Satellite-derived SO₂ flux time-series and magmatic processes during the 2015 Calbuco eruptions

Abstract. Quantifying time-series of sulphur dioxide (SO₂) emissions during explosive eruptions provides insight into volcanic processes, assists in volcanic hazard mitigation, and permits quantification of the climatic impact of major eruptions. While volcanic SO₂ is routinely detected from space during eruptions, the retrieval of plume injection height and SO₂ flux time-series remains challenging. Here we present a new numerical method based on forward- and backward-trajectory analyses which enable such time-series to be robustly determined. The method is applied to satellite images of volcanic eruption clouds through the integration of the HYSPLIT software with custom-designed Python routines in a fully automated manner. Plume injection height and SO₂ flux time-series are computed with a period of ~ 10 minutes with low computational cost.

Using this technique, we investigated the SO₂ emissions from two sub-Plinian eruptions of Calbuco, Chile, produced in April 2015. We found a mean injection height above the vent of ~ 15 km for the two eruptions, with overshooting tops reaching ~ 20 km. We calculated a total of 300 ± 46 kt of SO₂ released almost equally during both events, with 160 ± 30 kt produced by the first event and 140 ± 35 kt by the second. The retrieved SO₂ flux time-series show an intense gas release during the first eruption (average flux of 2560 kt day⁻¹), while a lower SO₂ flux profile was seen for the second (average flux 560 kt day⁻¹), suggesting that the first eruption was richer in SO₂. This result is exemplified by plotting SO₂ flux against retrieved plume height above the vent, revealing distinct trends for the two events. We propose that a pre-erupted exsolved volatile phase was present prior to the first event, which could have led to the necessary overpressure to trigger the eruption. The second eruption, instead, was mainly driven by syneruptive degassing. This hypothesis is supported by melt inclusion measurements of sulfur concentrations in plagioclase phenocrysts and groundmass glass of tephra samples through electron microprobe analysis.

This work demonstrates that detailed interpretations of sub-surface magmatic processes during eruptions are possible using satellite SO₂ data. Quantitative comparisons of high temporal resolution plume height and SO₂ flux time-series offer a powerful tool to examine processes triggering and controlling eruptions. These novel tools open a new frontier in space-based volcanological research, and will be of great value when applied to remote, poorly monitored volcanoes, and to major eruptions that can have regional and global climate implications through, for example, influencing ozone depletion in the stratosphere and light scattering from stratospheric aerosols.