Characterizing ash-atmosphere interactions for better hazard prediction from explosive volcanic eruptions

Aviation and other hazards result from high intensity, explosive eruptions that inject volcanic ash into the upper troposphere and lower stratosphere. Eruptions of VEI4 and higher occurred in the historical time and recent geologic past, and are expected to occur again, with potential hazards to modern infrastructure. Atmospheric transport, dispersion, and sedimentation of ash particles are controlled by fundamentally different processes than other particles normally transported in the atmosphere, due to the complex internal and external morphology of ash particles. We have found that the morphology of erupted ash depends on eruption energetics, and that this morphology, in turn, controls the behavior of ash in the atmosphere. Eruptions with Volcanic Explosivity Indices (VEI) of less than 4 typically undergo a single phase of bubble nucleation at depth during decompressive magma ascent, while more energetic eruptions (VEI>=4) experience a second phase of nucleation near the vent as a result of explosive decompression that raises oversaturation of inter-bubble water to above the critical threshold for nucleation of new bubbles. This second phase of nucleation results in very different ash morphology than found in less energetic eruptions, with a greater fraction of compound ash particles, smaller simple particles, and greater complexity of particles in general. The morphology of ash from such energetic eruptions affects transport behavior in the dry stratosphere, where suspended ash settles under the influence of gravity, but is modulated by hydrodynamic interaction with the surrounding air. In addition, the alteration of flow fields around ash particles can lead to particle-fluid-particle interactions that add additional complexity to ash behavior. This has direct application to atmospheric transport models that are currently inadequate for predicting the distribution of the distal post-eruption ash hazard. While numerous other factors affect long-range stratospheric ash transport, the character of the ash itself can at least be better incorporated into transport models as a step toward more accurately predicting volcanic ash hazards from future eruptions.