Automated, high time-resolution measurements of SO₂ flux at Soufrière Hills Volcano, Montserrat

We report here the first results from an automated, telemetered UV scanning spectrometer system for monitoring SO2 emission rates at Soufrière Hills Volcano, Montserrat. Two spectrometers receive light by way of a motor-driven stepping prism and telescope in order to make vertical scans of the volcanic plume. Spectral data from these spectrometers, situated 2,800 m apart and 4,500 m from the volcano, are relayed back to the observatory every 4-5 s via radio modems. A full scan of the plume is accomplished every 1-6 min by the (time-synchronised) spectrometers and a SO2 emission rate is calculated using the SO2 slant concentrations, scan angles and plume speeds estimated from the wind speed from a telemetered weather station near to the volcano. The plume's position and dimensions are calculated using the angular data from the two spectrometers. The plume height varies significantly diurnally and seasonally and is important in order to minimise the error on SO2 emission rates. The new scanning system (Scanspec) provides SO2 emission rates from 08:00 to 16:00 h local time every day. Preliminary results highlight a number of features of the SO2 time series and plume dynamics and give our first indications of the errors and limits of detection of this system. SO2 emission rates vary widely on all time scales (minutes, days, months). This new system has already provided the first real and consistent indication that SO2 emission rates vary on a minutes to hours basis, which can be correlated with volcanic activity (for example, rockfall and pyroclastic flow activity). It is anticipated that this system at Soufrière Hills will yield information on shallow processes occurring on short time scales (periods of minutes to hours) as well as deep processes relating to magma supply rates, which will be associated with longer wavelength SO2 signals of weeks to months.