## 4 Conclusions  
The main goal of this paper was to show the potential of mobile DIAL-Lidar systems for volcanological research such as the one reported in this work, designed for the remote measurement of both in-plume exceedance of volcanic CO\({}_{2}\) concentration and flux, considered as precursors of volcanic eruptions. Considering the difficulties and, in some circumstances, the hazardous in retrieving these parameters as well as the limits of conventional in situ instruments, the BILLI system and the related BRIDGE DIAL data analysis technique have been developed. Here, particular emphasis has been placed on the data collection and processing techniques, and the most significant findings of the experimental campaigns carried out over three years at the most hazardous Italian volcanic areas. Observing the results related to the three experimental campaigns, carried out from October 2014 to July 2016, it is possible to note a gradual refinement of the system performance and an improvement in the accuracy of results. In fact, the implementation of the photo-acoustic cell has made it possible to reduce the systematic uncertainty of CO\({}_{2}\) concentrations to 5.5% and to evaluate a cumulative CO\({}_{2}\) measurement uncertainty (considering both the statistical and systematic uncertainty) between 6% and 7.5%. The reported approach for the analysis of acquired data allowed us to retrieve, accurately and rapidly, range-resolved CO\({}_{2}\) concentration profiles and maps. Furthermore, and with the aim to improve the already high potential of our system, a further data processing routine was implemented and tested for independent measurements of the plume transport velocity. Nevertheless, this technique, although promising, still requires efforts to be refined. In this way, BILLI will be able to perform simultaneously measurements of CO\({}_{2}\) concentration, CO\({}_{2}\) flux and wind velocity, without additional costs or the support of other conventional devices for the same purpose. Both CO\({}_{2}\) concentration profiles and maps are fundamental to describe the spatio-temporal dispersion of the volcanic plume into the atmosphere. CO\({}_{2}\) flux time-series retrieved by BILLI (affected by an overall uncertainty of \(\approx\)25%) were extremely useful to volcanologists for a fast tracking of volcanic activity, particularly if compared with conventional acquisitions. The detected values in excess of CO\({}_{2}\) concentrations and fluxes were in good agreement with conventional measurements, carried out in the same time interval, but based on completely independent and significantly different approaches. This has proven the sensitivity and reliability of our system for the detection and monitoring of CO\({}_{2}\), not only in limited areas, but also over extended regions and for prolonged periods of time. In summary, the novelty/advantage of this work is that the proposed standoff DIAL system allows effective measurements (CO\({}_{2}\) excess of a few tens of ppm has been clearly detected) to be taken continuously and remotely (up to more than 4 km from the crater), therefore, from a safer location free from the risks to which operators are exposed during direct sampling. In addition, data have been acquired with much higher temporal (10 s) and spatial (5 m) resolution than conventional instruments (the plume was scanned in few minutes rather than over several hours). Then, a complete time-resolved plume evolution has been detected in several measurement sessions; this can be useful for the assessment of the wind velocity, a crucial parameter not only for the CO\({}_{2}\) flux retrieval, but also for air traffic control applications. These performances allow the operator to characterize the spatio-temporal evolution of the plume, thus providing--24 h and real time--accurate information on an important precursor of eruptions.