Articles | Volume 17, issue 17
Atmos. Chem. Phys., 17, 10709–10732, 2017
Atmos. Chem. Phys., 17, 10709–10732, 2017

Research article 12 Sep 2017

Research article | 12 Sep 2017

Atmospheric processes affecting the separation of volcanic ash and SO2 in volcanic eruptions: inferences from the May 2011 Grímsvötn eruption

Fred Prata1, Mark Woodhouse2, Herbert E. Huppert3, Andrew Prata4, Thor Thordarson5, and Simon Carn6 Fred Prata et al.
  • 1Visiting scientist, Department of Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, Oxford, UK
  • 2School of Mathematics, University of Bristol, Clifton, Bristol, UK
  • 3Institute of Theoretical Geophysics, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK
  • 4Department of Meteorology, University of Reading, Earley Gate, Reading, UK
  • 5Faculty of Earth Sciences, University of Iceland, Reykjavik, Iceland
  • 6Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI, USA

Abstract. The separation of volcanic ash and sulfur dioxide (SO2) gas is sometimes observed during volcanic eruptions. The exact conditions under which separation occurs are not fully understood but the phenomenon is of importance because of the effects volcanic emissions have on aviation, on the environment, and on the earth's radiation balance. The eruption of Grímsvötn, a subglacial volcano under the Vatnajökull glacier in Iceland during 21–28 May 2011 produced one of the most spectacular examples of ash and SO2 separation, which led to errors in the forecasting of ash in the atmosphere over northern Europe. Satellite data from several sources coupled with meteorological wind data and photographic evidence suggest that the eruption column was unable to sustain itself, resulting in a large deposition of ash, which left a low-level ash-rich atmospheric plume moving southwards and then eastwards towards the southern Scandinavian coast and a high-level predominantly SO2 plume travelling northwards and then spreading eastwards and westwards. Here we provide observational and modelling perspectives on the separation of ash and SO2 and present quantitative estimates of the masses of ash and SO2 that erupted, the directions of transport, and the likely impacts. We hypothesise that a partial column collapse or sloughing fed with ash from pyroclastic density currents (PDCs) occurred during the early stage of the eruption, leading to an ash-laden gravity intrusion that was swept southwards, separated from the main column. Our model suggests that water-mediated aggregation caused enhanced ash removal because of the plentiful supply of source water from melted glacial ice and from entrained atmospheric water. The analysis also suggests that ash and SO2 should be treated with separate source terms, leading to improvements in forecasting the movement of both types of emissions.

Short summary
This paper investigates the separation of gases and particles that frequently occurs during violent volcanic eruptions. This problem is important because atmospheric winds spread volcanic aerosols at great distances from the source, and wind shear then causes the aerosols to spread in different directions at different altitudes. This has important repercussions for accurately forecasting the movement of hazardous volcanic clouds. The May 2011 Grímsvötn eruption is analysed in great detail.
Final-revised paper