First deployment of active AirCore in a volcanic plume at Mount Etna

Abstract. Uncrewed Aerial Systems (UAS) are by now well established platforms for observations in volcanic plumes which are a challenging environment. Options for trace gas observations inside volcanic plumes still remain limited because sophisticated measurement techniques for high-precision observations of trace gases often require instrumentation that cannot be used on board UAS due to high weight and power consumption. UAS-borne sampling of air followed by post-flight analysis can  extend the number of observable trace gases as well as the measurement quality. Originally developed for stratospheric observations, AirCore sampling with long coiled tubes has proven to be a light-weight sampling technique to probe parts of the atmosphere that are otherwise difficult to access. Trace gas analysis of sampled air is done post-flight, most commonly with fast high-precision optical methods with continuous flow analysers, delivering high-quality and high-resolution trace gas mole fractions. While balloon-borne AirCore setups perform passive sampling making use of natural pressure differences, we used a UAS-deployable small active AirCore setup collecting air with a pump. In July 2024, this  setup was deployed on a UAS alongside electrochemical and optical gas sensors to probe the volcanic plumes of Mount Etna (Sicily, Italy), which was particularly active at the time. This was to our knowledge the first time that the AirCore sampling technique was used inside volcanic plumes. The air sample was successfully analysed with cavity-ring down spectroscopy for carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄). While CO₂ and often also CO mole fractions were markedly enhanced in the plume, no significant change of CH₄ was observed. The ratio of CO and CO₂ mole fraction enhancements was found to be rather low which might point at fast oxidation processes.