Combining scanning-DOAS and SO2 camera observations leads to more robust volcanic SO2 flux records.
crossref(2024)
Abstract
The volcanic SO2 flux is key indicator of magma influx into the shallower portions of magmatic plumbing systems, and as such is central to volcano monitoring. However, observations are challenged by a variety of technical and methodological caveats and limitations, requiring the use of a multiple-technique approach as a key to acquiring more robust SO2 flux time-series. Here, we compare ~9 years (2014 to 2022) of SO2 flux measurements at Stromboli obtained through (i) a near-vent SO2 camera (UV1 system, managed by UNIPA) and (ii) a network of scanning-DOAS spectrometers (FLAME network, managed by INGV-OE). The UV1 system operates in close proximity to the summit craters (~500 m), while the FLAME network intercepts the volcanic plume from a greater distance (~2 km). We find remarkable differences in the SO2 flux time-series streamed by the two observational techniques, with the FLAME daily averages fluxes being up to ~200% higher than those seen by the SO2 camera. By examining the individual components involved in the flux calculation, we find the SO2 integrated column amounts (ICAs) to match one each other within a factor 30%. Hence, the large mismatch between the two SO2 fluxes is primarily caused by large differences in the used plume speeds. By applying a simple dispersion model, we find the Stromboli’s plume to exhibit a non-Gaussian dispersion behaviour, in which in-plume SO2 concentrations are non-linearly diluted (upon atmospheric dispersion) as a function of wind intensity. In the “puffing” plume conditions our model suggests for Stromboli, use of wind speed as a proxy for real gas velocity may not be appropriate, as it can lead to a net overestimation of SO2 fluxes when observed by distally operating scanning-DOAS. In contrast, SO2 camera observations can provide more accurate plume transport results, but are challenged by radiative transfer issues in the optically opaque, proximal plume. Ultimately, we propose a new SO2 flux record for Stromboli that combines DOAS and SO2 camera and takes advantage of the specific advantages of both techniques. Our combined SO2 fluxes is obtained by combining the SO2 camera-derived plume velocities (obtained from the optical flow algorithm applied to high-frequency UV camera images in near-vent conditions) with the FLAME-derived ICAs, recalculated at source conditions using an experimentally derived plume dilution function. The resulting SO2 flux exhibits a stronger correlation with volcanic activity than obtained using any of the two techniques alone. We emphasize that integrating near-vent measurements through UV cameras with distal scanning-DOAS measurements may significantly improve our understanding of volcano degassing dynamics and behaviour.
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