## 3 Results and Discussion
### The Experimental Campaign of Mt. Etna  
The last experimental campaign was carried out at Mt. Etna Volcano (3329 m ASL), a hostile and only partially inhabited region located near Catania, in Sicily. Mt. Etna is the largest and most important volcano in Italy and one of the most active volcanoes in the world. For these reasons, it was the ideal place to test our system performances [42]. Similar to previous campaigns, the main goal was to measure the exceedance of in-plume CO\({}_{2}\) concentration and flux to provide useful information to volcanological research for a precocious alert to the population in the case of eruptions. The area chosen for our experiments was the northeast summit crater of the volcano (other craters were discarded due to fact that volcanic plume is too dilute or only partially visible) and is characterized by extremes of temperature, high humidity, and rates of rainfall, and the presence of acid vapors, re-suspended dust and particles, which are toxic for humans and dangerous to the instrumentation. Notwithstanding the harsh environment, BILLI operated practically continuously for nearly a week (from 28 July to 1 August 2016 including an initial instrumental setup phase). During our experiment, the system was placed on a trailer, loaded on a truck, positioned into the courtyard of the INGV (Italian National Institute of Geophysics and Volcanology)observatory \"Pizzi Deneri\" at 2823 m ASL, on the northeast side of the main crater. This fixed position, about 500 m below the volcano summit, was \(\approx\)3 km far from the most important craters [42]. Changing the elevation angle in the range 7-14\({}^{\circ}\) while fixing the azimuth angle (230\({}^{\circ}\)), BILLI vertically scanned the plume. Atmospheric profiles were recorded every 10 s, whereas each scan, obtained combining 24 profiles, was completed in -15 min. Taking into account the local conditions, both the setup (e.g., ON/OFF wavelengths) of the system and the data processing routine remained the same as the previous campaign, with the only exception of a reduction in the system weight (thanks to a rearrangement of the mechanical frame of the system). For these reasons, in this case, laser beam also reached longer ranges and the systematic uncertainty remained stable at 5.5%, whereas, as reported in the literature [39], the statistical uncertainty of lidar profiles increased with range. At 2.5 km, a mean range, it was about 2% [41; 42], while it exceeded 5% at 4.2 km [42]. Thanks to the measures already adopted in the previous campaign and reported in Section 2.1, the field setup at Mt. Etna, and the good weather conditions, a slight improvement in the system performances was observed. Thanks to the BRIDGE DIAL technique, it was possible to retrieve up to 220 scans (lasting 10-20 min) of CO\({}_{2}\) concentration in excess in the plume and, at the same time, the CO\({}_{2}\) flux time-series. The whole dataset covered six days of operation, for a maximum of 12 consecutive hours. Examples of CO\({}_{2}\) profiles, dispersion maps, and flux time-series are reported in Figure 5a-c, in that order [40]. During the campaign, several configurations of plume were recognized, which varied as a function of acquisition time. In fact, both the magnitude and the size of the CO\({}_{2}\) plume were subject to change with the elevation of the laser beam pointing angle and, obviously, they were also influenced by the presence of wind [42]. In general, for elevation angle values comprised within 7\({}^{\circ}\) and 9\({}^{\circ}\), it was quite common to detect narrow plume peaks (extending for about 500 m with magnitude values not exceeding 140 ppm) attached or strictly close to the first or the second rockface of Mt. Etna. In fact, at that elevation angle range, the laser beam altitude was lower than the summit crater.