F. Prata et al.: Separation of ash and SO
2
10711
Figure 1. The changing appearance of the Grímsvötn eruption col-
umn in photographs taken at the start of the eruption in clear skies at
20:10 UTC (20:10 local time) where the whitish appearance of the
column suggests condensed water vapour (a) and later (21:05 UTC)
in a dark (ash-rich), cloud-laden atmosphere (b). There is a clear in-
dication (lower photograph) of ash in the lower parts of the column.
Later photographs show mammatus clouds forming in the eruption
cloud, suggesting ice nucleation, and this may have contributed to a
more rapid loss of ash particles from the cloud. Photographs cour-
tesy of Ólafur Sigurjónsson. See also Supplement photographs.
visibility worsened and cloud also moved in from the north,
making visual identification of the plume difficult. Early
reports and some later analysis suggested that the plume
had reached perhaps 15–19 km (see also the Supplement
photographs). Ashfall was evident all around Grímsvötn and
was reported from the Reykjavik area in the southwest to
Tröllaskagi peninsula in the north. Figure 2 shows a map of
the region, indicating the location of Grímsvötn, some of the
towns, and the airport.
According to the status reports (http://earthice.hi.is/
grimsvotn_eruption_2011) issued by the Icelandic Meteo-
rological 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. On 24 May the column height was between 5
and 7 km and on 25 May it was below 5 km. Subsequently,
Figure 2. Map of Iceland showing towns, the road network, and the
location of Grímsvötn.
the columns dropped to 10 km and then by 26 May did
not exceed 8 km and mostly remained below cloud level,
with a white steam plume observed to reach 2 km. The
Grímsvötn eruption is believed to be the largest in Ice-
land since the eruption of Katla in 1918 and erupted a
tephra volume of ∼ 0.7 km
3
(Gudmundsson et al., 2012a)
(or ∼ 0.15 km
3
dense rock equivalent, based on our average
vesicularity measurements of ∼ 78 %) in just over 2 days
compared with the April–May 2010 Eyjafjallajökull erup-
tion, which erupted ∼ 0.27 km
3
over a 39-day period (Gud-
mundsson et al., 2012b). Sigmarsson et al. (2013) estimate
that a total of ∼ 0.2 Tg(S) was released to the atmosphere.
Stevenson et al. (2012) demonstrated that ash from Eyjafjal-
lajökull reached many parts of the United Kingdom, noting
that even quite large (> 50 µm radii) particles were collected
on the ground. The much larger Grímsvötn eruption was ex-
pected to send more ash towards Europe than the April–
May 2010 Eyjafjallajökull event that caused Europe-wide
aviation disruption.
3
Transport
Atmospheric dispersion models, so-called Volcanic Ash
Transport and Dispersion (VATD) models, are used to simu-
late the transport of volcanic ash in the atmosphere (Draxler
and Rolph, 2003, e.g. HYSPLIT). These models have been
quite successful for both small (e.g. Eyjafjallajökull) and
mid-size (e.g. Puyehue-Córdon Caulle) eruptions. They de-
pend on the precise details of the eruption parameters for
their accuracy and in particular on being able to specify the
mass eruption rate (MER), its vertical structure, and its tem-
poral variation. If these are incorrect, then the forecast disper-
sion is also incorrect. Many VATD models rely on an estima-
tion of the MER obtained from a parameterised relationship
between the MER and the height of the eruption column. The
most commonly used parametrization has the MER propor-
tional to the fourth power of the height (Sparks et al., 1997b;
www.atmos-chem-phys.net/17/10709/2017/
Atmos. Chem. Phys., 17, 10709–10732, 2017
10712
F. Prata et al.: Separation of ash and SO
2
Mastin et al., 2009) under the assumption of dry standard
atmosphere conditions, without the inclusion of ice and/or
water at the vent, which is likely a large factor in the case
of the Grímsvötn eruption. There are two significant conse-
quences for forecasting the transport if the height is in error.
First, for an atmosphere with significant wind shear in which
the height is incorrectly specified, the transport will be in-
correct. Second, as the MER depends strongly on height, the
estimated amount of ash may be significantly under- or over-
estimated, if the height is under- or overestimated. More so-
phisticated models of the MER are available (Mastin et al.,
2009), and recent work by Woodhouse et al. (2013) among
others has shown a dependence of the MER on wind shear for
bent-over plumes (Degruyter and Bonadonna, 2012). These
more detailed treatments also require more detailed observa-
tions of various parameters to specify the MER.
HYSPLIT ash dispersion runs using GDAS wind fields
were used to test the sensitivity of the transport to the
height of the eruption column during the Grímsvötn erup-
tion. Inspection of the photographs (see Supplement pho-
tographs, especially Fig. S2) shows that the initial plume is
just steam (water vapour) and condensed water vapour, but
by 19:30 UTC the column appears to be ash-laden and fully
developed by 20:00 UTC. At 20:30 UTC the plume at the top
of the column appears to lighten, suggesting that the transi-
tion from ash-rich to gas-rich in the upper plume was com-
plete. Estimating the heights from the photographs is diffi-
cult, but if it is assumed that the maximum height reached is
∼
19 km, then by 19:10 UTC it is ∼ 8 km and by 19:25 UTC
it is ∼ 16 km. For a column rising to ∼ 9 km or higher begin-
ning on 21 May at 19:15 UTC, the transport of ash is mostly
northwards and then spreads eastwards and westwards. Con-
versely, for ash emitted to a height of ∼ 3 km on 22 May
at 14:00 UTC, the transport is mostly southwards and then
eastwards towards Scotland and on to the southern part of
Scandinavia. These two scenarios are motivated by satellite
observations of volcanic emissions taken by polar and geo-
stationary instruments (see next section) and are therefore
indicative of what actually happened. During the event, the
London Volcanic Ash Advisory Centre (VAAC), among oth-
ers, forecast ash emissions towards the north and south, in
broad agreement with the HYSPLIT simulations. As we shall
see, the forecast of significant ash emissions to the north was
incorrect because these emissions were almost entirely SO
2
gas, and the forecast overestimated the amount of ash trans-
ported to the south and then eastwards, probably due to an
incorrectly specified vertical distribution of ash at the source.
A more complex model written specifically for volcanic
ash eruptions, FALL3D (Folch et al., 2009), was also used to
study the transport. The model is a Eulerian dispersion model
that includes several different parameterizations that can be
used to specify the source term. It is driven by input atmo-
spheric wind fields and has been used in many studies of
atmospheric transport of ash, particularly to investigate ash
fall. We used FALL3D initialized with NCEP wind fields at
6-hourly intervals starting at 19:00 UTC on 21 May 2011.
The source term was specified using one of the preset op-
tions, in this case a point source, and the plume options were
used. A forward run was performed on a grid of 0.2 × 0.2
◦
resolution, with a vertical scale of ∼ 0.1 km for a total du-
ration of up to 96 h. Here we are not concerned with test-
ing the model’s propensity to accurately simulate long-range
ash transport, but rather to investigate the hypothesis that the
source of ash was from a column collapse, with the ash in-
jection treated as an impulse. Some of the results of the sim-
ulations are shown in the Supplement section (Fig. S1), at
6-hourly intervals starting at 13:00 UTC on 22 May. Here we
summarize the main findings, noting that FALL3D is used
for ash forecasting and thus the results are typical of what is
found from using state-of-the-art VATD models.
The short duration of the source of ash results in a small
plume or cloud of ash dispersing to the south-southwest and
then turning southwards (Fig. S1 in the Supplement) and later
towards the east (not shown). The ash is eventually trans-
ported over the northern part of the United Kingdom, some-
what south of the path observed in SEVIRI, and then on to
the southern part of Scandinavia, where ground-based parti-
cle measurement stations recorded elevated levels of PM
10
(Prata and Prata, 2012). The FALL3D simulations also show
a small amount of ash transported towards the northwest at
higher levels, collocated with satellite observations of SO
2
.
This ash cloud, also observed in ash retrievals from SEVIRI,
the infrared atmospheric spectrometer interferometer (IASI),
and Moderate Resolution Spectroradiometer (MODIS), is
short-lived and dissipated by 20:00 UTC on 22 May.
1
The
initial amount of erupted very fine ash required to generate
an ash cloud consistent with the satellite observations cannot
be modelled accurately using the fourth power law, as this
produces almost 100 times too much total mass so that the
mass fraction of very fine ash must then be altered. A sce-
nario more consistent with the satellite estimates is that af-
ter the tephra-laden plume reached full development around
19:30–20:00 UTC, rising to height of ∼ 19 km, its carrying
capacity dropped dramatically over the next hour or so, re-
sulting in the erupted tephra separating from the gas phase of
the plume and spreading outwards as a gravitational current
at much lower altitude, i.e. around ∼ 8 (6–10) km a.s.l. This
change may have been induced by widening of the erupting
vent, inducing partial column collapse and formation of py-
roclastic density currents (PDCs). Combined effects of ash
aggregation and a co-PDC plume would have enriched ash
content of the outward-moving lower-level plume. FALL3D
is not currently configured to simulate these kinds of source
terms (areal); thus, the source term was scaled to match the
satellite retrievals, assuming a maximum height of 6 km and
with a suitable vertical distribution of ash. The MER scales
1
Here it may be assumed that the ash was still in the atmosphere
but at a concentration too low to be detected by current satellite
infrared measurements.
Atmos. Chem. Phys., 17, 10709–10732, 2017
www.atmos-chem-phys.net/17/10709/2017/