Airborne monitoring of Mount St. Helens by the USGS began
in May 1980 for sulfur dioxide emissions and in July 1980
for carbon dioxide emissions. A correlation spectrometer,
or COSPEC, was used to measure sulfur dioxide in Mount St.
Helens' plume. The upward-looking COSPEC was mounted in a
fixed-wing aircraft and flown below and at right angles to
the plume. Typically, three to six traverses were made
underneath the plume to determine the SO2 burden
(concentration x pathlength) within a cross-section of the
plume. Knowing the burden along with the plume width and
plume velocity (assumed to be the same as ambient wind
speed), we could then calculate the emission rate of SO2.
Carbon dioxide in the Mount St. Helens plume was measured
by an infrared spectrometer tuned to the 4.26 um CO2
absorption band. An external sample tube was attached to
the fuselage of a twin-engine aircraft to deliver outside
air to the gas cell of the spectrometer. The aircraft was
then flown at several different elevations through the
plume at right angles to plume trajectory to define plume
area and carbon dioxide concentration in a vertical cross-
section of the plume. These two parameters along with the
density of CO2 for the altitude of the plume and the plume
velocity (assumed as above to be equal to ambient wind
speed) were then used to calculate the CO2 emission rate
(Harris and others, 1981).
Malinconico, L.L., 1979, Fluctuations in SO2 emission
during recent eruptions of Etna: Nature, v. 278, p. 43-45.
Stoiber, R.E., Malinconico, L.L. and Williams, S.N., 1983,
Use of the correlation spectrometer at volcanoes, in
Tazieff, H. and Sabroux, J.C., eds., Forcasting Volcanic
Events: Amsterdam, Elsevier, p. 425-444.
Harris, D.M., Sato, Motoaki, Casadevall, T.J., Rose, W.I.
and Bornhorst, T.J., 1981, Emission rates of CO2 from
plume measurements, in Lipman, P.W. and Mullineaux, D.L.,
eds., The 1980 eruptions of Mount St. Helens, Washington:
U.S. Geological Survey Professional Paper 1250, p. 201-207.
Casadevall, T.J., Johnston, D.A., Harris, D.M., Rose, W.
I., Malinconcio, L.L., Stoiber, R.E., Bornhorst, T.J.,
Williams, S.N., Woodruff, Laurel and Thompson, J.M., 1981,
SO2 emission rates at Mount St. Helens from March 29
through December, 1980, in Lipman, P.W. and Mullineaux, D.
L., eds., The 1980 eruptions of Mount St. Helens,
Washington: U.S. Geological Survey Professional Paper
1250, p. 193-200.
Contact_Person: Kenneth A. McGee
Address_Type: mailing address
Contact_Voice_Telephone: (206) 696-7695
David A. Johnston Cascades Volcano Observatory
U.S. Geological Survey
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