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

Kenneth A. McGee
David A. Johnston Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Boulevard, Vancouver, Washington 98661, (206) 696-7695,

Thomas J. Casadevall
Mail Stop 903, U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, CO 80225-0046; (303) 236-1080,
U.S. Geological Survey Open-File Report 94-212

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.


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 Ám 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).


From May 1980 to September 1988, more than 1000 fixed-wing aircraft flights were made by the U.S. Geological Survey in order to measure and characterize gas emissions from Mount St. Helens. Sulfur dioxide was detected on the majority of these flights. However, toward the end of this time period, and particularly during the final two years of measurements, the sulfur dioxide burden was often below the detection limit of the COSPEC. On those days, the sulfur dioxide emission rate was arbitrarily assigned a value of 3 tonnes/day in the database. Carbon dioxide was routinely measured starting in July 1980. These measurements were discontinued in August 1981 after CO2 levels had declined to near background levels.

The data listing in this report contains all of the available daily SO2 and CO2 emission rates determined by the USGS from May 1980 through the end of the measurements in September 1988. On a few occasions, two gas- measurement flights were made in a single day. In those cases, two emission-rate values are listed for that day. Portions of this database have been presented earlier by Casadevall and others (1981, 1983), Harris and others (1981), McGee (1992a), and McGee and Sutton (in press). Other data pertaining to these measurements such as plume dimensions and wind information were earlier listed in McGee (1992b).


Emission rates from Mount St. Helens, 1980-1988 in tab-delimited ASCII format.

Editor's note: I have included graphs of the raw data purely to illustrate the general pattern they show.


We would like to acknowledge the efforts of numerous individuals who contributed to the success of the USGS airborne gas measurement program by participating in the gas flights. Funding for this work has been provided by the USGS Volcano Hazards Program and the Global Change and Climate History Program.


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