## 3 Experimental Section
### Analytical Methods  
Nitrogen, Ar, O\({}_{2}\) and H\({}_{2}\) were analysed with a Shimadzu 15A gas-chromatograph equipped with Thermal Conductivity Detector (TCD) and a 9 m, 5A molecular sieve column. Methane and C\({}_{1}\)-C\({}_{4}\) alkanes and alkenes were analysed with a Shimadzu 14a gas-chromatograph equipped with Flame Ionization Detector (FID) and a 10 m long stainless steel column (\(\phi\) = 2 mm) packed with Chromosorb PAW 80/100 mesh coated with 23% SP 1700. The alkaline solution, separated from the solid precipitate by centrifugation at 4,000 rpm for 30 min, was used for the determination of: 1) CO\({}_{2}\) as CO\({}_{3}\)[2-] by titration with 0.5 N HCl solution; 2) SO\({}_{2}\) as SO\({}_{4}\)[2-], after oxidation with H\({}_{2}\)O\({}_{2}\), by ion-chromatography (Metrohm Compact 761); 3) HCl, as Cl' by ion-chromatography. The solid precipitate was oxidized by H\({}_{2}\)O\({}_{2}\) to determine H\({}_{2}\)S as SO\({}_{4}\)[2-] by ion-chromatography [63; 64]. The analytical error is \(<\)5%.  
The VOCs were pre-concentrated and transferred from the sampling vials into the column headspace of a Thermo Trace GC Ultra gas chromatograph by using a manual SPME (solid-phase micro-extraction) device introduced through the silicon membrane of the glass vial to expose the gas mixtures to a divinylbenzene (DVB)-Carboxen-polyddimethylsiloxane (PDMS), 50/30 \(\upmu\)m, 2 cm long fibre assembly (Supelco; Bellefonte, PA, USA) for 15 min [65]. The usefulness of the SPME method [66] for the VOC analysis has widely been demonstrated [67; 68; 69]. The DVB-Carboxen-PDMS fibre was selected by its high retentive properties, a feature that is particularly appropriate for analysis aimed to the determination of the organic compounds of interest for the present paper. A Thermo Trace GC Ultra gas chromatograph coupled with a Thermo DSQ Quadrupole Mass Spectrometer was used for analytical separation and detection. The mass spectrometer operated in full scan mode, in the mass range 40-400 \(m/z\). The transfer-line temperature was set at 230 \({}^{\circ}\)C. The mass detector was equipped with EI set at 70 eV. The source temperature was 250 \({}^{\circ}\)C. The gas chromatograph wasequipped with a split/splitless injection port operating in the splitless mode with a dedicated SPME liner (0.75 mm i.d.). analytes were desorbed from the SPME fiber through direct exposure for 2 min in the GC injection port, heated at 230 \({}^{\circ}\)C. The chromatographic column was a 30 m \(\times\) 0.25 mm i.d. 0.25 \(\upmu\)m film thickness TV1-MS fused silica capillary column (Thermo). The carrier gas was helium set to a flow-rate of 1.3 mL/min in constant pressure mode. The column oven temperature program was the following: 35 \({}^{\circ}\)C (hold 10 min), rate 5.5 \({}^{\circ}\)C/min to 180 \({}^{\circ}\)C (hold 3 min), rate 20 \({}^{\circ}\)C/min to 230 \({}^{\circ}\)C (hold 6 min) [65]. Compounds were identified by comparison of the mass spectra with those of the NIST05 library (NIST, 2005).  
The VOCs identified by mass spectrometry were quantified using an external standard calibration procedure performed on the basis of calibration curves created by analyzing gaseous standard mixtures of the main VOC groups, _i.e.,_ alkanes, alkenes, aromatics, cyclics, chlorofluorocarbons, ketones, aldehydes and heteroamatics. The values of the Relative Standard Deviation (RDS), calculated from seven replicate analyses of a gaseous mixture in which the compounds of interest were present at a concentration of 2 ppmv, are \(<\)7%. Eventually, the detection limits were determined by linear extrapolation from the lowest standard in the calibration curve using the area of a peak having a signal/noise ratio of 5 [68].