Reconstruction of SO 2 flux emission chronology from space-based measurements

Abstract

Infrared satellite images measured with the MODIS instrument of the volcanic plume produced during the 2006 eruption of Mt. Etna were analysed to produce maps of SO 2 amount. We used these maps to reconstruct time series of SO 2 fluxes by integrating profiles of SO 2 orthogonal to the plume advection direction and multiplying with wind speeds from a meteorological model. These data were then compared with a reconstructed time series of SO 2 fluxes measured with the FLAME ground-based network of ultraviolet DOAS systems surrounding the volcano. We found weak agreement on 3rd December when little ash was emitted, but this agreement improved when a 0.3 m s − 1 wind speed correction factor was used. FLAME and MODIS results were in good agreement on the 6th December, and improved when a – 0.3 m s − 1 offset was applied. The corrected data revealed that the only period of time when FLAME and MODIS did not track together was coincident with the presence of ash, which interferes with the IR imagery and retrieval of SO 2. We highlight that combining two independent time series of SO 2 flux allows a precise determination of wind speed, if there is sufficient time-dependent structure in the SO 2 signal. The observed increase in SO 2 flux prior to the ash emission is interpreted as a quiescent release of an accumulated gas phase that drive eruptive activity, as previously suggested for the southeast crater system of Etna. In this case the SO 2 flux signal therefore acted as a precursor to the eruptive ash events. This work demonstrates that quantitative reconstruction of SO 2 flux time series is feasible using MODIS data, opening a new frontier in the use of satellite data to interpret volcanic processes, in particular in poorly monitored remote locations.