## 3 Results and Discussion
### The Experimental Campaign of Stromboli Volcano  
The second field experiment was conducted at Stromboli Volcano Island. The volcanic activity on the island is generally considered \"regular\" and mild (strombolian). Larger-scale vulcanian-style explosions occasionally occur. Although these events, locally referred to as \"major explosions\" or \"paroxysms\", are not frequent and are short-lived (from tens of seconds to a few minutes), they could be extremely dangerous for locals and authorized personnel, since they produce fallout of coarse pyroclastic materials over wide dispersal areas. Moreover, neither geophysical nor volcanological precursor signals were observed before these events occurred. This could be due to the fact that they originate deep in the crustal roots of the volcano plumbing system. However, it has been observed that the previously cited events are systematically preceded by days/weeks of anomalous CO\({}_{2}\)-rich gas leakage from the Stromboli's deep (8-10 km) magma storage zone. For these reasons, volcanologists are considering with increasing interest CO\({}_{2}\) flux emissions from the open-vent crater plume for the monitoring of volcanic activity and hence for the assessment and mitigation of the related hazards [41]. As in other volcanic areas, at Stromboli, the volcanic gas CO\({}_{2}\) from a combination of simultaneously measured SO\({}_{2}\) fluxes and plume CO\({}_{2}\)/SO\({}_{2}\) ratios has also been recently evaluated [7; 11]. Even though the remote measure of SO\({}_{2}\) flux is possible by implementing UV spectroscopy [16], the CO\({}_{2}\)/SO\({}_{2}\) ratio requires in situ direct sampling/measurements close to hazardous active vents. For the previous reasons, the implementation of the BILLI DIAL system for the remote observation of the volcanic CO\({}_{2}\) flux, from distant and less risky locations, is a real advantage. The system operated almost continuously from 24 to 29 June 2015 despite the harsh environmental conditions due to the presence of high humidity, acid vapors, and re-suspended dust. It was placed inside the fence of the local ENEL (main Italian electric company) power plant in the Scari area, \(\approx\)2.5 km from the degassing vents on the volcano summit, between the margins of morphological peaks known as Pizzo and Vancori, 918 and 924 m, respectively, above sea level (ASL). The laser beam typically scanned the azimuth angles from 235.3\({}^{\circ}\) to 253.6\({}^{\circ}\) and the elevation angles from 15.2\({}^{\circ}\) to 27.4\({}^{\circ}\). As stated in Section 2.1, in order to reduce the differential absorption cross section of CO\({}_{2}\), the transmitted wavelengths were changed with respect to the previous campaign (see Table 2). As a result, longer distances were reached by the laser beam. Furthermore, the implementation of a photo-acoustic cell filled with pure CO\({}_{2}\) allowed us to reduce the systematic uncertainty, dominated by instability in wavelength setting, to 5.5% (see Section 2.2.5). Compared to the previous experimental campaign, the pulse repetition rate remained unchanged (10 Hz), instead both the temporal resolution was slightly modified and set at 10 s because only 100 lidar returns (50 at \(\lambda_{\text{ON}}\) and 50 at \(\lambda_{\text{OFF}}\), interlaced) were averaged for each scan to increase the SNR. A further improvement of the SNR and, therefore, of the accuracy of both CO\({}_{2}\) concentration and flux measurements was possible by employing filtering algorithms and curve fitting techniques, in the preliminary data analysis steps (see Section 2.2.1). Furthermore, by combining about 50 profiles, it was possible to obtain both vertical and horizontal plume scans in less than 10 min. Eventually, by processing several repeated scans (usually 10) acquired at different elevations, we also retrieved three-dimensional tomographies of the volcanic plume [39; 40; 41].