Atmos. Chem. Phys., 17, 10709–10732, 2017
https://doi.org/10.5194/acp-17-10709-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric processes affecting the separation of volcanic ash and
SO
2
in volcanic eruptions: inferences from the May 2011
Grímsvötn eruption
Fred Prata
1
, Mark Woodhouse
2
, Herbert E. Huppert
3
, Andrew Prata
4
, Thor Thordarson
5
, and Simon Carn
6
1
Visiting scientist, Department of Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory,
University of Oxford, Oxford, UK
2
School of Mathematics, University of Bristol, Clifton, Bristol, UK
3
Institute of Theoretical Geophysics, Department of Applied Mathematics and Theoretical Physics, University of Cambridge,
Cambridge, UK
4
Department of Meteorology, University of Reading, Earley Gate, Reading, UK
5
Faculty of Earth Sciences, University of Iceland, Reykjavik, Iceland
6
Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI, USA
Correspondence to:
Fred Prata (fred_prata@hotmail.com)
Received: 1 February 2017 – Discussion started: 6 February 2017
Revised: 5 July 2017 – Accepted: 10 July 2017 – Published: 12 September 2017
Abstract. The separation of volcanic ash and sulfur dioxide
(SO
2
) gas is sometimes observed during volcanic eruptions.
The exact conditions under which separation occurs are not
fully understood but the phenomenon is of importance be-
cause 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 Vat-
najökull glacier in Iceland during 21–28 May 2011 produced
one of the most spectacular examples of ash and SO
2
sep-
aration, 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 photo-
graphic evidence suggest that the eruption column was un-
able 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 Scandi-
navian coast and a high-level predominantly SO
2
plume trav-
elling northwards and then spreading eastwards and west-
wards. Here we provide observational and modelling per-
spectives on the separation of ash and SO
2
and present quan-
titative estimates of the masses of ash and SO
2
that erupted,
the directions of transport, and the likely impacts. We hy-
pothesise 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 sug-
gests that ash and SO
2
should be treated with separate source
terms, leading to improvements in forecasting the movement
of both types of emissions.
1
Introduction
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 trans-
ported rapidly zonally as in the case of the eruption of
Puyehue-Córdon Caulle, southern Chile, during June 2011,
when ash and SO
2
travelled together, circling the Southern
Hemisphere at latitudes south of 30
◦
S. They can be trans-
ported vertically by air circulations as in the case of Nabro,
Eritrea, also in June 2011, when the monsoon circulation
Published by Copernicus Publications on behalf of the European Geosciences Union.
10710
F. Prata et al.: Separation of ash and SO
2
may have played a part in lifting SO
2
gas into the strato-
sphere (Bourassa et al., 2012), although Fromm et al. (2014)
provides a convincing case for direct gas injection. Prevail-
ing atmospheric winds can play a pivotal role in the transport
of ash and SO
2
as in the case of the April and May 2010
eruptions of Eyjafjallajökull, Iceland, during which large
amounts of ash were transported zonally and meridionally
over continental Europe, leading to major disruptions of
air traffic. The direction of transport is determined by the
strength and direction of the zonal and meridional wind fields
and these vary with height. Vertical wind shear varies with
location and time and is commonplace.
SO
2
gas and ash particles represent two major components
of vigorous volcanic activity and these may be emitted to-
gether or individually and this mix can and does vary with
time, due largely to the character of the volcanic activity and
the geological setting of the volcano. Since there is no guar-
antee that ash and SO
2
will be erupted at the same time, nor
that they will remain collocated in space and time, there is a
good reason to investigate the conditions under which these
emissions remain together and conditions under which they
separate. Magma composition is also a factor because lower
viscosity magmas fragment into coarser particles that sep-
arate more easily from gas. Separation has been observed
during the eruptions of Okmok and Kasatochi (Prata et al.,
2010), this Grímsvötn eruption (Sigmarsson et al., 2013;
Moxnes et al., 2014), and during the Eyjafjallajökull erup-
tion (Thomas and Prata, 2011), although collocated transport
is also often observed. Holasek et al. (1996) investigated ash
and gas separation through analogue laboratory experiments.
They found that gas and ash separation occurred through
buoyancy effects, with particle sedimentation leaving higher
gas concentrations above. Separation can also occur when
ash and SO
2
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.
Here we study the remarkable separation of SO
2
and ash
during the 21–28 May 2011 eruption of Grímsvötn, using
mostly satellite data but also ground-based remote sensing
measurements and model simulations. The separation was
the greatest possible. SO
2
reached high altitudes (> 10 km),
travelled northwards, and then spread eastwards and west-
wards, while the ash remained at low altitudes (< 4 km) and
travelled southwards before spreading eastwards, eventually
reaching the western coast of Norway. The separation led to
a poor forecast of the ash hazard to aviation and we suggest
how such errors can be avoided in the future.
The injection of SO
2
into the stratosphere and its subse-
quent conversion to sulfate aerosol are important for under-
standing the radiative impact of volcanic eruptions (Robock,
2000). If the SO
2
emission remains largely in the troposphere
and is of short duration (less than a few days), its potential
impact on radiative forcing is less because the residence time
of the resulting sulfate aerosol is shorter. Stratospheric sul-
fate aerosols, however, can potentially alter the radiative bal-
ance. It is shown here that the Grímsvötn SO
2
did indeed
penetrate into the stratosphere. The satellite data are used
to estimate the total SO
2
injected into the stratosphere and
the total mass of very fine ash injected into the lower tropo-
sphere. Here the term very fine is used to describe ash with an
effective radius < 16 µm, which represents the largest grain
size that infrared sensors and retrieval algorithms can quan-
tify with any certainty.
Our paper stresses the importance of a multidisciplinary
approach to the study of volcanic eruptions in the atmo-
sphere: volcanological insights, space-based observations,
dispersion modelling, and fluid dynamics are all required to
develop understanding of the dominant processes involved.
The paper is organized as follows. The chronology of the
eruption and important events are described, followed by a
short section on the transport and the tools used to determine
it. Next the satellite data are introduced and estimates of the
ash mass loading, the SO
2
amount, and cloud-top tempera-
ture and height are provided. The phenomenon of particle–
gas separation is discussed and observational evidence is
presented for the Grímsvötn eruption. An uncoupled plume
model, in which plume dynamics and plume microphysics
are examined separately, is used to provide insights into the
most significant processes relevant to particle–gas separa-
tion. Most of the satellite observations and some modelling
results are included in the Supplement. The main inferences
from the study are presented in a concluding discussion sec-
tion, and an Appendix provides a mathematical description
of the plume model employed.
2
Chronology of the Grímsvötn eruption
Grímsvötn is a subglacial volcano situated under the Vatna-
jökull glacier in southeastern Iceland. Like many Icelandic
volcanoes it has a long record of eruptive activity (Thordar-
son and Larsen, 2007), with the last notable event prior to
the May 2011 activity occurring between 1 and 6 Novem-
ber 2004.
On the afternoon of 21 May 2011 at around 17:30 UTC,
seismicity
and
thermal
anomaly
measurements
at
Grímsvötn indicated that an eruption was likely, and at
19:00 UTC the eruption penetrated the Grímsvötn subglacial
caldera. The first signs from satellite observations of a plume
entering the atmosphere were recoded by the Spin-Enhanced
Visible and Infra-Red Instrument (SEVIRI) on-board the
geostationary Meteosat Second Generation (MSG)-2 satel-
lite at 19:15 UTC on 21 May. The weather conditions at
the start of the eruption were good and photographs of the
plume (see Fig. 1 and the Supplement) as it emerged out
of the glacier at ∼ 19:10 UTC clearly showed a steam-rich
plume that later developed into an ash-laden plume reaching
several kilometres into the atmosphere. As evening fell,
Atmos. Chem. Phys., 17, 10709–10732, 2017
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