F. Prata et al.: Separation of ash and SO
2
10723
Figure 12. (a) MODIS/Aqua 250 m resolution 841–876 nm reflectance image showing the Grímsvötn ash column, ash column shadow, ash
layer, rope clouds, and gravity wave features. (b) Annotated version of (a) with inset plot showing reflectance along the straight black line
from the ash column towards the southeast. The blue-coloured arcs indicate the locations of wave-like fronts. The orange-coloured circle
indicates the outline of the ash column, which casts a strong shadow (westwards of the column) on the underlying clouds of ash and water
clouds. The apparent wavelength of the waves is ∼ 4–6 km. The solar zenith and azimuth and MODIS viewing zenith and azimuth angles
close to the column are 82.5, 56.4, 55.5, and −53.4
◦
, respectively. Image acquired at 05:15 UTC on 22 May 2011.
the neutral buoyancy height (Fig. 13d) and thus these small
particles could be carried to the plume top and into the lateral
intrusion. Similarly, 100 µm particles could be transported to
17 km a.s.l. However, if these fine particles aggregate during
their transport to produce larger grains, the velocity in the up-
per plume could be insufficient to support them. The velocity
of the plume in the region above the condensation level falls
below 5 m s
−
1
and therefore fine particles will take several
minutes to reach the plume top, ample time for aggregation
to occur (Veitch and Woods, 2001; Costa et al., 2010). For
particles of 500 µm and 1 mm diameter, the critical fallout ve-
locity is reached at altitudes of 11 km a.s.l. and 6.8 km a.s.l.,
respectively (Fig. 13c), which is below the neutral buoyancy
height.
The small diameter grains carried above the condensation
height are transported further (circulating in turbulent eddies)
in an environment conducive to rapid wet aggregation, and
therefore we expect the particle diameter to increase substan-
tially. If aggregates grow sufficiently rapidly, they will fall-
out before reaching the neutral buoyancy height. Deposits of
tephra on the Vatnajökull glacier show evidence of hailstones
infused with ash with diameters as large as 1–2 mm. Aggre-
gates of this size would readily fall from the plume and would
be unlikely to be re-entrained into the plume due to their size
and the width of the plume at the height at which fall out oc-
curs. Smaller aggregates will also fall out of the plume but
may not reach the ground proximal to the eruption column,
instead being transported laterally by the wind.
7
Conclusions
The vertical separation of gases and particles in volcanic
eruption columns occurs frequently and if it occurs in the
presence of wind shear it is inevitable that this results in a
lateral, distal separation of gases and particles. Wind shear
is ubiquitous and significant when eruption columns extend
to the tropopause and consequently it should be expected that
some separation will occur. Since gases and particles are also
not always released in unison, the time-varying nature of the
wind fields might also lead to separation, even for a steady,
low-level eruption column. Gases and particles also separate
within the column due to aggregation of particles and the
formation of mixed-phase particles of ice and ash that can
lead to rapid fallout, leaving lighter gases at higher levels
in the column. These interactions between the erupting vol-
canic column and the atmospheric environment in the vicin-
ity of the volcano are important for the short- and long-range
transport of gas and particles.
The presence of water in the erupting column, either
through additional meltwater or atmospheric entrainment,
promotes aggregation and facilitates the rapid removal of ag-
gregates from the plume. This lowers the concentrations of
ash in the upper parts of the column and may also lead to
errors in forecasting ash concentrations in the atmosphere
if these processes are not captured in transport models. The
photographs shown in Fig. 1 and the MODIS satellite image
shown in Fig. 5 provide strong observational evidence of a
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F. Prata et al.: Separation of ash and SO
2
Figure 13. Model prediction of the Grímsvötn plume at 0500 on
22 May 2011, assuming 10 wt % of water vapour at the source. (a)
Plume width (taken as twice the Gaussian half width from the plume
centreline) as a function of height. (b) Mass fraction of liquid wa-
ter and ice as functions of height. (c) Density of the plume ρ
p
and
atmosphere ρ
A
as functions of height. (d) Vertical velocity of the
plume at the nominal plume edge (taken as twice the Gaussian half
width from the plume centreline) and critical fallout velocities of
50 µm, 100 µm, 500 µm, and 1 mm particles as functions of height.
lower level skirt of ash moving away from the main ash-rich
column.
A combination of satellite observations (passive and ac-
tive) and dispersion modelling has been used to study the
separation of ash and SO
2
and it is apparent that such data
could be readily utilized in dispersion models by using as-
similation (Fu et al., 2017) or through the use of inversion
techniques (Stohl et al., 2011). Whether these techniques are
sufficiently sensitive to predict separation, or perhaps more
importantly column collapse and PDC generation, remains to
be investigated. The plume model that we use here to analyse
the transport of particles in the eruption column highlights
the importance of multiphase processes, particularly the role
of water in vigorous eruption columns. Clearly more detailed
and complex modelling is needed and we recommend that
future studies using VATDs consider gases and particles sep-
arately and improve parametrizations of the physics of erupt-
ing columns. Separation of gases and particles in volcanic
eruptions occurs frequently and it seems logical to treat, at
least much of the time, the sources separately. Partial column
collapse is not an exceptional event and suggests that this
process should be included as a mechanism for ash genera-
tion and subsequent transport in VATDs. The overwhelming
observational evidence for maximum separation with high-
level SO
2
travelling northwards and low-level ash travelling
southwards led to a re-evaluation of model forecasts during
the Grímsvötn event, which initially forecast ash collocated
with high-level SO
2
and covering a large geographic region
extending northwards and eastwards from Iceland towards
Greenland and the western Norwegian coast (see Fig. 14).
The volcanic ash advisory also shows a region of poten-
tially highly concentrated ash apparently extending to FL200
(20 000 ft or 7 km) travelling southwards and extending east-
wards towards Scotland. This erroneous forecast led to clo-
sure of airspace over parts of northern Europe and disruption
of some air traffic. During the event, observed concentrations
did not exceed 2 mg m
−
3
on arrival over northern Europe and
were mostly less than 1 mg m
−
3
(Tesche et al., 2012; Ans-
mann et al., 2012; Moxnes et al., 2014), suggesting that the
ash layer was not sufficiently concentrated to be a hazard to
aircraft not close to the source. However, we caution that
without agreed engine manufacturers’ tolerance limits, the
actual dangerous ash concentration (or dosage) is unknown.
Observational data of the kind presented here can be used
to constrain VATD models (Stohl et al., 2011) and such mod-
els should treat at least the gas and particle components sepa-
rately. A straightforward way to do this is to extend the meth-
ods proposed by Eckhardt et al. (2008), Kristiansen et al.
(2010), and Stohl et al. (2011) to include two sources. Im-
provements in dealing with the complex nature of the interac-
tion of the atmosphere with the erupting column are needed,
including better parameterizations of the aggregation process
(e.g. Textor et al., 2006; Costa et al., 2010; Telling et al.,
2013b; Folch et al., 2016), improved understanding of bent-
over plumes (Woodhouse et al., 2013), and improved mod-
elling of the effects of partial or total column collapse. While
perhaps less common, except in large eruptions (VEI > 4),
column collapse can lead to the generation of pyroclastic
density currents that can act as secondary sources for new
column generation, so-called co-ignimbrite plumes (Self and
Rampino, 1981). These can be vertically extensive (several
kilometres), ash-rich, and hence significant in forecasting
transport of the aviation hazard. It is suggested here that
one or more partial column collapses at Grímsvötn led to
surges of “cold” ash layers that eventually led to transport of
ash towards Scotland and southern Scandinavia. This source
mechanism is not currently included in ash dispersion mod-
els. During the Grímsvötn event the London VAAC used the
state-of-the-art dispersion model, NAME (Jones et al., 2007),
driven by a source term that relates the total mass erupted to
the fourth power of the column height. The fine mass fraction
is taken as a small percentage of the total mass; 5 % is often
used, but this is an unconstrained guess. Clearly, if the col-
umn collapses then this parametrization of the source term is
not appropriate.
Emissions of gases and particles into the atmosphere from
Icelandic volcanoes can have important consequences for
the local environment and also for Europe (Thordarson and
Self, 2003). The mechanisms and processes controlling the
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