Thursday, May 17, 2012

Gas monitoring

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Measurements of the gas emissions from the volcano have been carried out throughout the eruption. These measurements have constituted an important input to the assessment of volcanic risk. Sulphur dioxide (SO2) and hydrogen chloride (HCl) are the species routinely monitored, primarily by remote sensing techniques.

Sulphur dioxide emission rates were initially measured using a correlation spectrometer (COSPEC) from July 1995 through to December 2001. The COSPEC utilizes a sulphur dioxide absorption feature in the ultra violet (UV) spectrum. It does so by capturing sunlight via an upward pointing telescope. Measurements were taken 2-5 times per week, using car, boat or the helicopter as a platform for horizontal traverses under the gas plume. On rare occasions a fixed scanning technique is employed, this involves mounting the (COSPEC) instrument on a tripod and manually rotating the instruments telescope, perpendicular to the plume axis.

A new technique for measuring SO2 flux (DOAS) involving the use of miniature ultraviolet (UV) spectrometers was first tested in August-September 2001 in the traverse modes (similar to COSPEC), then deployed in full at the beginning of 2002 in fixed scanning modes. This new technique comprises of a network of three spectrometers in fixed positions scanning the sky between the hours of 8am-4pm each day. The data is continuously telemetered back to the MVO. This thus provided a more continuous SO2 dataset relative to the sporadic (COSPEC) measurements of before.

HCl emission rates have been measured indirectly using open-path Fourier Transform Infrared spectrometer (FTIR) measurements to obtain HCl: SO2 mass ratios in the plume allowing HCl emission rates to be calculated from SO2 flux rates obtained from the UV spectrometers. The mass ratio itself may be the most revealing piece of information and may be used to evaluate changes in eruption rate and growth of the dome. The emission rates of HCl and SO2 have also, most importantly, become a valuable indicator of pauses in dome growth.