Atmospheric processes affecting the separation of volcanic ash and SO 2 in volcanic eruptions:Inferences from the May 2011 Grímsvötn eruption

• Main Authors: Prata, Fred, Woodhouse, Mark J, Huppert, Herbert E, Prata, Andrew, Thordarson, TH, Carn, Simon A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Atmospheric processes; Volcanic eruptions; Volcanic ash; Sulphur dioxide; Vatnajökull; glacier; Iceland
• Online Access: https://hdl.handle.net/1983/b49d4209-db56-4cca-9aa2-e08167a77057; https://research-information.bris.ac.uk/en/publications/b49d4209-db56-4cca-9aa2-e08167a77057; https://doi.org/10.5194/acp-2017-95; https://research-information.bris.ac.uk/ws/files/102177407/acp_2017_95.pdf; https://research-information.bris.ac.uk/ws/files/102177409/acp_2017_95_supplement.pdf

The separation of volcanic ash and sulphur 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 to 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 that 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 Scandanavian 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 erupted, the directions of transport, and the likely impacts. We hypothesise that a partial column collapse or perhaps several 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 exsolved magmatic 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.