###### Abstract
These measures were affected by the following average errors: peak scattering ratio: \(\pm\)4-5% and integrated backscatter coefficient: \(\pm\)10% [32]. DIAL-Lidar (Differential Absorption Light Detection and Ranging), or simply DIAL, is based on the fast switching of the wavelength of laser pulses on- and off-absorption of the molecule under study. From the application of the Beer-Lambert law to the ratio of the lidar signals (detected radiation vs. range) retrieved at both wavelengths, the range-resolved molecule concentrations (ppm) can be obtained, thus determining the two-dimensional or even three-dimensional plume morphology. This achievement is beyond the capabilities of FTIR, COSPEC (Correlation Spectrometer), and DOAS [15], which provide the path length integrated concentrations (ppm m). DIAL has been deployed at the main Italian volcanoes with UV lasers for tracking the total fluxes of SO\({}_{2}\)[28; 33]. In particular, the first example of the quantification of volcanic SO\({}_{2}\) fluxes by using the DIAL (active) technique is reported in [33] where it was extensively compared with passive monitoring techniques (DOAS and COSPEC). In the previously mentioned work, the lidar-based SO\({}_{2}\) fluxes were 50% higher on average than those derived with other standard passive techniques (COSPEC and DOAS) commonly used in volcanology. This mismatch was interpreted as evidence of possible scattering-induced errors in the passive techniques. More recently, ATLAS (Agile Tuner Lidar for Atmospheric Sensing), a ground-based DIAL system, has been implemented to perform measurements of water vapor flux at the Stromboli volcano (Italy) [34]. To our knowledge, this was the first time that lidar retrieved water vapor concentrations in a volcanic plume. Finally, the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), an elastic backscatter lidar operated from a satellite, was used to observe volcanic ashes from 15 to 20 April 2010, just after the explosion of the Eyjafjallajokull volcano (Iceland) [35]. This has opened up new scenarios in future volcanic-lidar applications. However, the apparatuses previously described were costly, heavy, and not effective enough to be implemented in the long-term period. Even though DIAL provides unprecedented opportunities to volcanologists, it requires more advancement to become a reliable technique for routine observations. In order to fill these gaps, a new DIAL system, designed to measure the volcanic CO\({}_{2}\) flux, was developed as part of the ERC (European Research Council) Starting Grant Project \"BRIDGE\". The BrIdge volcanic Lidar (BILLI), assembled at the ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development) Research Center of Frascati by the FSN-TECFIS-DIM group (Diagnostics and Metrology Laboratory) [36], successfully retrieved three-dimensional tomographies of volcanic CO\({}_{2}\) concentration in the plumes at Italy's most hazardous volcanic areas: Pozzuoli Solfatara (Naples, Italy) [21; 37; 38], the Stromboli volcano (Aeolians Islands, Italy) [39; 40], and Mt. Etna (near Catania, Italy) [41; 42]. As far as we know, lidar measurements of CO\({}_{2}\) have been performed in volcanic plumes only in recent times, due to the previously mentioned technical challenges and the extreme environmental operating conditions [14; 21; 37; 38; 39; 40; 41; 42]. In the last decade, promising results have been obtained by using a variant of DIAL, known as the Integrated Path DIAL (IPDIAL), but mainly to monitor ambient CO\({}_{2}\) concentrations (e.g., power plants/industrial emissions), and an Open-Path laser Spectrometer (OPS) to measure volcanic CO\({}_{2}\) concentration and flux.