F. Prata et al.: Separation of ash and SO
2
10721
Figure 10. A schematic of the principle features of the
Grímsvötn eruption column. Large hydrometeors composed of ash
aggregates and mixed-phase ash and ice particles fall through the
column, competing with the upward force of the eruption, eventu-
ally causing the column to collapse as well as the generation of one
or more PDCs, pushing an outflow of ash-rich air into the lower
troposphere. Ash rises from the PDC and falls in the collapsing
column. A high-level plume of SO
2
with some ash penetrates the
tropopause. The height range of the gravity current (or pyroclastic
density current) is unknown but likely to extend from the ground to
the top of the observed ash skirt.
generation results from the development of a collapsing veil
of material (Carey and Bursik, 2015). The PDCs emerge
from the veil and follow the underlying topography. Either
way the phreatomagmatic nature of the eruption with the in-
jection of large amounts of water appear to be important in-
gredients leading to the observation of a skirt of ash propa-
gating at lower levels.
The outflow from this mechanism may have been rela-
tively fast; the AIRS satellite observations suggest that the
column had stopped rising by 04:17 UTC on 23 May. Our
photographic series show that the plume stopped rising in
the time frame 19:30–20:00 UTC on 21 May. It stayed rela-
tively elevated, i.e. between 15 and 19 km, and according to
daily observational reports it stayed at this level until mid-
morning of 22 May. On the same day by noon it had dropped
below 10 km and stayed there through 23 May. At the end of
that day it dropped below 5 km and more or less remained
below that height for the rest of the eruption. The low-level
ash layer persisted close to the south coast of Iceland for at
least 24 h before starting its journey further southwards and
then eastwards. Atmospheric transport processes (e.g. buoy-
ant transport, advection by the low-level winds, particle set-
tling) act on this ash cloud, but the low-level winds were not
strong and thus the ash moved slowly. The cloud may also
have been fed by new ash from the ongoing minor eruptions.
The ash transported southwards from Grímsvötn, which
begins within the first hour of the eruption, arises not directly
from the emissions at the vent but most likely from a pos-
sible partial collapse of the eruption column, which can no
longer be sustained, or from the generation of one or more
Figure 11. Photograph of a vertical section taken on the Vatna-
jökull glacier at a location where there was significant ash fall from
Grímsvötn, on 31 May 2011. There is evidence of millimetre-sized
hail in the deposit. Photo taken by Adam Durant during a visit or-
ganized by Fred Prata.
PDCs. The southward movement of the ash skirt can best
be seen in the MODIS image acquired on 23 May 2011 at
12:05 UTC (Fig. 9). The ash mass retrievals for this image
(and three later images) are shown in Fig. S4 (Supplement)
(top-left panel) in which three mass loading levels are indi-
cated: 0.2, 2, and 4 g m
−
2
.
Further support for rapid removal of ash before transport
is provided in the photograph shown in Fig. 11 taken on
31 May, just 8 days later on the Vatnajökull glacier near
Grímsvötn. The photograph shows a short vertical section
dug into the deposit with evidence of hail. The presence of
hail within the deposit has also been described by Arason
et al. (2011), who found hailstones of 1–2 mm size infused
with ash. Gudmundsson (2013) has estimated the amount of
water melted by the eruption. Furthermore, since no Jökulh-
laups were observed, it may be assumed that much of that
water went into the plume in the form of hot water vapour
(steam) and also contributed to ice and hail formation within
the column. A large amount of lightning was observed in the
eruption column and clouds, also suggesting the presence of
hydrometeors.
It is difficult to estimate whether the column collapsed
more than once but there does seem to be evidence that an ash
surge existed on the morning of 22 May. A MODIS image
acquired at 05:15 UTC on 22 May (∼ 10 h after the start of
the eruption) appears to show gravity waves emanating from
the column and a skirt of ash spreading southwards and then
curving around the northeastern coast of Iceland. The photo-
graphic evidence suggests that the process started much ear-
lier. These waves could have been formed when the column
sloughed, causing a cold ash surge driven by the buoyancy
force due to the vertical gradient in the density. Figure 12a
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Atmos. Chem. Phys., 17, 10709–10732, 2017
10722
F. Prata et al.: Separation of ash and SO
2
shows the 250 m resolution MODIS band 2 (841–876 nm)
image reprojected, calibrated to reflectance, and digitally en-
hanced to highlight various features.
The features are identified as the Grímsvötn eruption col-
umn (slightly east of the volcano location, indicated by the
red triangle), its shadow cast westwards onto a lower layer of
ash and meteorological cloud, an ash layer extending along
the south coast of Iceland, rope clouds, and gravity waves
(Fig. 12b). The reflectance (as a percentage) along a tran-
sect indicated by the black line is also shown in the inset in
Fig. 12b. Variations in reflectance along the transect occur
due to height variations in the cloud and the solar and sensor
viewing geometry. The approximate wavelength of the waves
is ∼ 4–6 km.
6
Insights on the mechanisms and conditions for ash
separation from a plume
The observations provide strong evidence that separation of
ash occurred predominantly in the convectively rising part of
the eruption column, where the motion is driven by a buoy-
ancy force arising from a density difference between the col-
umn and the atmosphere, rather than at the source or in the
laterally intruding ash cloud. Figure 1 shows convincing ev-
idence that separation occurs at the convective column. The
buoyant volcanic plume is a complex physical environment,
with multiple interacting phases, highly turbulent flow fields,
and coupled non-linear physical and chemical processes oc-
curring. Despite this complexity, much insight into the dy-
namics of volcanic plumes has been gained from mathemati-
cal models of turbulent buoyant plumes (Morton et al., 1956),
which have been extended to model thermodynamics and
transport of solids in volcanic plumes (see, e.g. Wilson et al.,
1978; Sparks, 1986; Woods, 1988; Glaze and Baloga, 1996;
Sparks et al., 1997a; Bursik, 2001; Woodhouse et al., 2013).
Here we use an integral model of volcanic plumes to
gain insight into the physical processes that could lead to
an abrupt separation of ash from the plume at Grímsvötn.
We adopt the integral model of Woodhouse et al. (2013),
which includes descriptions of the thermodynamics of phase
changes in water, the effect of atmospheric winds on the
plume dynamics, and detailed profiles of the atmospheric
structure during the eruption. Additional details of our mod-
elling approach are given in the Appendix and a derivation
of the system of equations adopted in our model are given in
Sects. 2 and 3 of Woodhouse et al. (2013).
Our hypothesis is that the separation of ash from the con-
vectively rising plume that was observed at high altitude was
due to rapid aggregation of ash particles, mediated by a rapid
condensation of water in the plume. The presence of (liquid)
water is likely to promote the aggregation of ash particles
by allowing the formation of liquid bridges between grains
(Brown et al., 2012; Van Eaton et al., 2012). The capillary
forces in the liquid connections are much stronger than elec-
trostatic attractions between dry grains (James et al., 2003),
and therefore it is possible that wet aggregates can endure a
collision that would cause dry aggregates to break apart. Ag-
gregation in the presence of liquid water or ice is extremely
efficient, with aggregation timescales less than 0.1 s (Veitch
and Woods, 2001; Costa et al., 2010). Costa et al. (2010)
demonstrate that a particle size distribution that initially has a
peak number density at 10 µm can evolve to produce a peak
in the number density at 100 µm in 60 s in an environment
with condensed water available.
Because there is not that much very fine ash in the column
to begin with to generate a sector-wide plume collapse we
cannot be sure that aggregation is the sole driver. The parti-
cles in the 100 µm size fraction contain less than 10 % of the
mass erupted at any one time, so that even if all of this ash
forms aggregates, the mass fraction is still small compared
to the total mass. In the proximity of the volcano the tephra
contains an abundance of lapilli size clasts (2–64 mm in di-
ameter), and over 50 % of the proximal tephra is lapilli, and
the fallout units are over 80 % lapilli (i.e. 2–64 mm clasts).
Only the surges that generate the PDC deposits contain a
substantial amount of ash, more then 90 %, because they do
not have the capacity to carry the lapilli clasts to start with.
However, most of the tephra deposited by the PDCs is in the
0.1 to 1 mm (100–1000 µm) range, or > 70 %. Observations
on accretionary lapilli (i.e. ash aggregates) indicate that this
size range is too big to partake in ash aggregation by cap-
illary forces plus electrostatic forces. The separation of the
very fine ash, moving laterally and deposited from a laterally
moving current, and the lapilli size material that is processed
vertically by being transported upwards and then falling out
is not fully understood.
Rather than modelling aggregation explicitly, which is
subject to great uncertainty, here we use a model of vol-
canic plumes to investigate whether conditions in the plume
are favourable for wet aggregation and an abrupt fallout of
solids. The maximum elevation of solid particles of a given
size can be estimated by balancing the average vertical ve-
locity of gaseous phases in the plume with the settling speed
of a particle (see the Appendix). This provides a simple, yet
robust, method of examining the consequence of aggrega-
tion; the estimated maximum fallout height is determined
only by the particle size, and the evolution of the particle
size distribution is not required. Figure 13 illustrates a typi-
cal prediction obtained from our model for the plume from
Grímsvötn at 05:00 UTC on 22 May 2011.
Gentle winds and the large mass flux of erupted material
result in a sub-vertical plume that is affected little by the
wind. The model identifies an abrupt transition in the plume
from dry conditions at lower levels (below approximately
10 km) to an environment with a substantial amount of con-
densed water, and the low atmospheric temperature results in
a predominance of ice with peak concentration in excess of
4 g kg
−
1
(Fig. 13b). The critical fallout velocity of 50 µm par-
ticles is reached at an altitude of 18 km a.s.l. (Fig. 13c) above
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