Airborne monitoring of Mount St. Helens by the USGS began in May 1980 for sulfur dioxide
emissions and in July 1980 for carbon dioxide
correlation spectrometer, or COSPEC, was used to measure sulfur dioxide
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).