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

Volcanic SO2 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 similar to 9 years (2014 to 2022) of SO2 flux records at Stromboli obtained through (i) a near-vent (similar to 500 m) UV Camera system and (ii) a network of DOAS spectrometers scanning the distal (similar to 2 km) bulk plume. We find a large (133 t/d on average) systematic offset between the SO2 flux time-series streamed by the two observational techniques, with the flux from the scanning spectrometers being similar to 200 % higher on average than UV Camera flux. We propose this mismatch to derive from a combination of (i) SO2 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) SO2 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 SO2 fluxes. Finally, we propose a novel integrated SO2 flux record, based on the combination of UV Camera-derived gas velocities and DOAS-derived SO2 integrated column amounts (back-calculated at source using an experimentally derived plume dilution function). We expect this SO2 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.