10728
F. Prata et al.: Separation of ash and SO
2
at Grímsvötn is likely to have contributed water vapour in
addition to that derived from magma. The sensitivity of the
model predictions to the source water mass fraction is exam-
ined in Fig. A2 in which the water vapour content is taken to
be 5, 10, and 15 wt %. Adding water at the source has a pro-
nounced effect on the condensed water content of the plume,
with both the mass fractions of the condensed phases increas-
ing and the level at which condensation occurs decreasing as
the source mass fraction of water vapour increases. When the
source is relatively dry, with n
0
=
0.05, condensation occurs
when the plume temperature is below 0
◦
C, so that liquid wa-
ter and ice are expected to form. In contrast, for both n
0
=
0.1
and n
0
=
0.15 the condensation occurs when the plume tem-
perature exceeds 0
◦
C, and therefore the vapour first con-
denses to water, with ice forming at higher altitudes as the
temperature decreases. We note that the source mass frac-
tion of water vapour strongly influences the buoyancy of the
erupted material at the source; for n
0
=
0.05 and n
0
=
0.1 the
erupted material is initially more dense than the atmosphere
and is driven upwards by momentum, whereas the material
is buoyant at the vent when n
0
=
0.15. Interestingly, the ve-
locity at the source when n
0
=
0.15 is greater than that when
n
0
=
0.1. However, the dependence of the fallout velocity on
the plume density means that the fallout height for each par-
ticle size decreases substantially as n
0
increases.
Figure A2 demonstrates that the potential for the separa-
tion of very fine ash from the plume, driven by wet aggrega-
tion, increases substantially as the source water vapour con-
tent increases. However, for the atmospheric conditions at the
time of the Grímsvötn eruption, the model predicts substan-
tial concentrations of condensed water for all of the source
conditions examined. Therefore, our hypothesis of water-
mediated aggregation and enhanced removal of ash from the
plume is robust to changes in the source conditions.
Figure A2. Sensitivity of model predictions of the Grímsvötn plume
at 05:00 UT on 22 May 2011 to increases in the source water vapour
content, with (a–d) n
0
=
0.05, (e–h) n
0
=
0.10, and (i–l) n
0
=
0.15.
(a, e, i) Plume width as a function of height. (b, f, j) Mass fraction
of liquid water and water ice as a function of height. (c, g, k) Den-
sity of the plume ρ
p
and atmosphere ρ
A
as functions of height. (d,
h, l) Vertical velocity of the plume at the plume edge and critical
fallout velocities of 50 µm, 100 µm, 500 µm, and 1 mm particles as
functions of height.
Atmos. Chem. Phys., 17, 10709–10732, 2017
www.atmos-chem-phys.net/17/10709/2017/
F. Prata et al.: Separation of ash and SO
2
10729
The Supplement related to this article is available
online at https://doi.org/10.5194/acp-17-10709-2017-
supplement.
Competing interests.
The authors declare that they have no conflict
of interest.
Acknowledgements.
Andrew Hogg and Jeremy Philips provided
advice on part of this work and we thank them for their valuable
insights. We also thank Antonio Costa and Arnau Folch for
providing us with the code to run the FALL3D model and the
NASA AIRS and MODIS science teams for access to the satellite
data and products. We acknowledge the use of data products or
imagery from the Land, Atmosphere Near real-time Capability for
EOS (LANCE) system operated by the NASA/GSFC/Earth Science
Data and Information System (ESDIS) with funding provided
by NASA/HQ. This work was conducted within the European
Commission FUTUREVOLC project. The work of HEH is partially
supported by a Leverhulme Emeritus Fellowship. Simon Carn
acknowledges support from NASA through grants NNX11AF42G
(Aura Science Team) and NNX13AF50G (MEaSUREs). We thank
Arnau Folch and John Stevenson for their reviews of our paper.
We are especially grateful to John Stevenson for providing such
thorough and thought-provoking comments. His comments have
helped us improve the paper.
Edited by: Anja Schmidt
Reviewed by: Arnau Folch and John Stevenson
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