## 2 Materials and Methods
### Experimental Equipment  
In situ measurements were implemented indoors and outdoors with the use of portable equipment. More specifically, within classroom C3, a SKC sampler was installed, operating at 4 L/min and equipped with a size-selective inlet for PM\({}_{10}\). Air pollutants were collected on quartz filters of 37 mm diameter. On the GRS, a low volume controlled flow ratio (2.3 m\({}^{3}\)/h) sampler (Derenda LVS 3.1/PNS 3.1-15) was used for outdoor sampling, based on the European standard method EN 12341:2014 [52]. Particles were collected on 47 mm preconditioned quartz filters, which were protected in plastic holders before and after sampling. Each filter corresponds to a daily measurement. Particle mass concentration was conducted gravimetrically using an electronic microbalance (Mettler Toledo MX-5), with a resolution of \(10^{-6}\) g, within a weighing room under controlled climatic conditions (temperature \(20\pm 1\)\({}^{\circ}\)C and relative humidity \(50\pm 5\%\)). Moreover, the instrumentation also included ambient temperature and relative humidity sensors (Tinytag Plus 2 thermo-hygrometers with a range from \(-25\) to \(85\)\({}^{\circ}\)C for temperature and from 0 to 100% for relative humidity and accuracy of \(\pm 0.6\)\({}^{\circ}\)C and \(\pm 3.0\%\), respectively). It should be noted that daily values of wind speed and direction were obtained by a local meteorological station (\(37^{\circ}57^{\prime}00^{\prime\prime}\)N, \(23^{\circ}43^{\prime}00^{\prime\prime}\)E). For organic/elemental carbon (OC/EC) analysis, a thermo-optical ECOC Sunset Laboratory analyzer was used. In the analyzer, samples were thermally desorbed from the filter medium under an inert helium atmosphere followed by an oxidizing atmosphere using carefully controlled heating lamps. A flame ionization detector (FID) was used to monitor the analysis. More specifically, the method for measuring OC and EC in ambient particulate matter samples deposited on filters was based on the volatilization and oxidation of carbon-containing PM components. After the quantification of the carbonaceous gases released, an optical correction for the charring of OC to EC in the process (the thermal-optical method) occurred. The described procedure is a thermal-optical transmittance/reflectance (TOT/TOR) method [53]. For the analysis, tissue quartz filters were used, while the EUSAAR2 protocol was followed. The instrument's limit of detection (LOD) was 0.2 ugC cm\({}^{-2}\), and the analytical uncertainty was equal to \(\pm\) (concentration \(\times\) 0.05) + instrument blank concentration. Water-soluble anions (CI-, NO\({}_{3}\)-, NO\({}_{2}\)-, Br-, PO\({}_{4}\)\({}^{3-}\) and SO\({}_{4}\)\({}^{2-}\)) were investigated using suppressed ion chromatography (IC). In particular, a DIONEX DC ICS-5000 system was used for the evaluation of ion concentrations, since it offers a full range of Reagent-Free(tm) IC (RFIC(tm)) components. RFIC-EG combines automated eluent generation and self-regenerating suppression. Regarding the chemical process elaborated, a tissue quartz filter was used for ultrasonic extraction, using 6 mL of Milli-Q water and 0.5 mL isopropanol. Then, the solution was injected to the ionic chromatographer  
Figure 1: Experimental sites of the school: (**a**) the green roof system (GRS) and (**b**) classroom C3. The LOD ranged between 0.01 and 0.11 \(\upmu\)g mL\({}^{-1}\). Uncertainty (95%, k = 2) ranged between 5.61% and 9.31%, respectively. PM sample analysis of metals (Al, As, Ba, Cd, Co, Cr, Cs, Cu, Fe, Mn, Ni, Pb, Sr, Tl, V and Zn) was performed according to the procedure described by [PERSON] et al. [54]. Samples were digested using a mixture of HNO\({}_{3}\) (suprapur 65%) and H\({}_{2}\)O\({}_{2}\) (suprapur 30%) in a microwave digestion system (CEM; Matthews NC USA). All digested samples were analyzed through inductively coupled plasma mass spectrometry (ICP-MS) by a Thermo Scientific ICAP Qc (Waltham, MA, USA).