Principles of COSPEC V at Mt. Erebus
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from Kyle, P.R., Sybeldon, L.M., McIntosh, W.C., Meeker, K., and Symonds, R., 1994, Sulfur dioxide emission rates from Mount Erebus, Antarctica, in Kyle, P.R., ed., Volcanological and Environmental Studies of Mount Erebus, Antarctica, Volume 66: Washington, D.C., American Geophysical Union, p. 69-82.

Sulfur dioxide emission rates [from Mt. Erebus] were determined using a Barringer COSPEC V correlation spectrometer. The COSPEC operates by dividing ultraviolet light into several different spectral bands and measuring the intensity of energy in those bands. A telescope on the COSPEC (field of view 23 milliradians by 7 milliradians) scans incident solar radiation. Four masks containing seven gratings slit each filter and separate the light according to wavelength. Two bands of radiation are of particular interest; the first is the band of radiation of the wavelength where energy is absorbed by atmospheric sulfur dioxide. The second is the band of radiation where the presence of atmospheric sulfur dioxide produces energetic radiation (Millian et al., 1985). The ratio of the energy intensities of these two sets of radiation, in the absence of extraneous sulfur dioxide, produces a base line for the analysis. The energy intensity ratio is proportional to the amount of sulfur dioxide present. The COSPEC electronically produces an output voltage signal proportional to the ratio.

The COSPEC contains an automatic gain control (AGC) to correct for changes in the intensity of ultraviolet radiation during the day. The AGC adjusts the sensitivity of the instrument based on the intensity of the incoming radiation. The instrument is frequently calibrated by measuring the energy intensity ratio when a fused quartz chamber containing a known quantity of SO2 is inserted into the field of view of the instrument against a SO2-free background.

Since the 1987/1988 austral summer field season, an automatic scanning head that provides constant scan angles and scan rates with minimal aberrations due to operator interaction was used to collect all of the data (Kyle and McIntosh, 1989). In the automated technique, data were collected by mounting the COSPEC on an automatic scanner and recorded into the memory of a Toshiba T1200 HB laptop computer utilizing the software program COSPEC. Data were reduced using the program ASPEC.

The distance from the COSPEC to the plume was determined using topographic maps. The scan angle for each data set was measured by means of an inclinometer or protractor mounted on the COSPEC. The distance to the plume and scan angle are used to calculate the width of the plume.

Much of the COSPEC data [collected at Mt. Erebus] was accompanied by simultaneous video recordings of the plume shot from the COSPEC site. The video records allow accurate determination of the plume velocity. Owing to the small volume of the volcanic plume at Erebus and the distance from the COSPEC site to the plume (approximately 2 km), COSPEC data were mainly collected on windless days when the plume rose vertically. In this case, plume velocities depend only on thermal inertia.

The distance from the COSPEC/camera site at the lower Erebus hut and the distance between topographical landmarks on the rim of Erebus are well known. Plume velocities were therefore calculated directly from video tapes. Some distinguishable feature of the plume (for example, a discoloration or the leading edge of a puff) was timed using a stopwatch as it traveled known distances on the video screen. When time and distance traveled are measured, the calculation of plume velocity is trivial. In circumstances where the video was unsuitable (owing to no visible plume, for instance) or when no video was shot, average plume velocities from COSPEC sessions immediately preceding and/or following the times of "video gaps" were used. Nearly all measurements of plume travel times made using this method have better than ±5% statistical error, thereby greatly improving accuracy in one of the greatest sources of error (see Table 1 below) involved with correlation spectrometry observations.

TABLE 1. Uncertainties in COSPEC Measurements at Mount Erebus.
Error Source	With Video, %	Without Video, %
Wind speed/rise rate	±5	±30
Distance to plume	±3	±3
Scan rate	±2	±2
Data reduction with ASPEC	±2	±2
Cumulative error (square root of the sum of the squares)	±2	±30
Error is based on reproducibility, except distance and scan rate errors, which are best estimates.