Characterization of Eyjafjallajokull volcanic ash particles and a protocol for rapid risk assessment

Abstract

On April 14, 2010, when meltwaters from the Eyjafjallajökull glacier mixed with hot magma, an explosive eruption sent unusually fine-grained ash into the jet stream. It quickly dispersed over Europe. Previous airplane encounters with ash resulted in sandblasted windows and particles melted inside jet engines, causing them to fail. Therefore, air traffic was grounded for several days. Concerns also arose about health risks from fallout, because ash can transport acids as well as toxic compounds, such as fluoride, aluminum, and arsenic. Studies on ash are usually made on material collected far from the source, where it could have mixed with other atmospheric particles, or after exposure to water as rain or fog, which would alter surface composition. For this study, a unique set of dry ash samples was collected immediately after the explosive event and compared with fresh ash from a later, more typical eruption. Using nanotechniques, custom-designed for studying natural materials, we explored the physical and chemical nature of the ash to determine if fears about health and safety were justified and we developed a protocol that will serve for assessing risks during a future event. On single particles, we identified the composition of nanometer scale salt coatings and measured the mass of adsorbed salts with picogram resolution. The particles of explosive ash that reached Europe in the jet stream were especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers. Edges remained sharp even after a couple of weeks of abrasion in stirred water suspensions.

Fig. 1.

Particle size and shape (EMPA backscattered electron images; both same scale) for (A) the explosive ash, (B) typical ash, and (C) particle size distribution. The explosive ash particles are sharp, even at nanometer scale.

Fig. 2.

(A–C) SEM images of (A) fresh explosive ash; edges are sharp, even on the adhering nanoparticles; (B) after stirring in pure water for 90 min; edges are still sharp and particles still adhere; (C) nanoparticles remain aggregated even after stirring, suggesting they are sintered. (D–F) AFS maps of the surface of one particle, such as that shown in A, constructed from data from 10,000 force/distance curves, collected using a standard silicon tip; (D) image representing topography from maximum force data (dark marks the smooth particle surface) with adhering nanoparticles (closer to the observer, lighter color); (E) the same area constructed with adhesion force data; the large particle surface (pink to red) is most adhesive, black represents the middle adhesion range, and the adhering particles (blue) are not sticky; (F) the same area showing elasticity; the surface (green to pale blue) is most elastic and the particles (dark blue) are rigid. Hassenkam et al. (37) explain AFS in more detail.

Fig. 3.

An explosive ash particle glued to an AFM cantilever (A) fresh and (B) after exposure to water for a total of 45 s; particles remain attached while (C) salt is lost. The tiny mass gain (approximately 4 pg) results when the epoxy absorbs water. SEM images of (D) a fresh, typical ash particle with layers and rounded salt condensates; (E) the surface of another particle before, and (F) the same site after exposure to water; material has been removed on the terraces above and below the step edge. A slightly different angle of view for E and F produces slight distortion.

Fig. 4.

AFM images of the surface of an explosive ash particle (A) fresh and (B) after exposure to water for 24 h. Smooth, flat, salt layers were removed and a secondary phase nucleated (such as at arrow), probably goethite.

Fig. 5.

SEM image of (A) a fresh particle of typical ash glued to an AFM tip with epoxy; salt layers and rounded condensates are visible. After exposure to water, salt dissolved and the particle fell from the cantilever, leaving (B) a small fragment of salt behind; its mass was approximately 6 pg. (C) After 100 s in water, the salt completely dissolved, leaving (D) only epoxy.