## 3 Results and Discussion
### Plasma Diagnostics
It is noteworthy to mention that the OES spectra obtained at different plasma powers and different gas flow rates are very similar in terms of observed emission lines, indicating that similar radiative excited plasma species are present under all investigated conditions. Figure 3: Optical emission spectra of the plasmas in the Me–Me and Me–L electrode configurations at a gas flow rate of 4 L min\({}^{-1}\) and a discharge power of 90 \(\pm\) 2 W.  
Figure 2: **(a)** An example of current–voltage waveforms in the Me–Me electrode configuration measured for a gas flow rate of 4 L min\({}^{-1}\) and a plasma power of 20 \(\pm\) 2 W with an inset showing a zoom-in of the main plasma pulse. **(b)** Plasma frequency in the chopping gap as a function of plasma power for different air flow rates. **(c)** Examples of plasma images at 4 L min\({}^{-1}\) flow rate and plasma power of 20, 50, and 90 W with plasma volumes of 2.7 \(\pm\) 0.1, 4.7 \(\pm\) 0.6, and 39.5 \(\pm\) 3 mm\({}^{3}\), respectively, after an exposure time of 5 ms. **(d)** Plasma power averaged over all gas flow rates in the Me–Me and Me–L setup configurations versus applied power. In addition to a qualitative analysis of the excited plasma species, quantitative studies of the gas temperature were conducted via OES. Figure 4a represents T\({}_{\text{gas}}\) in the Me-Me and Me-L configurations as a function of plasma power for gas flow rates of 4, 8, and 12 L min\({}^{-1}\) (the results for all flow rates under investigation are shown in the SI, Figure S5). As can be seen, the Me-L system always shows a higher gas temperature compared with the Me-Me system at all gas flow rates and plasma powers (\(\Delta\)T\({}_{\text{gas}}\)\(\approx\) 433 \(\pm\) 30 K). This can be attributed to an increase in the H\({}_{2}\)O content in the plasma zone, which has higher vibrational-vibrational (V-V) and vibrational-translational (V-T) relaxation rates than N\({}_{2}\) and O\({}_{2}\), resulting in a fast gas heating and plasma thermalization [45; 46]. At the same time, a decrease in gas temperature is observed with increasing gas flow rate for both plasma systems, as illustrated in Figure 5b. From the literature, it is well known that one of the mechanisms to sustain plasma and prevent it from the so-called streamer-to-spark transition, avoiding undesirable energy losses on gas heating and plasma constriction, is to increase the gas flow rate [47]. Therefore, the decrease in the gas temperature with gas flow rate can be prescribed to the same phenomena, which leads the plasma back to the streamer mode. In addition, it is essential to note that the gas temperature measured via this method represents the temperature inside the plasma filament (the region with the highest emissivity). Therefore, considering that the number of plasma pulses and the plasma-treated gas volume increase with plasma power, it can be concluded that the overall gas volume between the two electrodes could be heated significantly. Overall, this all has to be considered when evaluating the final EC of the processes. Figure 4: (**a**) The gas temperature estimated via OES for different gas flow rates as a function of plasma power. (**b**) The average gas temperature over the plasma power range under investigation as a function of gas flow rates. Figure 5: Electron density profile for a single current peak at a flow rate of 4 L min\({}^{-1}\) and a plasma power of 20 \(\pm\) 2 W in the Me–Me system. Electron density (n\({}_{\rm e}\)) is one of the main plasma characteristics determining plasma physical properties, namely, the ionization degree (\(\sim\)plasma regime). In this work, n\({}_{\rm e}\) was estimated to reveal the discharge operational regimes, employing the current-voltage method and considering a plasma channel diameter of 150 \(\upmu\)m [48; 49].