Multicopter measurements of volcanic gas emissions at Masaya (Nicaragua), Turrialba (Costa Rica) and Stromboli (Italy) volcanoes: Applications for volcano monitoring and insights into halogen speciation

Volcanoes are a natural source of several reactive gases (e.g. sulfur and halogen containing species), as well as non-reactive gases (e.g. carbon dioxide). Besides that, halogen chemistry in volcanic plumes might have important impacts on atmospheric chemistry, carbon to sulfur ratios and sulfur dioxide fluxes are important established parameters to gain information on subsurface processes. In this study we demonstrate the successful deployment of a multirotor UAV (quadcopter) system with custom-made lightweight payloads on board for the compositional analysis and gas flux estimation of volcanic plumes. The various applications and their potential with such new measurement strategy are presented and discussed on example studies at three volcanoes encompassing flight heights of 450 m to 3300 m and various states of volcanic activity. Field applications were performed at Stromboli Volcano (Italy), Turrialba Volcano (Costa Rica) and Masaya Volcano (Nicaragua). Two in-situ gas-measuring systems adapted for autonomous airborne measurements, based on electrochemical and optical detection principles, as well as an airborne sampling unit, are introduced. We show volcanic gas composition results including, abundances of CO 2 , SO 2 and halogen species. The new instrumental set-ups were compared with established instruments during ground-based measurements. For total SO 2 flux estimations a small differential optical absorption spectroscopy (DOAS) system measured SO 2 column amounts on transversal flights below the plume, showing the potential to replace ground-based manned operations. At Stromboli volcano, short-term fluctuation of the CO 2  / SO 2 ratios could be determined and confirm an increased CO 2  / SO 2 ratio in spatial and temporal proximity to explosions by airborne in-situ measurements. Reactive bromine to sulfur ratios of 0.19 × 10 −4 to 9.8 × 10 −4 were measured in-situ in the plume of Stromboli volcano downwind of the vent.