DESCRIPTION:
Correlation Spectrometer (COSPEC) Measurements

MSH83_USGS_scientists_COSPEC_on_SugarBowl_10-21-83.jpg USGS scientists use a COSPEC (correlation spectrometer), to measure sulfur-dioxide gases being emitted from the dome. Setup on Sugar Bowl, located on the northwest flank of Mount St. Helens. USGS Photograph taken on October 21, 1983, by Lyn Topinka. [medium size] ... [large size] ... [TIF Format, 18 M]

Gas emissions are measured regularly in conjunction with seismicity and ground deformation to monitor eruptive activity. Mount St. Helens continuously emits volcanic gas from fumaroles on and around the dome. Most of the gas emitted by the volcano is water vapor, but emissions also include sulfur dioxide, carbon dioxide, hydrogen, and lesser amounts of helium, carbon monoxide, hydrogen sulfide, and hydrogen chloride.

Gas studies include (1) frequent airborne measurements of sulfur dioxide and, in 1980 and 1981, carbon dioxide in the plume and (2) less frequent sampling of gases from crater fumaroles. The emissions of sulfur dioxide are measured in the plume by a correlation spectrometer (COSPEC) designed originally for pollution studies. The instrument measures the amount of solar ultraviolet light absorbed by sulfur dioxide in the plume and compares it with an internal standard. Three to six traverses are made beneath the plume at right angles to the plume trajectory several times each week to calculate daily emission rates.

... the bulk sulfur dioxide (SO2) emissions both at Kilauea's summit and at Pu'u O'o are measured by the correlation spectrometer (COSPEC) in metric tons per day. COSPEC compares the spectra of the incoming light with the known spectra of sulfur dioxide to determine the amount of the gas in the volcanic plume. The COSPEC is aimed directly at a volcanic plume, either from the ground or from an airplane. Large numbers of measurements yield results that are statistically significant. ...

Emission rates of gases are studied to estimate the quantity of SO2 and CO2 being emitted from a volcano, and are carried out by making measurements in the volcanic plume. Sulfur dioxide emission rates have been used to infer the volume of degassing magma and magma supply. Sulfur dioxide and carbon dioxide measurements are both reported as fluxes with units of metric tonnes per day, although the two gases are measured in different ways.

Sulfur dioxide is measured remotely using a correlation spectrometer (COSPEC). Scattered solar ultraviolet energy of specific wavelengths is absorbed in proportion to SO2 concentration and plume thickness. This absorption, with units of pathlength-concentration, is measured by the COSPEC and calibrated with the absorption of an SO2 gas standard in the instrument. The product of the plume light-absorption profile and wind speed yields the SO2 emission rate. The technique is described by Casadevall and others (1981; 1983; 1987), Stoiber and others (1983), and Millan and others (1976). This method is used routinely and worldwide to make volcanic SO2 measurements. Measurements can be made from the ground or the air. Wind speed is determined by using a hand-held anemometer for ground-based measurements. Airborne SO2 and CO2 measurements use wind speeds measured by a variety of methods including those reported by local airports. Alternatively, wind speed is determined by comparing true air speed, flying with and against the wind, and true ground speed. Accurate determination of wind speed is critical for reliable emission-rate determinations. Airborne measurements of SO2 tend to be more reliable than ground-based measurements, because wind speed is determined at the site of measurement.

Ground-based COSPEC measurements can be made from a vehicle by traversing beneath the plume while pointing the instrument up through it. Alternatively, the COSPEC can be mounted on a tripod near the vent to scan either vertically or horizontally through the plume.

Airborne SO2 measurements are made by flying below and at right angles to the plume trajectory. Multiple traverses for either ground-based or airborne measurements are averaged to calculate SO2 emission rates, which are then scaled up to a daily rate.

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