## 4 Conclusions  
The purpose of this paper is to investigate the levels and sources of indoor and outdoor PM\({}_{10}\) in a refurbished school building within an urban environment. As studies of IAQ in energy retrofitted educational buildings are limited, our research aims to fill this gap by providing results for the case of a preliminary school retaining a GRS, located in a densely built area of Athens, Greece. Additionally, this study enhances literature on the investigation of coarse airborne particle sources within a school microenvironment. For this reason, an integrated 25-day experimental campaign took place from January to June 2017, divided into two periods (warm and cold). Measurements of PM\({}_{10}\) were implemented simultaneously on the GRS and in a classroom directly underneath the roof in order to compare mass concentration and chemical composition of the recorded particles between the two experimental sites. Some interesting conclusions are summarized as follows:  
* The location of the school, both source and climate wise, played a major role in formulating its indoor and direct outdoor air quality, as traffic and residential heating were found to be the most important sources, together with resuspension. * The existence of the GRS roof did not seem to have an impact on air quality. This is an expected result as this is the only building within many kilometers with vegetation on the roof. However, more experimental campaigns need to be implemented in order to estimate if a GRS can severely mitigate concentrations of particulate matter. * Concentrations of PM\({}_{10}\) remained at relatively low levels and did not exceed the daily limit of 50 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\), with a very few exceptions. Seasonal variations seem to affect results. * During the cold period, higher PM\({}_{10}\) concentrations were measured on the GRS, probably due to biomass burning, central heating systems and traffic. * On the other hand, during the warm period, PM\({}_{10}\) were found to be higher indoors, mainly because of resuspension of particles from human activities. * No significant correlation between ambient and internal PM\({}_{10}\) was found depicting the importance of indoor activities. * Concerning meteorological factors, elevated wind speed led to decreased levels of PM\({}_{10}\), while T and RH did not affect significantly the results. * OC + EC accounted for the larger percentage (51.92% indoors and 57.92% outdoors) of total measured PM\({}_{10}\). * Closed windows were related to elevated OC levels within the classroom because of resuspension. * On the contrary, EC concentrations were measured higher on the GRS, the I/O ratio being notably lower than unity, since EC is strongly associated with outdoor sources such as combustion processes. The calculation of the OC/EC ratio showed higher values indoors, depicting the influence of multiple possible indoor sources of organic materials, such as small pieces of paper, skin debris and clothing fibers. * The investigation of water soluble anions demonstrated that the use of cleaning products as well as resuspension and infiltration of SOA dominated the indoor air quality, while marine aerosols and SOA dominated the outdoor air quality. * The most abundant metals in both experimental areas were found to be the earth crust elements Fe and Al, while penetration of ambient air seemed to play a key role in the enrichment of the indoor environment in trace metals. * Source apportionment results highlighted six main factors as emission sources of PM\({}_{10}\) both indoors and outdoors. It seems that the largest percentage of contribution is related to combustion products from vehicles and biomass burning. The second most important source of pollution is associated with resuspension of particles in the classroom C3 due to activities of pupils. Furthermore, the contribution of asphalt residues from the brakes and tires of the vehicles is also important.