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Measuring volcanic SO2 gas from the air
with a Correlation Spectrometer

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Traverses made beneath volcanic plume

Sketch showing aircraft traversing beneath a volcanic plume
Sketch by M. Doukas
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COSPEC mounted in airplane with periscope sticking through window
Solar ultraviolet energy passes through a periscope mirror into a COSPEC mounted in the airplane.
Sketch shows an airplane traversing beneath a volcanic plume downwind from an active volcano. To measure sulfur dioxide gas, the aircraft flies below the plume at right angles to the path of the plume with a vertically-aimed COSPEC (see photo, upper right). Because volcanic vents are typically higher than the surrounding terrain, it is often possible to get beneath the plume. Scientists usually make 3-6 passes beneath the plume to calculate the amount of SO2 in the plume. As the plane passes beneath the plume, the amount of ultraviolet energy absorbed by the plume is measured by the COSPEC.

SO2 concentration in plume and wind speed determined

Sketch showing aircraft traversing beneath a volcanic plume A COSPEC measures ultraviolet energy. This graph shows a COSPEC record from a single traverse beneath a volcanic plume. Before and after the plane passes beneath the plume, scientists trigger calibration marks (note peaks), which are made with a SO2 gas standard mounted inside the instrument. The jagged chart trace between the calibration marks represents the amount of ultraviolet energy absorbed by SO2 molecules within the plume.

During the flight, scientists and the pilot must also estimate wind speed by one or more methods. Traditionally, wind speeds are calculated by comparing true air speed, flying with and against the wind (along axis of of the plume), with true ground speed. New methods utilizing GPS technology are now available. Wind speed is usually the chief variable that determines the precision of SO2 gas measurements.

SO2 (tonnes/day) = (SO2 concentration in plume) x (cross-sectional area of plume) x (plume velocity) x (mathematical constants)

Reference

Doukas, M.P., and Gerlach, T.M., 1995, Sulfur dioxide scrubbing during the 1992 eruption of Crater Peak, Mount Spurr volcano, Alaska, in Keith, T.E.C., ed., The 1992 eruptions of Crater Peak vent, Mount Spurr volcano, Alaska: U.S. Geological Survey Bulletin B-2139, p. 47-57 (report is available online.)

Methods of monitoring volcanic gases

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Last modification: Tuesday, 18-Jul-2000 16:52:19 EDT (SRB)