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Since sulfur dioxide in volcanic eruption clouds was first measured by the total ozone mapping spectrometer (TOMS) in 1983 [Krueger, 1983], SO2 released from dozens of eruptions has been quantified. Such measurements are invaluable for addressing such problems as global climate change and dynamics, understanding the physical and chemical processes of magma chambers and volcanic eruptions, and volcanic hazards. However, validating the TOMS SO2 retrievals has been difficult due to the fleeting nature of volcanic clouds. An important goal of this validation program is to extend the range of eruptions that can be accurately measured by both ground and satellite instruments.

New Total Ozone Mapping Spectrometer (TOMS) instruments launched in 1996 have provided new and better information about ozone distribution, sulfur dioxide concentrations following large volcanic eruptions, the distributions of ultraviolet-absorbing aerosols (including ash plumes) in the troposphere, and the flux of ultraviolet radiation reaching the Earth's surface. The instruments gathered important data on ozone depletion in the Antarctic in the austral springs of 1996 and 1997 and in the Arctic in the Northern Hemisphere spring of 1997. Ash clouds associated with forest fires in the United States and in Indonesia, as well as the eruption of the Soufriere Hills volcano in Montserrat in December 1997, were also observed. NASA's Earth Probe (EP) satellite, launched in July 1996, carries one TOMS instrument. A second TOMS instrument flew on the Japanese Advanced Environmental Orbiting Satellite (ADEOS), known as “Midori,” launched in August 1996 from Japan. Unfortunately, because the ADEOS spacecraft failed in June 1997, only 10 months of data are available from the ADEOS TOMS instrument (see http://mentor.eorc.nasda.go.jp/ADEOS). The Earth Probe satellite flew in a 500 km Sun-synchronous orbit, which was chosen to complement the 800 km altitude of the ADEOS spacecraft by providing improved horizontal resolution for viewing aerosol sources (26 km x 26 km at nadir instead of the 42 km x 42 km resolution of ADEOS). Because of the lower orbit, data gaps did not allow for full daily maps of the sunlit Earth from EP TOMS. In order to restore the complete global coverage lost with the ADEOS failure, the EP spacecraft was boosted to a 740 km orbit in early December 1997.

The Cerro Hudson volcano in southern Chile (45.92°S, 73.0°W) emitted large ash and sulfur dioxide clouds on August 12–15, following several days of minor activity [Global Volcanism Network Bulletin, 1991]. The SO2 clouds were observed using (preliminary) near real-time data from the Total Ozone Mapping Spectrometer (TOMS) as they encircled the south polar region. The injection of SO2 into the stratosphere has essentially created a gigantic chemical tracer that could provide new insights into the wind patterns and seasonal circulation around the Antarctic region. around the Antarctic region. The TOMS instrument, on board the National Aeronautic and Space Administration's Nimbus 7 satellite, measures the ratio of backscattered Earth radiance to incoming solar irradiance in the ultraviolet spectrum. Although originally designed to measure ozone, it was later discovered that the TOMS instrument could also detect and quantify SO2 [Krueger, 1985]. After this discovery, measurements from TOMS were examined for SO2 emissions for all recorded volcanic eruptions since Nimbus-7 was launched in October 1978, and current data are analyzed as new eruptions occur. The satellite is in a polar, Sun-synchronous orbit so that it crosses the equator at local noon and observes the whole sunlit Earth in approximately 14 orbits each day. Total column amounts of SO2 are determined that represent the amount of gas affecting the reflection of ultraviolet light through a column of the atmosphere from the satellite to the reflecting surface, Earth, given in terms of milli atmospheres centimeter (1000 milli atm cm = a gas layer 1-cm thick at STP). The mass of SO2 is calculated by integrating over the cloud area to obtain a volume, then converting to tons.