Aggregation-dominated ash settling from the Eyjafjallajökull volcanic cloud illuminated by field and laboratory high-speed imaging

The recent Eyjafjallajökull (Iceland) eruption strikingly under-lined the vulnerability of a globalized society to the atmospheric dispersal of volcanic clouds from even moderate-size eruptions. Ash aggregation controls volcanic clouds dispersal by prematurely remov-ing fi ne particles from the cloud and depositing them more proxi-mally. Physical parameters of ash aggregates have been modeled and derived from ash fallout deposits of past eruptions, yet aggregate sedimentation has eluded direct measurement, limiting our ability to predict the dispersal of volcanic clouds. Here we use fi eld-based, high-speed video analysis together with laboratory experiments to provide the fi rst in situ investigation and parameterization of the physical fea-tures and settling dynamics of ash aggregates from a volcanic cloud. In May 2010, high-speed video footage was obtained of both ash par-ticles and aggregates settling from the Eyjafjallajökull volcano erup-tion cloud at a distance of 7 km from the vent; fallout samples were collected simultaneously. Experimental laboratory determinations of the density, morphology, and settling velocity of individual ash par-ticles enable their distinction from aggregates. The combination of fi eld and experimental analyses allows a full characterization of the size, settling velocity, drag coeffi cient, and density distributions of ash aggregates as well as the size distribution of their component par-ticles. We conclude that ash aggregation resulted in a tenfold increase in mass sedimentation rate from the cloud, aggravating the ash haz-ard locally and modifying cloud dispersal regionally. This study pro-vides a valuable tool for monitoring explosive eruptions, capable of providing robust input parameters for models of cloud dispersal and consequent hazard forecas