_Applied_  
_sciences_  
_Article_  
**Chemical Composition and Source Apportionment of PM\({}_{\mathbf{10}}\) in a Green-Roof Primary School Building**  
**[PERSON] 1\({}^{\copyright}\)**, **[PERSON] 2\({}^{\copyright}\)**, **[PERSON] 3\({}^{\copyright}\)**, **[PERSON] 2\({}^{\copyright}\)**, **[PERSON] 4, **[PERSON] 3, **[PERSON] 2 and [PERSON] 1\({}^{,,}\)**\({}^{\copyright}\)  
1 Department of Applied Physics, Faculty of Physics, University of Athens, Building Physics 5, University Campus, 15784 Athens, Greece; [EMAIL_ADDRESS]  
2 Atmospheric Chemistry & Innovative Technologies Laboratory, Institute of Nuclear & Radiological Sciences & Technology, Energy & Safety, National Centre for Scientific Research \"Demokritos\", Ag. 15310 Attiki, Greece; [EMAIL_ADDRESS] (D.S.); [EMAIL_ADDRESS] (T.M.); [EMAIL_ADDRESS] (K.B.)  
3 Laboratory of Environmental Chemistry, Department of Chemistry, Section III, National and Kapodistrian University of Athens, Panepistimiopolis, 15784 Athens, Greece; [EMAIL_ADDRESS] (S.K.); [PERSON]ADDRESS] (A.S.)  
4 Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Lofos Koufou, 15236 Athens, Greece; [EMAIL_ADDRESS]  
* Correspondence: [EMAIL_ADDRESS]; Tel.: +30-2107276847  
Received: 9 November 2020; Accepted: 25 November 2020; Published: 27 November 2020  
**Featured Application: Introductory Article of the MDPI Special Issue \"New Challenges for Indoor Air Quality\".**  
**Abstract:** Research on air quality issues in recently refurbished educational buildings is relatively limited. However, it is an important topic as students are often exposed to high concentrations of air pollutants, especially in urban environments. This study presents the results of a 25-day experimental campaign that took place in a primary school located in a densely built-up area, which retains a green roof system (GRS). All measurements refer to mass concentrations and chemical analysis of PM\({}_{10}\) (particulate matter less than 10 micrometers), and they were implemented simultaneously on the GRS and within the classroom (C3) below during different periods of the year. The results demonstrated relatively low levels of PM\({}_{10}\) in both experimental points, with the highest mean value of 72.02 \(\upmu\)g m\({}^{-3}\) observed outdoors during the cold period. Elemental carbon (EC) was also found be higher in the ambient environment (with a mean value of 2.78 \(\upmu\)g m\({}^{-3}\)), while organic carbon (OC) was relatively balanced between the two monitoring sites. Moreover, sulfate was found to be the most abundant water soluble anion (2.57 \(\upmu\)g m\({}^{-3}\)), mainly originating from ambient primary SO\({}_{2}\) and penetrating into the classroom from windows. Additionally, the crustal origin of particles was shown in trace metals, where Al and Fe prevailed (9.55% and 8.68%, respectively, of the total PM\({}_{10}\)). Nevertheless, infiltration of outdoor particles within the classroom was found to affect indoor sources of metals. Finally, source apportionment using a positive matrix factorization (PMF) receptor model demonstrated six main factors of emissions, the most important of which were vehicles and biomass burning (30.30% contribution), along with resuspension of PM\({}_{10}\) within the classroom from human activities (29.89% contribution). Seasonal variations seem to play a key role in the results.  
**Keywords:** particulate matter; indoor air quality; schools; source apportionment; green roof