###### Abstract
Instead, standoff detection of volcanic CO\({}_{2}\) has only been accomplished during eruptions of mafic volcanoes, where ground-based Fourier transform infra-red (FTIR) spectrometers can take advantage of magma/hot rocks as efficient light sources [19]. Conversely, monitoring of the far more usual \"passive\" CO\({}_{2}\) releases from quiescent volcanoes has implied access to risky summit craters for direct measurement of fumaroles [20] or in situ sampling of plumes by implementing Multi-GAS (Multi-component Gas Analyzer) systems [21] or Active-FTIR [22]. The employment of modern systems and networks for volcanic gas observations [8,20,23] has made it possible to make significant progress in direct sampling techniques for volcano monitoring. This has resulted in an improvement in time resolution of traditional ground-based volcanic gas observations [24]. Moreover, taking advantage of fast advancements in optoelectronics, especially in the last two decades including new coherent sources, detection systems, and spectrometric devices, we have at present many optical remote techniques, both passive and active, ground-based, airborne, and even space-borne, that are applicable to issues involving Earth CO\({}_{2}\) degassing [12,14]. The improvements of volcanic gas measurements in terms of numbers, quality, and time duration are providing evidence that CO\({}_{2}\) is the gas most directly linked to \"pre-eruptive\" degassing processes [21]. As confirmation of this fact, precursory increases in CO\({}_{2}\) plume flux have been recently detected at several volcanoes [25] such as at Mt. Etna, the most active and dangerous volcano in Europe [26]. Taking into account the potentialities given by data on the volcanic CO\({}_{2}\) flux for predicting eruptions, the ability to remotely monitor this flux with a high time resolution is an important step ahead in current volcanology. Considering the stringent requirements of volcanological research and the great advantages of active optical remote sensing (e.g., measurement not dependent of sunlight or other radiation sources, safe monitoring distance, probing of sites difficult to access, large areal coverage, higher accuracy than passive techniques, etc. [14]), ground-based lidar systems have represented, since their creation, a valuable option for the remote detection of volcanic gases and aerosols. Such systems share many advantages with seismic and deformation monitoring with respect to direct sampling; in a nutshell,they can perform measurements from a safe area, while also allowing (semi)-continuous operation during eruptions. Moreover, gas amounts can be retrieved non-invasively in near real-time, obviating the need for successive laboratory analysis, thus avoiding possible sample contamination, and--unlike in situ sampling--standoff sensing techniques measure integrated or range-resolved gas concentrations through cross-sections of the plume, thus providing a more informative characterization of composition and flux of the volcanic plume [8]. Lidar systems have increasingly been used since the early 1980s for the valuation of volcanic hazards (e.g., during the Soufriere and Mt. St. Helens eruptions in 1979 and 1981, respectively) [8]. Over the last 30 years, several lidars have been deployed to obtain data on the concentrations and fluxes of sulfate aerosol [27; 28; 29] and ash [30]. Furthermore, laser remote sensing systems have been used to detect volcanic particles in the troposphere at Mt. Etna (Italy) [31] and in the lower stratosphere during the Pinatubo eruption (Philippines) [32]. In particular, the lidar at Garmisch-Partenkirchen was one of the first examples of volcanic lidar implemented to observe the spread of the stratospheric volcanic plume (from 10 to 28 km) by evaluating the time-series of vertical profiles of the scattering ratio and vertically integrated column of the backscatter coefficient using a Nd:YAG laser operating at 532 nm.