## 3 Results and Discussion
### The Experimental Campaign of Mt. Etna
Therefore, the profiles' behavior could be justified by the proximity of the rockface behind the plume that, de facto, prevented the complete dispersion of the volcanic cloud into the atmosphere and, at the same time, made the wind contribution negligible. The situation changed by increasing the elevation angle, as is shown by the profile reported in Figure 5a, which was acquired in the free atmosphere at 15:19 (local civil time) with a 12\({}^{\circ}\) elevation angle and a 230\({}^{\circ}\) azimuth angle [42]. In Figure 5a, it is possible to note two peaks, relative to CO\({}_{2}\) plume, highlighted by the red boxes. As expected from previous considerations, the first peak, localized between 1250 and 2550 m, was extremely wide (it extends for more than 1 km) and its magnitude exceeded the value of 150 ppm. Instead, the second plume peak, around 250 ppm, was detected beyond 4 km (precisely from 4060 m to 4140 m) from the system location and represents the farthestplume peak ever detected by our system. The magnifications of both plumes are clearly visible in the lower part of the figure. The measurement uncertainty (obtained by the quadratic sum of both statistical and systematic uncertainties) is equal to 6% and 7.5%, respectively [42]. Instead, the portion of the profile between the two mentioned plume peaks showed several oscillations. These latter were due to the residual presence of instrument noise. For this reason, it was neglected. The behavior of this plume could be justified by the fact that, for such elevation angles and altitudes, the plume was completely dispersed in the free atmosphere and scattered by a moderate presence of the wind [42]. Figure 5b shows an example of CO\({}_{2}\) dispersion vertical map, acquired on 31 July 2016, from 12:47 p.m. to 1:03 p.m. This time slot was selected for the relevance of the results. The tomography represents the in-plume concentration exceedance as a function of elevation angle and range of acquisition. The time evolution of the plume is clearly visible, highlighted by the red, orange, yellow, and green spots with respect to the dark blue color that, instead, represents the CO\({}_{2}\) concentration of the natural background. As already stressed, at lower elevation angles, the plume was circumscribed between the first and second rockface of Mt. Etna encountered by the laser beam. The rockface regions have been highlighted by the semitransparent yellow dotted boxes in the lower side of each scan. Instead, in the free atmosphere, the smoke plume rose and developed in both the vertical and horizontal direction, covering a wide area, after a complete random dispersion. Plume fluctuations in the free atmosphere were also probably due to a combination of the thermal updraft, from the main degassing vents on the volcano summit, and on a smaller scale, to the presence of northwest wind blowing during the measurement session. Furthermore, differences between subsequent maps were also probably due to rapid, local, and random fluctuations of particles and gases emitted by the volcano [42]. Dispersion maps such as the one shown in Figure 5b provide the basis for the computation of the volcanic carbon dioxide flux. In Figure 5c, an example of CO\({}_{2}\) flux series, lasting 6 h (from 12:22 p.m. to 6:08 p.m. local civil time), is reported. As in the Stromboli volcano campaign, CO\({}_{2}\) flux was obtained by multiplying the plume transport velocity with the plume CO\({}_{2}\) molecular density. The first term was obtained with a permanent UV camera network, while the second one was inferred by summing the carbon dioxide excess over the cross section of the volcanic plume [42]. Then, following the procedure reported in Section 2.2.4, it was possible to retrieve the CO\({}_{2}\) flux. The CO\({}_{2}\) flux varied from 13 to 92 kg/s (1235 to more than 8000 tons/day) during the measurement interval, with an average value of 40 \(\pm\) 13 kg/s (3900 \(\pm\) 1287 tons/day).