Since their first deployment in November 1978, the Total Ozone Mapping Spectrometer (TOMS) instruments have provided a robust and near-continuous record of sulphur dioxide (SO2) and ash emissions from active volcanoes worldwide. Data from the four TOMS satellites that have flown to date have been analysed with the latest SO2/ash algorithms and incorporated into a TOMS volcanic emissions database that presently covers 22 years of SO2 and ash emissions. The 1978-2001 record comprises 102 eruptions from 61 volcanoes, resulting in 784 days of volcanic cloud observations. Regular eruptions of Nyamuragira (DR Congo) since 1978, accompanied by copious SO2 production, have contributed material on approximately 30% of the days on which clouds were observed. The latest SO2 retrieval results from Earth Probe (EP) TOMS document a period (1996)(1997)(1998)(1999)(2000)(2001) lacking large explosive eruptions, and also dominated by SO2 emission from four eruptions of Nyamuragira. EP TOMS has detected the SO2 and ash produced during 23 eruptions from 15 volcanoes to date, with volcanic clouds observed on 158 days. The EP TOMS instrument began to degrade in 2001, but has now stabilized, although its planned successor (QuikTOMS) recently failed to achieve orbit. New SO2 algorithms are currently being developed for the Ozone Monitoring Instrument, which will continue the TOMS record of UV remote sensing of volcanic emissions from 2004 onwards.Volcanic eruptions vary greatly in style, duration and vigour, but all sub-aerial eruptions involve the emplacement of material, typically including water vapour and other gases, silicate ash, and aerosols, into the atmosphere above the eruption vent. The detection, analysis and tracking of the ensuing volcanic clouds and plumes is crucial for effective mitigation of volcanic hazards such as airborne ash (e.g. Casadevall 1994), understanding of magmatic degassing processes (e.g. Scaillet et al. 1998;Wallace 2001) and quantify-

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The 1994 eruption of Rabaul, in Papua New Guinea, involved a small plinian eruption at Vulcan and a vulcanian eruption on the opposite side of the caldera at Tavurvur. Vulcan's ash leachates indicate seawater interaction that is consistent with earlier observations of low sulfur dioxide emissions and the presence of ice crystals in the initial plinian eruption cloud. In contrast, Tavurvur ash leachates indicate no seawater interaction, and later sulfur dioxide emissions remained high despite low-level eruptive activity. Silicic melt inclusions indicate that the andesitic melt contained about 2 weight percent water and negligible carbon dioxide. Mafic melt inclusions in Tavurvur ash have water and carbon dioxide contents that vary systematically over the course of the eruption. The mafic melt inclusions suggest that a mafic dike intruded from below the silicic chamber and provide further evidence that mafic intrusions drive caldera unrest.

Satellite-based ultraviolet remote sensing of volcanic eruptions has produced quantitative measurements of the mass of sulfur dioxide and ash in volcanic clouds by accounting for ozone absorption and Rayleigh scattering in the atmos phere. These retrieval techniques were developed with data from the total ozone mapping spectrometer (TOMS) instruments on American, Russian, and Japanese satellites. The sulfur dioxide retrievals have been validated against groundbased Brewer and COSPEC measurements. The ash mass retrievals are in agreement with AVHRR two-band infrared ash retrievals. Daily satellite moni toring has detected, tracked, and quantified S0 2 emissions from a wide range of eruptive activity, from highly explosive to effusive types, and has produced an unprecedented 20-year record of global volcanism. Primary findings from the TOMS data are (1) observations of "excess sulfur" over that liberated during liq uid-phase degassing have indicated the existence of a volatile phase in pre empted magma; (2) indirect evidence for co-erupted H 2 S gas from apparent in crease in S0 2 mass in drifting clouds; (3) insights into the removal rates of S0 2 from the atmosphere, interactions with co-emitted ash particles, and responses to meteorological conditions; and (4) potential operational application of sulfur di oxide and ash detection for aviation hazard mitigation.