A Compilation of Sulfur Dioxide and Carbon Dioxide Emission-Rate Data from Mount St. Helens during 1980-88

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. The use of correlation spectroscopy
for determining the sulfur dioxide output of volcanoes is well
established and the technique has been discussed in detail by
a number of investigators (Malinconico, 1979; Casadevall and
others, 1981; Stoiber and others, 1983).
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).