Abstract
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.
Highlights
Vigorous volcanic eruptions emit copious amounts of gases and particles into the atmosphere, where they are transported by the winds, potentially in all directions
According to the status reports issued by the Icelandic Meteorological Office (IMO) and the Institute for Earth Sciences (IES) in Iceland, the column reached the greatest heights during 21–22 May and were estimated to be between 15 and 19 km
The reason for the large differences between infrared atmospheric spectrometer interferometer (IASI) and Spin-Enhanced Visible and Infra-Red Instrument (SEVIRI) retrievals is under investigation, but IASI has a greater sensitivity to ash due to the higher spectral resolution, and the assumptions used in the retrievals are different (Clarisse and Prata, 2016)
Summary
Vigorous volcanic eruptions emit copious amounts of gases and particles into the atmosphere, where they are transported by the winds, potentially in all directions. They can be transported rapidly zonally as in the case of the eruption of Puyehue-Córdon Caulle, southern Chile, during June 2011, when ash and SO2 travelled together, circling the Southern Hemisphere at latitudes south of 30◦ S. Separation can occur when ash and SO2 are emitted in separate explosive pulses when either the energy of the eruption has changed, emplacing the materials at different heights, or the atmospheric conditions have changed in the intervening period These processes are complex and difficult to predict for individual events. The main inferences from the study are presented in a concluding discussion section, and an Appendix provides a mathematical description of the plume model employed
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