Settling dynamics of ash aggregates from the Eyjafjallajökull (Iceland) eruption plume illuminated by high-speed video analysis

The recent Eyjafjallajökull eruption strikingly underlined the economic vulnerability of a globalized society to the atmospheric dispersal of volcanic plumes from even moderate-size eruptions. Ash aggregation has already been recognized as a primary control factor in volcanic plume dispersal, because aggregates settle at a terminal velocity higher than that of individual particles and prematurely remove from the plume fine particles that otherwise would settle further away from the volcano. Critical information on the terminal fall velocity and grain size distribution of ash aggregates can be modelled or derived from ash fallout deposited from past eruptions, but it still eluded direct measurement, thus limiting our capability to predict the dispersal of volcanic plumes. Here we use field-based high-speed video footage and laboratory experiments to provide the first direct parameterization of the physical features and settling dynamics of ash aggregates from a volcanic plume. On May 2010 we recorded high-speed video footages of both ash particles and aggregates settling from the Eyjafjallajökull volcano eruption plume at a distance of 7 km from the vent, simultaneously collecting fallout samples (see Andronico et al., this meeting). Experimental determinations of the density, morphology, and settling velocity of ash particles allowed us to distinguish them from the aggregates. Combining footage and experimental analyses we were able to fully characterize the size, settling velocity, drag coefficient, and density distributions of ash aggregates as well as the size distribution of their component particles. The above methodology offers a new eruption monitoring tool capable to provide robust input parameters for models of plume dispersal and consequent hazard forecast.