## 3 Materials and Methods  
We used a multi-GAS unit [24; 25] to measure in-plume concentrations of major volatile species at Mount Garet's southeast crater (~797 m a.s.l.). The instrument was composed of a Gascard nondispersive infrared (NDIR) CO\({}_{2}\) spectrometer from Edinburgh Sensors and City Technology SO\({}_{2}\), H\({}_{2}\)S and H\({}_{2}\) electrochemical sensors (see supplementary material S1 for details on sensor's manufacture models, sensor's accuracy and repeatability, calibration ranges and gas standards). Gas sensors were calibrated before and after the field campaign at the Earth and Marine Sciences Department (DiSTeM) of the University of Palermo. The system also included temperature (T) and relative humidity (Rh) KVM3/5 Galltec-Mela sensors. All components and respective power sources were contained inside a weather-resistant plastic case, with inlet/outlet ports to provide access to ambient air. In the field, volcanic gas was pumped through the in-series connected sensors using a small pump with a flow rate of 1.8 l/min. In-plume gas concentrations were measured non-continuously for approximately 4 h (see Figure 2 for the whole concentration time series of CO\({}_{2}\) and SO\({}_{2}\)) at a frequency of 1 Hz. Each measurement routine was preceded by instrument warm-up (3 min) and two minutes of ambient air flow to flush residual volcanic gas that remained trapped within the circuit. Multi-GAS data were processed using the Ratiocalc software [47]. We selected specific acquisition time windows showing good temporal correlation (R\({}^{2}\) \(\geq\) 0.70) between volatile concentrations measured simultaneously [48; 49; 50] (see Figure 3a for different gas species). Molar ratios and errors reported in Table 1 were calculated from the slope and the R\({}^{2}\) value of the best-fit line (linear regression, such as the example of Figure 3b, in black), respectively. Following an identical procedure, bulk compositionsand associated errors reported in Tables 1 and 2 were obtained from all data points for a given x/SO\({}_{2}\) pair. The x/SO\({}_{2}\) dot-to-dot ratio versus in-plume SO\({}_{2}\) concentration [47; 51] (Figure 3a) is also shown to demonstrate that ratios, widely scattered at low SO\({}_{2}\) concentrations, converge to a less-varying ratio value (statistically indistinguishable from bulk x/SO\({}_{2}\) ratios reported in Table 1) at high SO\({}_{2}\) concentrations (30-70 ppmv). \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Gas Species**} & **Molar Ratio** & \multirow{2}{*}{**R\({}^{2}\)**} & \multirow{2}{*}{**Error (\(\pm\))**} & \multicolumn{2}{c}{**SO\({}_{2}\) Max**} & \multirow{2}{*}{**Nr. of Samples**} \\  & **x/SO\({}_{2}\)** & & & & **(Plume Marker in ppm)** & \\ \hline H\({}_{2}\) & 0.04 & 0.73 & 0.006 & 36.1 & 713 \\  & 0.04 & 0.70 & 0.007 & 39.9 & 804 \\  & 0.03 & 0.86 & 0.009 & 56.8 & 288 \\  & 0.03 & 0.78 & 0.005 & 61.9 & 534 \\  & 0.03 & 0.74 & 0.007 & 56.3 & 480 \\ \hline Bulk composition & 0.03 & & & & \\ \(\pm\) & 0.006 & & & & \\ \hline H\({}_{2}\)O & 40.5 & 0.70 & 7.4 & 63.2 & 431 \\  & 40.0 & 0.73 & 5.7 & 43.8 & 1006 \\  & 47.8 & 0.78 & 6.3 & 61.9 & 1387 \\  & 55.5 & 0.86 & 3.7 & 39.9 & 544 \\  & 53.2 & 0.71 & 4.7 & 39.9 & 1538 \\ \hline Bulk composition & 47.2 & & & & \\ \(\pm\) & 5.7 & & & & \\ \hline CO\({}_{2}\) & 0.85 & 0.82 & 0.076 & 36.6 & 557 \\  & 1.07 & 0.71 & 0.182 & 37.8 & 473 \\  & 0.65 & 0.87 & 0.045 & 50.9 & 451 \\  & 1.02 & 0.76 & 0.265 & 31.8 & 439 \\  & 1.06 & 0.70 & 0.653 & 37.3 & 725 \\ \hline Bulk composition & 0.87 & & & & \\ \(\pm\) & 0.24 & & & & \\ \hline H\({}_{2}\)S & 0.13 & 0.93 & 0.011 & 52.9 & 526 \\ \hline H\({}_{2}\)O/CO\({}_{2}\) & 68.6 & 0.75 & 10.0 & 61.9 & 1019 \\ \hline \end{tabular}
\end{table} 
Table 1: Individual x/SO\({}_{2}\) molar ratios measured by the multi-component gas analyzer system (multi-GAS) in the Mount Garet’s plume. These correspond to time windows showing good temporal correlation between gas species (R\({}^{2}\geq\) 0.70) and strong volcanic gas signal (\(\geq\)30 ppm of SO\({}_{2}\)).