## 2 Results and Discussion
### VOC Composition  
Up to 129 different non-methane VOCs, pertaining to the alkane (27 compounds), aromatic (21 compounds), cyclic (17 compounds), alkene (15 compounds), _Cl_-bearing (13 compounds), _O_-bearing (ketones, aldehydes, organic acids and alcohols; 36 compounds) and heteroaromatic (7 compounds) groups, were determined (Table 2; gas concentrations are expressed in ppb by volume and referred to the dry gas phase). The total VOC concentrations in gases with a dominating magmatic contribution (samples #1-47, hereafter \(M\) gases) are relatively low (from 61 to 1664 ppbv), whereas they range from 180 to 1235942 ppbv (Table 2) in those gases (#48-141) characterised by prevalent hydrothermal contribution (hereafter \(H\) gases). Hydrothermal reservoirs, even when associated with volcanic systems, are commonly recharged by fluids circulating within organic-bearing sedimentary rocks. This organic source is then transformed into VOCs through biogenic and thermogenic processes [32]. Therefore, relatively high VOC concentrations in the \(H\) gases are expected. On the contrary, the organic-rich component constitutes a minor fraction of the \(M\) gases, since it is generally destroyed by high-temperature, oxidizing fluids released from the magmatic melts [13]. The relative percentages (mean values) of the different groups of VOCs can provide preliminary indications to distinguish the \(M\) and \(H\) gases: in the \(M\) gases, alkane, _Cl_-bearing, aromatic, heteroaromatic and alkene compounds are present in almost comparable amounts (31, 28, 16, 16 and 9% of total VOCs, respectively), whereas cyclic and _O_-bearing species represent a small VOC fraction (<0.04%) (Figure 1a). The organic fraction of the \(H\) gases is largely dominated by alkanes and aromatics (54 and 24%, respectively) with minor cyclic, alkene, heteroaromatic and O-bearing compounds (from 0.7 to 1.4%), and traces (<0.04%) of Cl-bearing compounds (Figure 1b). These evidences are consistent with recent investigations that have highlighted a recurrent relation between VOC speciation and thermodynamic conditions at the fluid source in gas discharges from volcanic and geothermal systems [33; 34; 35; 36]. Predominance of alkanes and aromatics in both the \(M\) and \(H\) gases was considered to reflect the proceeding of \"reforming\" processes, which in geothermal areas, as well as in hydrothermal systems commonly surroundings active volcanoes, are favoured by the large availability of catalytic agents, such as free acids, alluminicates and sulphur gas species [37; 38]. Pyrolysis of organic material was found to produce mostly alkanes and, secondarily, aromatics [39; 40], whereas Fischer-Tropschreactions were invoked for the production of light alkanes and, at a minor extent, alkenes [41]. The presence of halocarbons in volcanic gas emissions was attributed to either the product of pyrolysis of adjacent vegetation [42, 43] or, alternatively, air contamination [44, 45]. Conversely, organic geochemical evidence supported a pristine abiogenic origin by high-temperature gas-phase radical reactions [46, 47]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & & & & & **Cl-** & **O-** & & \\ n\({}^{\circ}\) & **sample** & **Alkanes** & **Aromatics** & **Cycles** & **Alkenes** & **Bearing** & **bearing** & **heteroaromatics** & **sum** \\ \hline  
4 & 1.9 & 0.0 & 1.8 & 15 & 0.01 & 34 & 61 \\ \hline \end{tabular}
\end{table} 
Table 2: Composition of main VOC groups. Concentrations are in ppbv. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & & & & & & & **Cl-** & **O-** & & \\
**n\({}^{\circ}\)** & **sample** & **Alkanes** & **Aromatics** & **Cycles** & **Alkenes** & **Bearing** & **bearing** & **heteroraormatics** & **sum** \\ \hline  
& 244 & 0.5 & 43 & 102 & 10704 \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._