Airborne ice nuclei near an active volcano

Abstract

ACTIVE volcanoes have been suggested as being sources of atmospheric ice nuclei^1,2, the rare yet crucial particles that initiate much of the Earth's precipitation. Other studies indicate that some volcanoes are not adding ice nuclei to the atmosphere or may be deactivating natural ice nuclei^3–5. We report here measurements of atmospheric ice nucleus concentrations upwind and within the effluent plume of St Augustine, an actively venting island volcano situated at the base of the Aleutians in the mouth of Cook Inlet, Alaska (59°20′N, 153°30′W). St Augustine erupted violently on January 23, 1976 and then again on February 4, 1976. We find no evidence for ice nuclei production by this volcano. Between and after the major eruptions the volcano actively vented giving off gaseous and particulate matter. Sulphur dioxide concentrations in the plume were >5p.p.m. (J. Moyers, unpublished) and the total particulate loading was ∼100 μg m⁻³ (W. Zoller, unpublished) during the venting stages. A photograph of the venting volcano (February 1, 1976), taken as the National Center for Atmospheric Research (NCAR) Electra aircraft (instrument boom visible to left) prepared to enter the plume on a sampling run, is shown in Fig. 1. During this and other sampling runs ice nuclei in the air were captured on Millipore membrane filters using the NCAR SAMOVAR Air Filtration System⁶.