Eos, Vol. 77, No. 41, October 8, 1996 New Methods Make Volcanology Research Less Hazardous PAGES 393, 396-397 Peter Francis, Adam Maciejewski, Clive Oppenheimer, and Charles Chaffin Volcanic gases are important to the scien- tific community for many reasons: the Earth's secondary atmosphere originated through volcanic degassing, and volcanic gases still play a crucial role in the Earth system. Occa- sional massive eruptions pump such large quan- tities of acid gas into the stratosphere that the resulting aerosols modify global climate for months or years. Also, volcanic gases escaping from magma bodies act as "messengers" that warn of impending eruptions and convey in- sight into magma chamber processes. Volcanologists are interested in both the fluxes and the compositions of exhaled gases, but collecting in situ measurements on active volcanoes is hazardous. Several vol- canologists were killed by an unexpected eruption of Galeras volcano in Colombia in January 1993, including our colleague Geoff Peter Francis and Adam Maciejewski, Depart- ment of Earth Sciences, The Open Univer- sity, Walton Hall, Milton Keynes, MK6 7AA, United Kingdom; Charles Chaffin, Department of Chemistry, Kansas State University, Manhat- tan, KS 66506; Clive Oppenheimer, Depart- ment of Geography, University of Cambridge, Cambridge CB2 3EN, United Kingdom Brown. This tragedy emphasized the need for remote methods for studying volcanoes. Two approaches are being explored: satellite re- mote sensing and ground-based techniques. Progress is being made using open-path Fourier transform infrared (OP-FTIR) tech- niques for gas studies. During a field cam- paign on Mount Etna and Vulcano in Italy, we found it was possible to measure SO2 and HC1 concentrations in plumes over distances up to 1.85 km, and discovered that the trace gas SiF4 could easily be detected. This hith- erto little known gas may provide a useful guide to vent temperatures. FTIR spectros- copy has the potential to provide a powerful tool for monitoring volcanoes. Even when samples are collected directly from vents, analysis of volcanic gases is diffi- cult because of contamination by ambient air. With remote techniques, complete analy- ses are impractical, because the atmosphere already contains large amounts of the two most important components of volcanic gases: H2O and CO2. Gas species that have very low background atmospheric concentra- tions, such as SO2 and HC1, are much easier to analyze. The standard technique for re- mote gas measurement is the correlation spectrometer (COSPEC). This instrument measures the absorption of solar ultraviolet radiation by SO2 in a volcanic gas plume. COSPEC has provided an invaluable vol- canological aid for more than 20 years, but it only measures SO2 concentrations. Given that any gas plume is being constantly di- luted downwind of the vent, concentration measurements of a single gas are only mean- ingful if these can be interpreted in terms of its flux. Increases in the SO2 flux from a vol- cano are important indicators of potential eruptions, since depressurization causes SO2 to exsolve from an ascending body of fresh magma. However, estimation of the flux of a gas requires knowledge of the dimensions of the plume and wind speed, as well as the con- centration of the gas. All three parameters can be difficult to determine. If the concentrations of two or more gases can be measured simultaneously, then plume chemistry can be monitored without estimates of flux. Changing gas concentra- tion ratios in the plume provide direct in- sights into the activity of the magmatic/ hydrothermal systems. In an ideal world, vol- canologists would have both SO2 flux data provided by COSPEC and data on the concen- trations of other gas species. Using Infrared to Detect Gases OP-FTIR spectroscopy provides one means of measuring a number of gas species simultaneously. It relies on detecting the characteristic absorptions of different gases in the infrared part of the spectrum. The po- tential for using natural infrared radiation from hot volcanic sources for gas studies was first noted by Naughton et al. [1969], who ob- served a lava fountain at Kilauea with a spec- trophotometer working between 2.5 and 14.5 fim, and were able to detect sulphur dioxide. FTIR spectroscopy is capable of much higher spectral resolution. Although it is not new, the technique has flourished recently be- cause concerns over air pollution have cre- ated a demand for portable equipment that can monitor trace gases. It was first applied to volcanology in 1991 at the Aso volcano in Japan, where Notsu and his group detected fumarolic SO2 from a distance of 4 km [Notsu et al., 1993]. Later, concentrations of both SO2 and HC1 were measured in the gas plume emanating from the growing lava dome at Mt. Unzen [Morietal, 1993]. Apyroclastic flow from this dome took 60 lives, including those of several journalists and three well-known volcanologists. Notsu's team observed from the safety of a topographic high point 1.3 km away. The equipment that our group used for the OP-FTIR spectroscopy method consists of a small spectrometer mounted on a 10" New- tonian astronomical telescope and hooked up to a portable computer. Radiation from a hot source traveling through the gas plume is Fig. 1. Researchers collecting field data with the Fourier transform infrared spectrometer on Mt. Etna, from a point about 2 km from and 500 m below the summit crater, which is concealed by the prodigious gas plume. In this case, an artificial infrared source was placed on the ash slopes in the middle distance at a range of about 500 m. This page may be freely copied.