National Aeronautics and Space Administration

Abstract. Volcanic clouds are an important natural hazard to aircraft and host chemical reactions that interest both volcanologists and atmospheric scientist. Ice has been suggested as a possible component of eruption clouds, but there has been no direct evidence for its presence. Here we report the detection, using a satellite-borne infrared sensor, of >2 million tonnes of ice in the cloud produced by the September 1994 eruption of Rabaul volcano, in Papua New Guinea. The cloud also contained relatively low levels of sulphur dioxide (80±50 kilotonnes), compared with other stratospheric eruption clouds. The unusual aspects of this cloud may be related to the entry of sea water into the volcanic vent, and its participation in the eruption column. Past eruptions the occurred in similar (coastal) settings, such as those of Krakatau and Santorini, might have has less effect on the atmosphere than their volume alone would suggest, because the presence of ice may decrease the residence time of ash and sulphur in the atmosphere. In addition, the ability of ice to mask the characteristic spectral signature of volcanic ash will increase the difficulty of designing airborne ash detection systems for aviation safety.