F. Prata et al.: Separation of ash and SO
2
10717
Figure 5. MODIS true-colour 250 m resolution image of the
Grímsvötn eruption column, showing the shadow cast on the ground
and cloud below (to the N of the column). Note also the ash layer off
the south coast that appears detached from the main column, sug-
gesting that it is no longer being fed by ash from the vent. Image:
MODIS/Aqua, 22 May 2011, 13:15 UTC.
by Yang et al. (2007a) for the Ozone Monitoring Instrument
(OMI), and Clarisse et al. (2008) for IASI. AIRS data provide
an excellent view of the SO
2
dispersion; see the Supplement
Fig. S2. SO
2
was first detected in AIRS data at 03:35 UTC
on 22 May, which was the first overpass of the Aqua satellite
platform over Iceland following the initial Grímsvötn erup-
tion. A large cloud of SO
2
gas was detected over the Vatna-
jökull glacier, slightly displaced to the north of Grímsvötn.
In subsequent AIRS overpasses the SO
2
cloud grew larger
and spread predominantly northwards, reaching the Green-
land coast by 04:17 UTC on 23 May, ∼ 12 h later. The SO
2
cloud then spread westwards and eastwards while still propa-
gating northwards into a long filament. The SO
2
layer height
cannot be inferred directly from the AIRS retrievals, but the
direction of travel and transport modelling suggests a height
of ∼ 8–10 km, which implies the SO
2
was stratospheric.
The mass of upper troposphere–lower stratosphere (UTLS)
SO
2
calculated from the AIRS data is shown in Fig. 7. The
maximum SO
2
mass was found to be ∼ 0.24 ± 0.05 Tg at
14:00 UTC on 23 May 2011. Although the AIRS retrievals
are only strictly valid for the UTLS, in this case it is most
likely that the majority (> 90 %) of the SO
2
was located in
the UTLS. Identification of a volcanic layer in CALIOP li-
dar data was difficult initially, suggesting that conversion to
SO
2−
4
aerosol was not yet sufficient to provide a good signal
and that few ash particles were collocated with the SO
2
.
3
3
The CALIOP lidar is insensitive to SO
2
gas, but backscatter
depolarization and colour ratio values from both SO
2−
4
and ash par-
ticles can often be identified for strong layers.
Table 2. SO
2
total mass estimates from four different satellite in-
struments (two infrared and two ultraviolet) from 22 to 26 May
2011.
Instrument
Date in May 2011
22
23
24
25
26
Total mass (Tg)
AIRS
0.10
0.24
0.18
0.12
0.11
IASI
1
0.23
0.32
OMI
0.15
0.28
0.29
0.25
0.20
GOME-2
2
0.03
0.11
0.13
0.18
0.23
1
L. Clarisse (personal communication, 2015).
2
A. Richter,
http://www.iup.uni-bremen.de/scia-arc/.
At least three other satellite sensors detected the high-level
SO
2
cloud: OMI on the Aura platform, GOME-2 on Metop-
A, and IASI also on Metop-A. Table 2 shows estimates of
the daily SO
2
mass from each of the sensors. OMI observa-
tions are shown in the Supplement Fig. S3. Sigmarsson et al.
(2013) estimated the sulfur budget for the Grímsvötn erup-
tion and made use of satellite SO
2
measurements.
Although there is some disparity between the esti-
mates from the different sensors, when the effects of dif-
ferences in height sensitivity, timing, field-of-view sizes,
and swath overlap are taken into account, the values fall
within the expected error bounds. The means and stan-
dard deviations for 22, 23, 24, 25, and 26 May are
0.128 ± 0.08, 0.238 ± 0.09, 0.200 ± 0.08, 0.183 ± 0.07, and
0.180 ± 0.06 Tg, respectively. We therefore conclude that
between 0.13 and 0.24 ± 0.1 Tg SO
2
was released into the
UTLS by Grímsvötn during the period 22–26 May 2011,
about half the total estimated amount released to the atmo-
sphere. Carn et al. (2016) estimated a maximum SO
2
load-
ing of ∼ 0.38 Tg and Sigmarsson et al. (2013) estimated
∼
0.2 Tg(S) or ∼ 0.4 Tg(SO
2
).
4.4
Ash
Volcanic ash retrievals were performed using the methods
outlined by Wen and Rose (1994) and Prata and Prata (2012).
Data from MODIS, AIRS, and IASI, all on polar-orbiting
platforms, were used to determine brightness temperatures
and, ultimately, fine (effective radii < 16 µm) ash mass load-
ings and particle sizes. Geostationary data from SEVIRI pro-
vided measurements every 15 min from which brightness
temperatures in five infrared channels could be used to detect
and quantify the very fine ash component. Figure S4 (Supple-
ment) shows ash mass and effective particle size retrievals
from SEVIRI at 6-hourly intervals on 23 May 2011.
The mass of very fine ash was estimated using SEVIRI
images by averaging in hourly intervals (four estimates per
hour) and adjusting the estimates for changes in viewing an-
gle, which can cause an error in the cloud-top temperature es-
timation. Mass is estimated by identifying only ash-affected
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