## 3 Results and Discussion
### The Experimental Campaign of Pozzuoli Solfatara  
The first field experiment was conducted at the Phlegraen Fields volcano [21; 37; 38]; this is a rising caldera including Pozzuoli and the suburbs of Naples, among the most inhabited zones of Italy. The bradyseismic crises of the volcano produced thousands of shallow earthquakes, and its activity has been often, even if not continuously, active in the last few decades. Scientists and stakeholders observed these recent crises, despite them not evolving into an eruption, bearing in mind the related important hazard mitigation problems and the linked necessary interventions. These issues have become even more critical taking into account that ground uplift restarted in 2012, with the intensification of degassing activity, in the caldera center and at the main surface hydrothermal manifestations of Solfatara and Pisciarelli. Current uplift/degassing unrests observed in Pozzuoli are probably due to intermittent injections of carbon dioxide-rich gas in the hydrothermal system of the volcano, indicating that fresh magma is supporting a strong inflow of gas from deep layers. Here, attention has been focused on the degassing fumarolic vent of Pisciarelli that, besides a small augmentation in discharge temperature (from less than 95 \({}^{\circ}\)C up to 110 \({}^{\circ}\)C), has visibly escalated in degassing activity over the past decade (the carbon dioxide concentration in fumaroles has also risen), making it the most active among the Italian fumaradic vent areas. Therefore, quantifying the CO\({}_{2}\) output from Pisciarelli is vital for the interpretation and possibly the prediction of future changes in this volcanic zone [21]. The system measured nearly continuously during both 15 and 16 October 2014, after the experimental setup on 14 October. During the measurement operations, BILLI performed a horizontal scan of the almost vertical plume in the 198\({}^{\circ}\)-232\({}^{\circ}\) range of azimuth angles (see Figure 3a). The elevation of 12\({}^{\circ}\) was used mostly for the performed scans, where the plume structure was fully resolved and the lidar echoes were more intense. Controls proved that at lower and higher elevations, the plume was often either optically too thick or too spread, so only a few additional scans at 0\({}^{\circ}\), 6\({}^{\circ}\), and 15\({}^{\circ}\) were completed. A total of 200 lidar returns (100 at \(\lambda_{\text{ON}}\) and 100 at \(\lambda_{\text{OFF}}\), see Table 2) were ascribed to a single profile and averaged to improve the SNR; in this way, the sampling frequency of the lidar echoes was 0.05 Hz, which corresponded to a 20-s time resolution. As already discussed, the space resolution of BILLI was 5 m. The combination of about 25 profiles yielded a total scan of the plume in around 10 min [21; 37; 38]. Thanks to the BRIDGE DIAL technique (see Section 2.2) it was possible to retrieve, globally, up to 40 scans of CO\({}_{2}\) concentration in excess in the plume and, at the same time, the CO\({}_{2}\) flux time-series. The whole dataset covered almost three days of operation, for a maximum of nine consecutive hours. Examples of the CO\({}_{2}\) dispersion map and flux time-series are reported in Figure 3a,b, respectively [21]. Figure 3a illustrates the results of a typical scan (using the polar diagram) through the plume. When the laser beam intercepted the plume, peak CO\({}_{2}\) concentrations of up to 5000 ppm were measured. The plume was typically crossed at azimuth angles of 205-225\({}^{\circ}\) and at distances in the range of 50-70 m from the system. At azimuth angles <205\({}^{\circ}\) and >225\({}^{\circ}\), or at distances <50 and >70 m, BILLI retrieved the carbon dioxide concentration usually found in ambient air, thus providing confirmation that the laser beam was able to scan the whole plume. This result directly corresponded to the most vigorously degassing fumarolic vent of Pisciarelli.