ICE IN THE 1994 RABAUL ERUPTION CLOUD - IMPLICATIONS FOR VOLCANO HAZARD AND ATMOSPHERIC EFFECTS

VOLCANIC clouds are an important natural hazard to aircraft(1), and ho st chemical reactions that interest both volcanologists(2,3) and atmos pheric scientists(4-6). Ice has been suggested as a possible component of eruption clouds', but there has been no direct evidence for its pr esence. Here we report the detection, using a satellite-borne infrared sensor, of greater than or similar to 2 million tonnes of ice in the cloud produced by the September 1994 eruption of Rabaul volcano, in Pa pua New Guinea. The cloud also contained relatively low levels of sulp hur dioxide (80+/-50 kilotonnes), compared with other stratospheric er uption clouds. The unusual aspects of this cloud may be related to the entry of sea water into the volcanic vent, and its participation in t he eruption column. Past eruptions that occurred in similar (coastal) settings, such as those of Krakatau and Santorini, might have had less effect on the atmosphere than their volume alone would suggest, becau se the presence of ice may decrease the residence time of ash and sulp hur in the atmosphere. In addition, the ability of ice to mask the cha racteristic spectral signature of volcanic ash will increase the diffi culty of designing airborne ash detection systems for aviation safety.