[1] Large-scale volcanic eruptions produce fine ash (<200 mm) which has a long atmospheric residence time (1 hour or more) and can be transported great distances from the volcanic source, thus, becoming a hazard to aircraft and public health. Ash particles have irregular shapes, so data on particle shape, size, and terminal velocities are needed to understand how the irregular-shaped particles affect transport processes and radiative transfer measurements. In this study, a methodology was developed to characterize particle shapes, sizes, and terminal velocities for three ash samples of different compositions. The shape and size of 2500 particles from (1) distal fallout ($100 km) of the 14 October 1974 Fuego eruption (basaltic), (2) the secondary maxima ($250 km) of the 18 August 1992 Spurr eruption (andesitic), and (3) the Miocene Ash Hollow member, Nebraska (rhyolitic) were measured using image analysis techniques. Samples were sorted into 10 to 19 terminal velocity groups (0.6-59.0 cm/s) using an air elutriation device. Grain-size distributions for the samples were measured using laser diffraction. Aspect ratio, feret diameter, and perimeter measurements were found to be the most useful descriptors of how particle shape affects terminal velocity. These measurement values show particle shape differs greatly from a sphere (commonly used in models and algorithms). The diameters of ash particles were 10-120% larger than ideal spheres at the same terminal velocity, indicating that irregular particle shape greatly increases drag. Gas-adsorption derived surface areas are 1 to 2 orders of magnitude higher than calculated surface areas based on measured dimensions and simple geometry, indicating that particle shapes are highly irregular. Correction factors for surface area were derived from the ash sample measurements so that surface areas calculated by assuming spherical particle shapes can be corrected to reflect more realistic values.

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Satellite SO 2 and ash measurements of Mount Spurr's three 1992 volcanic clouds are compared with ground-based observations to develop an understanding of the physical and chemical evolution of volcanic clouds. Each of the three eruptions with ratings of volcanic explosivity index three reached the lower stratosphere (14 km asl), but the clouds were mainly dispersed at the tropopause by moderate to strong (20-40 m/s) tropospheric winds. Three stages of cloud evolution were identified. First, heavy fallout of large (1500 mm) pyroclasts occurred close to the volcano (!25 km from the vent) during and immediately after the eruptions, and the cloud resembled an advected gravity current. Second, a much larger, highly elongated region marked by a secondary-mass maximum occurred 150-350 km downwind in at least two of the three events. This was the result of aggregate fallout of a bimodal size distribution including fine (!25 mm) ash that quickly depleted the solid fraction of the volcanic cloud. For the first several hundred kilometers, the cloud spread laterally, first as an intrusive gravity current and then by wind shear and diffusion as downwind cloud transport occurred at the windspeed (during the first 18-24 h). Finally, the clouds continued to move through the upper troposphere but began decreasing in areal extent, eventually disappearing as ash and SO 2 were removed by meteorological processes. Total SO 2 in each eruption cloud increased by the second day of atmospheric residence, possibly because of oxidation of coerupted H 2 S or possibly because of the effects of sequestration by ice followed by subsequent SO 2 release during fallout and desiccation of ashy hydrometeors. SO 2 and volcanic ash travelled together in all the Spurr volcanic clouds. The initial (18-24 h) area expansion of the clouds and the subsequent several days of drifting were successfully mapped by both SO 2 (ultraviolet) and ash (infrared) satellite imagery.