New Scanning DOAS Installed at Mount St. Helens

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The combination of a continuous MultiGAS station within Mount St. Helens' crater and the new scanning DOAS represents state-of-the-art in geochemical monitoring.

During daytime hours, an instrument the size of a small telescope scans the sky from horizon to horizon in a conical geometry above Mount St. Helens’ crater. It collects sunlight, which is recorded by a spectrometer inside an instrumentation hut. The recorded data are sent via radio to the Cascades Volcano Observatory, where automated analysis software checks for characteristic absorption features from volcanic gases in the light path. Curious scientists also check the data.

The instrument is a Scanning Differential Optical Absorption Spectrometer (Scanning DOAS). On June 16, 2021, the first Scanning DOAS instrument was installed on a volcano in the U.S.—at Mount St. Helens. It is part of the global Network for Observation of Volcanic and Atmospheric Change (NOVAC), a community of volcano observatories that run scanning DOAS instrumentation at approximately 45 volcanoes around the world.

The Scanning DOAS primarily targets sulfur dioxide (SO2). Rather than measuring the concentration of this gas at the instrument location, this remote sensing technique can actually measure the volcanic emission rate - typically given in tons of SO2 emitted into the atmosphere per day.

Mount St. Helens is quiet at the moment but if SO2 is detected in the future, the emission rates of other major volcanic gases (H2O, CO2, H2S) can be determined by multiplying the DOAS-determined SO2 emission rates with the abundances of other emitted volcanic gases relative to SO2, as provided by a MultiGAS instrument installed on the lava dome in the crater (that monitoring station is called SNIF).

The combination of continuous MultiGAS and DOAS instrumentation represents the state-of-the-art in geochemical monitoring of high-temperature volcanic vents.

Field team installs scanning DOAS at Mount St. Helens

USGS Cascades Volcano Observatory Physical Science Technician Brian Meyers installs a data telemetry antenna on a volcano monitoring station at Mount St. Helens. A DOAS scanner is mounted above the flat-panel antenna at the top of the mast. A fiber optic cables transmits the light collected by the DOAS scanner to a spectrometer inside the fiberglass hut. Data is transmitted to CVO for analysis.

(Credit: Christoph Kern. Public domain.)