Volcanic Gas Emissions from Soufrière Hills Volcano, Montserrat

Abstract:

Soufrière Hills Volcano in Montserrat, part of the Lesser Antilles island arc, erupted in 1995 and has continued to show signs of heightened activity since, with cycles of extrusive activity separated by pauses. The latest extrusive period ended in 2010, with the current pause the longest since the onset of the eruption. Although surface activity remains low, continuous monitoring data reveal that SO₂ fluxes are still relatively high (1 – 5 kg s^-1). Deformation data also show an ongoing, although slowing, inflation signal, raising questions about the ongoing state of the volcano and the potential for future eruptive cycles.

Measurements of SO₂ flux are taken using UV scattered sunlight spectroscopic measurements. Spectra are recorded by miniature UV spectrometers, from which the SO₂ slant column in the field of view is retrieved via Differential Optical Absorption Spectroscopy (DOAS). SO₂ fluxes are then computed by traversing or scanning the spectrometer across the volcanic plume to produce a cross-section, which is multiplied by the plume speed to determine the flux. Such measurements have several sources of error, including the plume speed used, assumptions on the plume geometry and atmospheric scattering of light (known as “light dilution”). One advantage, however, is the sensitivity of these instruments to other volcanic trace gases, including bromine monoxide (BrO). Soufrière Hills Volcano was the location of the first detection of BrO in a volcanic plume, sparking significant interest in the role of volcanic bromine emissions on atmospheric chemistry, particularly for ozone depletion.

We will present recent observations of SO₂ flux at Soufrière Hills Volcano from both traverses and scanning measurements, discussing the technical difficulties in monitoring gas fluxes and sources of uncertainty. We will also present measurements of the plume chemistry, including BrO, and discuss these results in the context of the ongoing activity at Soufrière Hills Volcano.