SO ₂ emissions during the 2021 eruption of La Soufrière, St Vincent, revealed with back-trajectory analysis of TROPOMI imagery

Abstract Determining SO 2 emission time-series from explosive eruptions can provide important insights into the driving magmatic processes, however accurate measurements are difficult to collect. Satellite-based platforms provide SO 2 imagery, however translating this to the altitude- and time-resolved emission history required to unravel volcanic processes is a major challenge. This means SO 2 emission time-series are rarely quantified for major eruptions, producing a gap in our understanding of explosive volcanism. Here, we combine SO 2 imagery collected by the TROPOspheric Monitoring Instrument (TROPOMI) with PlumeTraj, a back-trajectory analysis toolkit, to reconstruct the SO 2 emission prior to, and during, the explosive eruption of La Soufrière volcano, St Vincent, in April 2021. Precursory SO 2 emissions were quantified the day before the eruption, with emission rates in agreement with ground-based measurements. We estimate initial magma sulfur contents by comparing the measured SO 2 emissions with erupted magma volumes, finding that the initial explosion was sulfur poor (730 ppm S) compared to the main eruption phase (up to 3400 ppm S). This suggests that the initial explosion cleared old, previously degassed magma resident in the shallow plumbing system, followed by the eruption of the main, mostly un-degassed magma source.