Improved volcanic SO2 flux records from integrated scanning-DOAS and UV Camera observations.

Volcanic SO 2 flux is a key indicator of magma influx into shallower portions of magmatic plumbing systems, and as such is central to volcano monitoring. However, observations have traditionally been challenged by a variety of technical and methodological caveats and limitations, to overcome which it is required an intercomparison of different observational techniques and, where possible, their integration. Here, we compare ~9 years (2014 to 2022) of SO 2 flux records at Stromboli obtained through (i) a near-vent (~500 m) UV Camera system and (ii) a network of DOAS spectrometers scanning the distal (~2 km) bulk plume. We find a large (133 t/d on average) systematic offset between the SO 2 flux time-series streamed by the two observational techniques, with the flux from the scanning spectrometers being ~200 % higher on average than UV Camera flux. We propose this mismatch to derive from a combination of (i) SO 2 flux underestimation by the UV Camera, as caused by incomplete coverage of the plume (due to topography of the crater area) and radiative transfer issues in the optically dense, near-vent plume, and (ii) SO 2 flux overestimation by the distal scanning spectrometers', caused by non-ideal (incomplete) atmospheric dilution of source-released gas puffs during atmospheric transport. Our analysis suggests this latter process to be dominant, imparting a positive wind speed dependence and a marked seasonality to the distal scanning spectrometers' fluxes, and causing them to significantly overestimate the source SO 2 fluxes. Finally, we propose a novel integrated SO 2 flux record, based on the combination of UV Camera-derived gas velocities and DOAS-derived SO 2 integrated column amounts (back-calculated at source using an experimentally derived plume dilution function). We expect this SO 2 flux time-series to be less affected by external factors (e.g., meteorological, illumination and volcano topography conditions) than using any of the two techniques alone and hence a better proxy of volcano behaviour. We recommend testing of the integrated UV Camera-scanning-DOAS method at other volcanoes to explore its utility for improved volcanic degassing characterization. • We conducted a long-term comparison (~ 9 years) between SO 2 daily flux records at Stromboli obtained through a near-vent (~500 m) UV Camera system and a network of scanning-DOAS spectrometers scanning the distal (~2 km) bulk plume. • We find a large systematic offset between the SO 2 flux time-series streamed by the two observational techniques, with the flux from the scanning spectrometers being ~200 % higher on average than UV Camera flux. • We propose this mismatch to derive from a combination of (i) underestimation factors related to the UV Camera due to topographic and radiative transfer issues and (ii) a flux overestimation by scanning-DOAS caused by non-ideal (incomplete) atmospheric dilution of source-released gas puffs during atmospheric transport. • We propose a novel integrated SO 2 flux record, based on the combination of UV Camera-derived gas velocities and DOAS-derived SO 2 integrated column amounts (back-calculated at source using an experimentally derived plume dilution function).