Variation in OMI SO2 measurements between extrusive and non-extrusive periods of Soufrière Hills volcano, Montserrat

Ground-based instruments have been used to monitor sulphur dioxide (SO2) emissions from Soufrière Hills volcano, Montserrat since the beginning of the eruption in 1995. According to these measurements, SO2 emission has been approximately constant, over a timescale of years, whereas lava extrusion has been discontinuous. The controls on the shorter timescale variability of SO2 emissions, particularly those relating to lava extrusion, are not well understood. Satellite-based observations of SO2 emissions (using the Ozone Monitoring Instrument, OMI, on board the NASA-Aura satellite) are particularly useful during periods of enhanced volcanic activity, when excessive ash in the plume precludes ground-based measurements. On the 6th October 2009, Soufrière Hills volcano began its fifth phase of lava extrusion. This phase culminated in a partial dome collapse, a small lateral blast to the north, and a large plume of gas and ash on the 11th February 2010. The total collapsed volume was estimated at ∼40-50 million m3 and the SO2 emission, measured by OMI, was 0.058Tg. The SO2 emitted from 6th October 2009 to 11th February 2010 was monitored daily using the OMI sensor. Whilst lava extrusion was occurring, SO2 was detected by OMI on 33% of the days. Once extrusion ceased, SO2 was only detected on 8% of the days. There are a number of possible hypotheses to explain this difference: an increased SO2 emission rate from the volcano during lava extrusion; the explosive lofting of gas and ash; and increased buoyancy of the plume due to higher vent temperatures. The latter two explanations result in the SO2 being emplaced at greater altitudes, resulting in a stronger signal for OMI. Discriminating between these factors is of importance to the modelling of the processes of magma and volatile emission. We evaluate the evidence and mechanisms for the linkage between SO2 emissions and lava extrusion.