## 3 Results and Discussion
### The Experimental Campaign of Stromboli Volcano
Thanks to an improved and fully automated version of the BRIDGE DIAL data processing routine (taking into account local conditions and the changes in system setup), it was possible to retrieve up to 180 scans of CO\({}_{2}\) concentration in excess in the plume and, at the same time, the CO\({}_{2}\) flux time-series. In addition, thanks to a further data processing routine (see Section 2.2.3), it was possible to estimate the plume transport velocity and, hence, the local wind velocity. The whole dataset covered almost six days of operation, for a maximum of 12 consecutive hours. Examples of CO\({}_{2}\) dispersion map, wind profiles, and flux time-series are reported in Figure 4a,b, respectively [39; 40; 41]. Figure 4a illustrates the results of a typical vertical scan through the plume as a function of range (X axis), elevation angle, and altitude (Y axes). The tomography reported above was extrapolated from the measurement session performed on 26 June 2015 between 11:26 and 11:46 (local civil time), and it represents a dataset acquired as a single group of measurements slowly stepping up in elevation angle (from 16.8\({}^{\circ}\) to 20\({}^{\circ}\)) over the course of about 20 min, since, during that time, the plume displacement was extremely clear. The carbon dioxide concentration in the zone under study is indicated by the color scale, with peaks exceeding 60 ppm. Here, the volcanic plume was encountered in the 2150\(-\)2600 m range, from 17 to 19.5\({}^{\circ}\) elevation angles, and a fixed azimuth of 237.8\({}^{\circ}\). Regions highlighted with red boxes and named as A, B, C, and D were posited to be the same set of scatterers being tracked over time. Therefore, they represent a useful example for summarizing the time evolution of inhomogeneities relative to the volcanic plume. To appreciate these displacements, it is necessary to specify that particles detected in region A need a certain amount of time to move to the regions B (about 3 min), C (about 6 min), and D (about 9 min). As observed in the field, these movements were mainly due to the presence of wind blowing from north-northwest [40]. Considering both vertical and horizontal scans of the whole campaign, the beam hit the plume at azimuth angles in the range 237-251\({}^{\circ}\), elevation angles in the range 16.5-20\({}^{\circ}\) (which means altitude of 700-830 m) and at distances in the range 2200-2500 m. For azimuth angles <237\({}^{\circ}\) and >251\({}^{\circ}\), or for distances <2200 and >2500 m, BILLI retrieved the carbon dioxide concentration we usually find in ambient air, thus providing confirmation that the laser beam was able to scan the whole plume. This result was confirmed by direct observations [37; 39]. The quality of the results (as in the example shown in Figure 4a), combined with the high temporal resolution of the acquired data, allowed us to perform, almost simultaneously with in-plume CO\({}_{2}\) concentrations, the first independent measurement of plume transport velocity (v\({}_{\text{p}}\)) and therefore of the wind velocity, using the correlation method mentioned in Section 2.2.3 and detailed in a previous work [40]. As already mentioned, the assessment of the wind velocity could be crucial for both the CO\({}_{2}\) flux retrieval and air traffic control applications. In this regard, a set of data (lasting about 90 min) was selected from the morning of 26 June 2015. An example of vertical wind profile was superimposed on the same dataset (lasting about 20 min) of the CO\({}_{2}\) concentration dispersion map reported in Figure 4a, showing a high consistency with the time evolution of the in-plume spatial aerosol inhomogeneities (see the previously mentioned A, B, C, D regions highlighted with red boxes). The global weighted average value of wind velocity was found equal to \(1.1\pm 0.4\) m/s for the whole measurement test session.