PM\({}_{10}\). Figure 5: Chemical profiles of 6 PMF factors. Figure 6: Contribution percentages of each factor to the total mass of PM\({}_{10}\). The first factor corresponded to secondary sulfate particles, as it was characterized by 65.95% of SO\({}_{4}\)\({}^{2-}\) and lower percentages of organic fractions. The formation of secondary SO\({}_{4}\)\({}^{2-}\) in the atmosphere was due to the oxidation reactions of the primary pollutant SO\({}_{2}\)[108]. Therefore, this chemical component of PM\({}_{10}\) was mainly related to particle transport episodes from more distant sources [109; 110]. This factor contributed 15.79% to the total of PM\({}_{10}\) sources. The second factor was a mixture of dust from soil elements and car residues and was characterized as traffic non-exhaust (Traffic NEX). In particular, the contribution of earth crust elements Mn and Fe [111] in this factor reached 40.84% and 33.26%, respectively, while heavy metals such as Pb, Cu and Cd originating mainly from brakes and tires of vehicles on the road [112] contributed 38.84%, 31.91% and 17.55%. These particles were usually resuspended. This factor presented a low contribution to the total PM\({}_{10}\) mass concentration (7.49%). The third factor was associated with the production of secondary NO\({}_{3}\)\({}^{-}\) (58.24%) with an additional contribution of EC (29.46%). This source was mainly due to primary NO\({}_{2}\), which was emitted into the atmosphere as a combustion product, transported over long distances and converted to NO\({}_{3}\)\({}^{-}\), as [PERSON] et al. [113] report. It is also worth noting that the existence of EC highlighted the presence of incomplete combustion products. This source demonstrated the lowest contribution to the total PM\({}_{10}\) (3.66%). It appeared mostly during the cold season, probably due to higher NO\({}_{2}\) emissions from cars and central heating from nearby dwellings, in combination with different meteorological conditions [114]. The fourth factor corresponded to a mixed source, as it was related to both vehicle exhaust and biomass burning (i.e., neighboring fireplaces (Biomass and Traffic EX)). In particular, the contributions of OC and EC to this factor were found to be 68.96% and 63.13%, respectively. The high percentage of OC combined with the elevated values of the OC/EC ratio in the external environment (Section 3.2) increased the probability of PM\({}_{10}\) emissions originating from biomass combustion processes, as the research of [PERSON] et al. [115] highlighted. Moreover, as [PERSON] et al. [116] and [PERSON] [117] reported, the high percentage of EC was associated with PM\({}_{10}\) emissions as products of incomplete combustion of gasoline or diesel in the engines of vehicles. This result was also strengthened by a 25.20% contribution of Cu found for this factor, a metal which is commonly associated with traffic emissions, as [PERSON] et al. [118] and [PERSON] et al. [119]. mention. It is important to note that this factor presented the highest contribution to the total PM\({}_{10}\) sources with a percentage of 30.30%. This means that about 1/3 of the total measured particulate matter originated from this particular source. Additionally, seasonality also played a key role in the results, as amid the cold period, the contribution of the fireplaces from the neighboring dwellings was intense. On the contrary, during the warmer days, the external average concentration was mainly due to the exhaust of vehicles. The fifth factor represented mainly indoor PM\({}_{10}\), which were resuspended in classroom C3 by the various activities of the students.