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[1] Water is the dominant component of volcanic gas emissions, and water phase transformations, including the formation of ice, can be significant in the dynamics of volcanic clouds. The effectiveness of volcanic ash particles as ice-forming nuclei (IN) is poorly understood and the sparse data that exist for volcanic ash IN have been interpreted in the context of meteorological, rather than volcanic clouds. In this study, single-particle freezing experiments were carried out to investigate the effect of ash particle composition and surface area on water drop freezing temperature. Measured freezing temperatures show only weak correlations with ash IN composition and surface area. Our measurements, together with a review of previous volcanic ash IN measurements, suggest that fine-ash particles (equivalent diameters between approximately 1 and 1000 mm) from the majority of volcanoes will exhibit an onset of freezing between $250-260 K. In the context of explosive eruptions where super-micron particles are plentiful, this result implies that volcanic clouds are IN-rich relative to meteorological clouds, which typically are IN-limited, and therefore should exhibit distinct microphysics. We can expect that such ''overseeded'' volcanic clouds will exhibit enhanced ice crystal concentrations and smaller average ice crystal size, relative to dynamically similar meteorological clouds, and that glaciation will tend to occur over a relatively narrow altitude range.

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Theoretical models are developed for the sedimentation from the margins of a particleladen, axisymmetric, turbulent, buoyant plume, in a still environment and for an axisymmetric turbulent momentum jet. The models assume that the mass of each individual size fraction of sediment carried in a parcel of fluid decreases exponentially with time. For relatively coarse particles, the fallout models predict that the sediment deposition beyond a distance r on the ground expressed in log units should decay linearly with distance away from the vent for the momentum jet and should decrease with r I/3 for the buoyant plume. The exponential decay constant J is proportional to the terminal fall velocity Vt of the particles in both cases and inversely proportional to the square root of the initial momentum flux M o for the jet fallout (Jj o• Vt Mo' 1/2) and to the third power of the initial buoyancy flux Fo for the plume fallout (Jp o• Vt Fo'1/3). Smaller particles areaffected by reentrainment caused by the turbulent eddies sweeping ambient fluid back into the plume or jet and thus reincorporating some particles that were released from the flow at greater heights. This is taken into account by introducing a reentrainment coefficient, {•, into the theoretical models with the assumption that the coefficient has a constant value for a plume of given strength. In new experiments, fallout occurs from the margins of particle-laden, fresh water, buoyant jets, and plumes in a tank of salty water, and sedimentation is measured on the tank floor. Two experiments were weakly affected by reentrainment and show excellent agreement with the simple theory. For smaller particles and increasingly buoyant plumes and strong jets, particle reentrainment is important. The experimental data are fitted by the new reentrainment theory, confirming that values of the reentrainment coefficient are approximately constant for a given flow. A settling number, 13, is defined as the ratio of the characteristic velocity of the jet or plume to the particle settling velocity. For 13 > 1, reentrainment seems to reach an equilibrium state for which the reentrainment coefficient is a constant of value 0.1 for jets and 0.4 for plumes, irrespective of flow strength or particle size. The plume experiments indicate that the value of the reentrainment coefficient is strongly dependent on plume strength and particle size for 13 slightly less than 1. The general principles of sedimentation from turbulent plumes and jets are applied to the fallout of pumice from volcanic eruption columns and of metalliferous particles from black smokers on the ocean floor. For volcanic eruptions, the results provide an explanation for the near vent overthickening of tephra fall deposits and imply that lithic and pumice fragments from small Iapilli up to at least I m diameter blocks are efficiently reentrained into eruption columns. The size of particles reentrained in hydrothermal plumes is predicted to vary from less than 100 gm in weakly buoyant plumes up to over 1000 I. tm in megaplumes...

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Volcanic eruptions are events that rapidly and suddenly disperse gases and ne particles into the atmosphere, a process most conveniently studied from the synoptic satellite perspective, where remote sensing o¬ers a practical tool for spatial and temporal measurements. Meteorological satellites o¬er approximately 20 years of archived data, which can be analysed for measurements of masses of SO 2 and ne volcanic ash in spatial two-dimensional arrays and integrated with other meteorological data. The satellite data o¬er a tool to study volcano{atmosphere interactions in a quantitative way. They provide information of unique value for understanding the fate and transport of ne silicates with signi cant health hazards and for addressing the problem of volcanic cloud hazards to jet aircraft. Studies of satellite data have demonstrated the following.(1) Volcanic clouds from convergent plate boundary volcanoes contain large and variable excesses of SO 2 . c

An 80,000 km 2 stratospheric volcanic cloud formed from the 26 February 2000 eruption of Hekla (63.98° N, 19.70° W). POAM-III profiles showed the cloud was 9-12 km asl. During 3 days this cloud drifted north. Three remote sensing algo rithms (TOMS S0 2 , MODIS & TOVS 7.3 urn IR and MODIS 8.6 urn IR) estimat ed -0.2 Tg S0 2 . Sulfate aerosol in the cloud was 0.003-0.008 Tg, from MODIS IR data. MODIS and AVHRR show that cloud particles were ice. The ice mass peaked at -1 Tg -10 hours after eruption onset. A -0.1 Tg mass of ash was detected in the early plume. Repetitive TOVS data showed a decrease of S0 2 in the cloud from 0.2 Tg to below TOVS detection (i.e.O.Ol Tg) in -3.5 days. The stratospheric height of the cloud may result from a large release of magmatic water vapor early (1819 UT on 26 February) leading to the ice-rich volcanic cloud. The optical depth of the cloud peaked early on 27 February and faded with time, apparently as ice fell out. A research aircraft encounter with the top of the cloud at 0514 UT on 28 February, 35 hours after eruption onset, provided validation of algorithms. The aircraft's instruments measured -0.5-1 ppmv S0 2 and -35-70 ppb sulfate aerosol in the cloud, 10-30% lower than concentrations from retrievals a few hours later. Different S0 2 algorithms illuminate environmental variables which affect the qual ity of results. Overall this is the most robust data set ever analyzed from the first few days of stratospheric residence of a volcanic cloud.