Airborne sulfur dioxide (SO2) gas sampling of the Cook Inlet volcanoes (Mt. Spurr, Redoubt, Iliamna, and Augustine, fig. 1) began in 1986 when several measurements were carried out at Augustine volcano during the eruption of 1986 (Rose and others, 1988). More systematic monitoring for SO2 began in March 1990 and for carbon dioxide (CO2) began in June, 1990 at Redoubt Volcano (Brantley, 1990 and Casadevall and others, 1994) and continues to the present. This report contains all of the available daily SO2 and CO2 emission rates determined by the U.S. Geological Survey (USGS) from March 1990 through July 1994. Intermittent measurements (four to six month intervals) at Augustine and Iliamna began in 1990 and continues to the present. Intermittent measurements began at Mt. Spurr volcano in 1991, and were continued at more regular intervals from June, 1992 through the 1992 eruption at the Crater Peak vent to the present.
A correlation spectrometer (COSPEC) was used to measure SO2 in the Cook Inlet volcanic plumes. The upward-looking COSPEC was mounted in a fixed- wing aircraft and flown below and at right angles to the plume. The first traverse (or orbit) during a measurement was used to determine the size, location and direction of the volcanic plume. In some cases no SO2 plume was detected by the instrument and a value of zero was reported as the SO2 rate. Even though SO2 may have been sensed by the operator's nose (more sensitive than the instrument), a rate of zero was still reported. Typically, three to six traverses were made underneath the plume to determine an average SO2 burden (concentration multiplied by the pathlength) within a cross-section of the plume. Knowing the burden, the plume width and plume velocity (assumed to be the same as ambient wind speed), the emission rate of SO2 was calculated. The resultant value of SO2 is reported after values of more than one standard deviation from the average have been removed. All emission rates are reported in metric tons/day (t/d) above the background level upwind of the volcano. The use of correlation spectroscopy for determining the SO2 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 Cook Inlet volcanic plumes was measured by an infrared spectrometer (MIRAN) tuned to the 4.26 micrometers CO2 absorption band. Volcanic CO2 is defined as that gas detected within a volcano's plume that is in excess of the concentration of ambient CO2 in the atmosphere. An external sample tube was attached to the fuselage of a twin-engine aircraft to deliver outside air to the gas cell of the MIRAN. The aircraft was then flown at several different but increasing elevations through the plume. These traverses were at right angles to the plume trajectory and defined plume area and CO2 concentration in a vertical cross-section of the plume. Plume area, CO2 density at the mean altitude of the plume, and the plume velocity (assumed to be equal to the ambient wind speed), were then used to calculate the CO2 emission rate. The resulting calculated CO2 emission rate is not an average of several measurements (as is the case for SO2 emission values), but one value. During times of low emission rates and low wind speeds, orbits within a kilometer of the vent were required, which gave low results because of slow instrumental response times. Thus, some CO2 measurements reported here are indicated to be minimum values. All emission rates are reported in metric tons/day (t/d) above the background level upwind of the volcano. The use of infrared spectroscopy for determining the CO2 output of volcanoes is discussed in detail by Harris and others (1981).
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