F. Prata et al.: Separation of ash and SO
2
10727
Table A1. Source parameter value used in the plume model for 0500
on 22 May 201.
Parameter (symbol)
Value
Vent radius (L
0
)
200 m
Source gas mass fraction (n
0
)
0.05
Source temperature (T
0
)
1000 K
Vent altitude (z
0
)
1725 m
longer supported by the plume (i.e. u
s
=
0), we find that
π d
2
8
ρ
p
C
D
u
2
p
=
π d
3
6
ρ
s
g,
(A2)
and therefore the particle falls out of the plume when
u
p
4dρ
s
g
3ρ
p
C
D
1/2
≡
u
c
(d),
(A3)
in which u
c
(d)
is the critical fallout velocity for a particle
of diameter d. The value of the drag coefficient depends
on properties of the particle, particularly shape, and on the
Reynolds number of the flow field in which it is carried (Wil-
son and Huang, 1979). Furthermore, the drag coefficient of
aggregates may differ from that of individual particles (James
et al., 2003). Here we take a representative value of C
D
=
1,
noting that u
c
(d)
is not strongly sensitive to the value of
the drag coefficient. The variation in density of solids (from
∼
700 kg m
−
3
for vesicular pumice to ∼ 3200 kg m
−
3
for
glass shards) does not greatly alter the critical fallout veloc-
ity calculated using our reference density (1200 kg m
−
3
) with
changes in the value by a factor of 0.76 to 1.6.
We assume that the radial profile of the mean axial velocity
of the plume is Gaussian,
u
p
(r, z) = u(z)
exp −r
2
/R
2
,
(A4)
in which r is the radial distance from the centreline of the
plume and R is a characteristic radial length scale. The ra-
dial distance r = 2R is taken as representative of the plume
width, and at that point the local mean axial velocity of the
plume is less than 2 % of the centreline value.
A2
Model results
At 05:00 UTC on 22 May 2011, the C-band weather radar
at Keflavík International Airport recorded a plume height of
19.3 km. The mass flux of erupted material is estimated by
matching the model prediction of the plume height to the
radar observation with fixed values of the vent radius, gas
mass fraction, and temperature at the source (Table 3). The
resulting source mass flux estimate is Q
0
=
9.5 × 10
7
kg s
−
1
.
Figure A1 shows time series of the plume height and con-
densation level, the maximum mass fractions of liquid water
and ice in the plume, and the critical height at which par-
ticles fall out of the plume for four particle diameters on
Figure
A1.
Model
predictions
of
the
properties
of
the
Grímsvötn plume on 22 May 2011. (a) Plume-top height and
condensation level in the plume. (b) Maximum mass fractions of
liquid water and water ice. (c) Critical height at which particles fall
out of the plume for particles of 50 µm, 100 µm, 500 µm, and 1 mm
diameter.
22 May 2011. Plume-top heights are derived from a fixed C-
band radar and a mobile X-band radar. The variation in the
condensation level in the plume follows that of the plume-
top height. The mass fractions of liquid water and ice in the
plume do not vary substantially (with the exception of a de-
crease in the ice content at 09:00 and 10:00 UTC) despite
changes in the condensation height, and there is a plenti-
ful supply of condensed water in the plume throughout this
period of the eruption. There are pronounced differences in
the critical fallout heights of particles of different diameters.
Particles of 50 µm diameter are carried near the plume-top
height, above the condensation level. Often 100 µm diame-
ter particles are carried above the condensation level, but we
note that between 09:00 and 10:00 UTC on 22 May the criti-
cal fallout velocity of these particles is reached at low levels
in the plume. The larger particles (diameters of 500 µm and
1 mm) consistently fall out below the condensation level.
The period between 09:00 and 10:00 UTC on 22 May is
distinctive in the relatively low plume height, ice content, and
low critical fallout height for particles of a diameter greater
than 500 µm. The low plume height requires a reduced mass
flux from the source and therefore relatively low velocities in
the plume. Thus, the critical fallout velocity of a particle oc-
curs at lower altitudes. Therefore, during this period the fall
out of relatively small diameter particles could occur with-
out significant wet aggregation; dry aggregation in the lower
plume might be sufficient to remove very fine ash.
The model source conditions used above (Table 3) have
a relatively dry source with a water vapour mass fraction of
5 wt %. However, the melting of glacier ice around the vent
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Atmos. Chem. Phys., 17, 10709–10732, 2017