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

Abstract

The recent Eyjafjallajökull (Iceland) eruption strikingly underlined 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 removing fi ne particles from the cloud and depositing them more proximally. 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, highspeed video analysis together with laboratory experiments to provide the fi rst in situ investigation and parameterization of the physical features and settling dynamics of ash aggregates from a volcanic cloud. In May 2010, high-speed video footage was obtained of both ash particles and aggregates settling from the Eyjafjallajökull volcano eruption 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 particles 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 particles. We conclude that ash aggregation resulted in a tenfold increase in mass sedimentation rate from the cloud, aggravating the ash hazard locally and modifying cloud dispersal regionally. This study provides a valuable tool for monitoring explosive eruptions, capable of providing robust input parameters for models of cloud dispersal and consequent hazard forecast.

The operational eEMEP model version 10.4 for volcanic SO<sub>2</sub> and ash forecasting

“…Both model and satellite retrievals, used as input to the inversion, operate today with assumed equal ash characteristics during the whole eruption period. Other processes, such as fine ash aggregation that increases the gravitational settling speed and reduces the atmospheric residence time (Brown et al, 2011), were also observed during the Eyjafjallajökull eruption (Taddeucci et al, 2011), but are not included in the model at this time.…”

A review of volcanic ash aggregation

“…The timescales of aggregate formation are short: for example, AP1 aggregates were deposited within five minutes of the onset of the 1990 eruption of Sakurajima volcano, Japan (Gilbert and Lane, 1994) In long-lived ash clouds (>1 day), cloud water concentrations fall close to background levels due to mixing and sedimentation (e.g., Schumann et al, 2011), and electrostatic forces may then play an important role in particle binding (James et al, 2003). Aggregation played an important role in the settling of ash from eruption clouds generated during the 2010 eruption of Eyjafjallajökull volcano, Iceland (Taddeucci et al, 2011). Ground observations indicate that in May 2010 fine ash reached the surface as both particle clusters and accretionary pellets (e.g., Fig.…”

Atmospheric Electrification in Dusty, Reactive Gases in the Solar System and Beyond

“…The dirty thunderstorm mechanism requires ice particles in the plume and is only likely to be important for plumes which reach altitudes with temperatures that allow freezing to occur electric potential gradient necessary to generate lightning discharges. Clustering can be particularly effective in the presence of prevalently fine ash-laden jets exiting volcanic conduits 15 thus facilitating ash aggregation in the plume (Taddeucci et al 2011). Further charging by the formation of hydrometeors (i.e.…”