F. Prata et al.: Separation of ash and SO
2
10713
as H
4
, in which H is the column height; thus, the MER is
reduced by a factor of ∼ 100 if the column height is reduced
from 19 to 6 km.
Stevenson et al. (2013) studied Grímsvötn ash deposits
in Scotland and northern England using pollen traps, tape-
on-paper measurements, rainwater samples, and air quality
measurements. The majority of ash deposition was found in
Scotland in agreement with the observations presented here.
However, they find larger median grain sizes of 19–23 µm
and maxima of 80 µm significantly higher than the effec-
tive radii found in the atmosphere from the satellite mea-
surements. Since Stevenson et al. (2013) sample ash on the
ground and have a bias that precludes making measurements
of grain sizes < 10 µm and the infrared retrievals are less
sensitive to particles with effective radii > 10 µm, the two
measurement strategies are largely incompatible. Very fine
ash was also detected by the infrared sensors over Green-
land during the early phase of the eruption on 22 May (Prata
and Rose, 2015, see Fig. 52.11). This ash signal quickly dis-
sipates and the ash was high in the atmosphere (above the
tropopause) and collocated with the SO
2
. There is no evi-
dence from the satellite measurements that any of this ash
reached the United Kingdom or Europe. Any ash arriving in
Europe from this part of the plume would have had to de-
scend from the stable stratosphere and, having travelled for
4–5 days, would be of low concentration and have a very
small effective radius.
3.1
Separation of the dispersing volcanic cloud
The observational evidence for significant separation of ash
and SO
2
is unequivocal, but was this entirely due to wind
shear? Certainly at some stages during the eruption the col-
umn reached at least 16 km and wind data show that wind
shear was present, which suggests the emissions would dis-
perse in different directions. During the period between the
early morning of 22 May and late afternoon of 23 May, satel-
lite, radar, and photographic evidence shows that the column
changed height and at times extended to less than 10 km.
Atmospheric Infrared Sounder (AIRS) near-real-time bright-
ness temperatures can be used to retrieve upper level SO
2
in the atmosphere. A series of six images of this product
are shown in Fig. S2. These products are based on bright-
ness temperatures at specific wave numbers that correspond
to SO
2
absorption locations, and a radiative transfer model is
used to fit the spectral features to the SO
2
column amount
(Prata and Bernardo, 2007). A more simplified product is
also used, indicating SO
2
as negative temperature differ-
ences, in which
T < −
6 K. Positive differences found in
these images are due to other causes and strongly positive
values are generally unusual. The causes of positive differ-
ences are mostly associated with water vapour and thermal
contrast effects. There is no other literature discussing posi-
tive differences in this product that we are aware of. Figure 3
AIRS temperature difference
22 May 2011 03:35 UT
23 May201104:17UT
-
Figure 3. AIRS brightness temperature difference images for over-
passes on 22 and 23 May 2011. Panel (a) shows 03:35 UTC,
22 May. Panel (b) shows 13:17 UTC, 22 May. Panel (c) shows
04:17 UTC, 23 May. Negative values suggest absorption by SO
2
gas. The red-coloured spot (positive temperature differences) seen
in panels (a) and (b) situated near the Grímsvötn vent may be due
to the ash-rich column.
shows three of these brightness temperature difference prod-
ucts.
The SO
2
feature is clearly evident in these images, but
there is also a positive difference coincident with the lo-
cation of the Grímsvötn vent. We speculate that the posi-
tive temperature differences correspond to the location of
the ash-rich eruption column, slightly displaced from the
upper-level dispersing SO
2
. Interestingly, by 04:17 UTC on
23 May 2011, this positive anomaly has disappeared. The
brightness temperature difference is determined from two
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Atmos. Chem. Phys., 17, 10709–10732, 2017
10714
F. Prata et al.: Separation of ash and SO
2
channels at 1361.44 and 1433.06 cm
−
1
situated inside and
outside of the strong ν
3
SO
2
absorption band (Prata and
Bernardo, 2007). The reason for the positive difference over
the column is unclear, but we suggest that because these
two channels sound the atmosphere with peak contributions
at different heights, a positive difference will be observed
when the top of the column is below the upper-level sounding
channel (1433.06 cm
−
1
) peak and above or near the weight-
ing function peak of the lower-level channel (1361.44 cm
−
1
).
This interesting observation suggests that it may be possible
to utilize the AIRS spectral information to determine ash col-
umn heights for opaque columns. The changes in height of
the ash column and vertical emplacement of ash is a signifi-
cant process that affects the subsequent direction of transport
of volcanic ash.
4
Satellite data analyses
Satellite instruments have been used extensively to mon-
itor volcanic emissions (Prata, 2009). Both ash and SO
2
gas can be quantified using measurements made in the in-
frared (e.g. Clarisse et al., 2010; Prata and Prata, 2012;
Carn et al., 2005) and in the ultraviolet (e.g. Carn et al.,
2016), while ultraviolet-visible reflected light can be used to
identify volcanic aerosols (aerosol optical depth), and pas-
sive microwave measurements can be used to detect large
(millimetre-sized) volcanic particles. These passive measure-
ments have been recently supplemented by an active space
borne lidar that can provide much-needed vertical informa-
tion. The Cloud-Aerosol Lidar with Orthogonal Polarization
(CALIOP) on board the CALIPSO polar-orbiting platform
provides vertical structure information from backscattered
light along the sub-satellite point but has a long repeat cycle,
which inevitably means limited coverage for rapidly evolv-
ing, spatially limited events such as low to medium size vol-
canic eruptions. Table 1 provides a list of the satellite instru-
ment data used in this study, including the salient character-
istics of these instruments.
4.1
Cloud-top height
An estimate of the column height of eruptive material is crit-
ical to understanding the movement of ash away from its
source. Initial estimates of the height of the ash column,
based on radar measurements by the IMO (Petersen et al.,
2012), suggested that it had reached ∼ 20 km. However, the
satellite data analysed here, together with radiosonde tem-
perature profiles made at Keflavík airport, indicate that the
column maximum height did not exceed ∼ 16 km and var-
ied between ∼ 8 and ∼ 16 km from onset at 19:00 UTC on
21 May 2011 until a lowering sometime after 05:15 UTC
on 22 May. The radar has a vertical resolution of between
2 and 5 km at a range of 75 km (Petersen et al., 2012, see
their Fig. 2); thus, a radar estimate of 15–25 km is broadly
consistent with the satellite estimate. The measurements re-
ported by Petersen et al. (2012) from radars situated at Ke-
flavík (∼ 257 km away) and on a mobile platform show an
oscillatory behaviour of the plume top on 21–22 May, with
a considerable drop in height to 10 km around 20:00 UTC
on 22 May, followed by a drop to 6 km (or lower) at
∼
11:00 UTC on 23 May and another drop to less than 3 km
at ∼ 16:00 UTC. The height does not exceed 8 km thereafter.
At the start of the Grímsvötn eruption a tall and opti-
cally thick column extended several kilometres into the at-
mosphere. During this phase, the large opacity of the cloud
makes ash retrieval using infrared and ultraviolet radiation
very difficult and it is likely that the cause of the opacity is
due to condensed water vapour and steam in the column. Par-
ticle sizes are large (millimetre to centimetre size) and hot
gases, principally water vapour, dominate the emissions. The
evolution of the column can be studied using single-channel
infrared measurements (Fig. 4) by making the assumption
that the cloud is behaving as a black body and the brightness
temperatures correspond to the cloud-top temperature.
The black body assumption is generally quite good, but
because the cloud may overshoot, undercooling may occur
(Woods et al., 1995), leading to cloud tops with infrared tem-
peratures many degrees Kelvin below the background atmo-
spheric temperature. Radiosonde data from Keflavík airport
(see map Fig. 2) were used to relate the infrared brightness
temperatures to cloud-top height. The radiosonde data show a
tropopause at ∼ 8.5 km and a dry layer between 3 and 4 km
2
.
The winds are towards the southwest and south-southwest
up to about 4 km, then westerlies up to the tropopause and
high-level winds from the south. A tall column of ash enter-
ing this highly sheared atmosphere will suffer transport in at
least three different directions. Plume evolution can also be
estimated from spatial changes in the brightness temperature
images and is a useful way to identify the onset of column
growth.
The shadow cast by the Grímsvötn eruption clouds seen
on some MODIS images can also be used to estimate cloud-
top height (Prata and Grant, 2001). A MODIS/Aqua 250 m
resolution image acquired at 13:15 UTC on 22 May shows
the Grímsvötn column with a strong shadow cast onto the
ground and cloud below (Fig. 5).
Utilizing the geometry of the satellite and sun-viewing di-
rections and the contrast difference between the dark shadow
and brighter cloud and/or ice below, the highest parts of the
ash column in this image are estimated to be 16 ± 1 km,
with other parts of the column having lower tops. It can also
be seen that there is a tephra layer to the south of the col-
umn that appears to be detached and dispersing southwards.
This layer may have arisen from a less vigorous phase of the
eruption (when the column top was lower) or was possibly
formed from ash rising off a PDC or from a partial column
2
These heights may be a little higher, ∼ 1–1.5 km over the high
terrain of the Vatnajökull glacier.
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