## 2 Site Description and Techniques for Data Analyses
### Gravimetric and Chemical Analysis  
Detailed description of chemical analysis methods could be found in [PERSON] et al. (2011). PM\({}_{2.5}\) daily samples were collected by using two sequential low-volume samplers (2.3 m\({}^{3}\)h\({}^{-1}\), EN12341), using two different substrates: Teflon collected by a Thermo ESM Andersen (FH95 SEQ) sampler and quartz fibre by an Explorer Plus (Zambelli) sampler. Each sampling starts at midnight. In total 100 Teflon substrate samples and 100 quartz substrate samples were collected from June to October 2012. Wind, atmospheric pressure, relative humidity, and temperature were measured from the meteorological station located close to the study site.  
After stabilizing for 48 h in controlled environment (constant humidity and temperature), all filters were weighted with an analytical balance (Sartorius CP2/F, reading precision 1 \(\upmu\)g) before and after sampling. The daily concentrations of quartz and Teflon substrates were similar (Pearson correlation coefficient of 0.87 and an average value of 15.1 \(\upmu\)g m\({}^{-3}\) and of 14.2 \(\upmu\)g m\({}^{-3}\) for Teflon and quartz substrates, respectively). Teflon substrates measurements showed lower uncertainty than those on quartz substrates, then they were taken as reference for PM\({}_{2.5}\).  
Teflon samples were used to measure water-soluble species (Na\({}^{+}\), K\({}^{+}\), Mg\({}^{2+}\) and Ca\({}^{2+}\), SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)-, NH\({}_{4}\)\({}^{+}\), Cl\({}^{-}\)), water-soluble inorganic carbon (WSIC) and water-soluble organic carbon (WSOC). Two aliquots of 5 mL of deionized water were used to extract the soluble ionic species by ultrasonic agitation for 20 min. Chemical analysis was performed using a Dionex DX500 (Dionex, Sunnyvale, CA, USA) Ion Chromatography system, and the method is reported in [PERSON] et al. (2011).  
WSOC and WSIC were measured using a Shimadzu TOC analyser (model 5050, Shimadzu corporation) with high-temperature (680 \({}^{\circ}\)C) Pf-catalysed oxidation coupled to non-dispersive infrared (NDIR) gas detection of CO\({}_{2}\)([PERSON] et al., 2011).  
Quartz samples were used for the determination of 16 metal concentrations and OC and EC measurements. 8 elements (Ni, Cu, V, Mn, As, Pb, Cr, Sb) were analysed by Graphite Furnace Atomic Absorption Spectroscopy (GF-AAS, Perkin Elmer Analyst 600 System) and 8 elements (Ca, Mg, Na, K, Fe, Al, Zn, and Ti) by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Varian Liberty 110 spectrometer). 1 mL of H\({}_{2}\)O\({}_{2}\), 2 mL of HF and 4 mL of HNO\({}_{3}\) were used for acid digestion in a MILESTONE MLS 1200 MEGA (FKV) microwave oven. 0.7 g of H\({}_{3}\)BO\({}_{3}\) was added after cooling to complex fluoride ions and to dissolve fluoride salts ([PERSON] et al., 2011).  
Organic/Elemental carbon (OC and EC) were analysed by a carbon aerosol analyser (Sunset Laboratory Inc., Tigard, OR, USA) ([PERSON] et al., 2011) using NIOSH protocol ([PERSON] et al., 2011). The carbonaceous fraction of PM\({}_{2.5}\) was detected on a 1.5 cm\({}^{2}\) quartz substrate punch. Uncertainties in EC and OC measurements were of about 5% \(\pm\) 0.2 \(\upmu\)g cm\({}^{-2}\). For sampling of quartz substrate a denuder was not used ([PERSON] et al., 2011; [PERSON] et al., 2011).