F. Prata et al.: Separation of ash and SO
2
10715
Table 1. Satellite instruments used in this study to identify volcanic emissions. The instruments are AVHRR: advanced very high resolution
radiometer, MODIS: Moderate Resolution Infrared Spectroradiometer, AIRS: Atmospheric Infrared Sounder, IASI: infrared atmospheric
spectroradiometer interferometer, SEVIRI: Spin-stabilised Enhanced Visible and Infra-Red Instrument, OMI: Ozone Monitoring Instrument,
and CALIOP: Cloud-Aerosol Lidar with Orthogonal Polarization.
Instrument/platform
Spatial
Temporal
Parameter
(A)ctive or (P)assive
resolution
resolution
(km
2
)
(hours)
AVHRR/Metop-A, B
∼
1.1 × 1.1
∼
3
Ash
P
MODIS/Terra/Aqua
0.25 × 0.25–1 × 1
∼
6
Ash/SO
2
P
AIRS/Aqua
∼
13 × 13
∼
12
SO
2
/Ash
P
IASI/Metop-A
∼
10 × 10
∼
12
SO
2
/Ash
P
SEVIRI/MSG-2
∼
3 × 3–10 × 10
0.25
Ash
P
OMI/Aura
∼
13 × 24
24
SO
2
/AAI
P
CALIOP/CALIPSO
∼
0.1 × 0.3
16 days
Aerosols
A
collapse. Whatever the exact mechanism involved, this low-
level tephra plume is effectively independent of the tephra
fed into the upper column at source and could therefore be
treated as a separate source for the purpose of forecasting its
movement. The layer is evident on later MODIS images (see
later Fig. 9) and it is this low-level tephra layer that eventu-
ally travels eastwards towards Scotland and on to southwest-
ern Scandinavia.
4.2
Space-borne lidar measurements
The CALIOP instrument on-board the polar-orbiting
CALIPSO platform is a polarization-sensitive, elastic
backscatter
lidar
capable
of
providing
high-vertical-
resolution (∼ 60 m) attenuated backscatter profiles of clouds
and aerosols as well as cloud-top heights. The instrument
transmits linearly polarized light and measures the return
signal at 532 and 1064 nm. The components perpendicular
and parallel to laser polarization are measured separately at
532 nm. Details of the instrument, the science applications,
and an example of its use in a volcanic study may be found
in Hunt et al. (2009), Vaughan et al. (2009), and Winker
et al. (2012). The lidar is near-nadir pointing, has a ground
footprint diameter of 70 m, and a repeat time of 16 days,
which limits the number of times the lidar beam coincides
with a target of interest. Between 21 and 23 May 2011, 10
CALIOP coincidences were identified for the Grímsvötn ash
and SO
4
2−
clouds. Figure 6 shows an example of a CALIOP
pass on 23 May when the CALIPSO trace intersected an
ash cloud to the south of Iceland and a SO
4
2−
layer to the
north. Panel (a) shows indices based on coincident AIRS
brightness temperature difference measurements, using an
index to indicate ash (orange/red colours) or SO
2
(shades of
blue).
Details of the ash and SO
2
indices can be found in Prata
et al. (2015) and Hoffmann et al. (2014), respectively. Fig-
ure 6b shows a MODIS/Aqua true-colour image acquired
at the same time as the AIRS measurements. Panel (c)
shows the CALIOP attenuated backscatter signal measured
at 532 nm. The black horizontal line indicates the height of
the tropopause determined from GMAO (Global Modelling
and Assimilation Office) reanalysis data (Rienecker et al.,
2008). The strips at the base show collocated AIRS pix-
els along the CALIOP track where ash and SO
2
have been
identified. Between ∼ 59.9 and ∼ 62.7
◦
N (left-most white-
coloured ellipse), a tropospheric ash cloud is detected in the
AIRS data and the CALIOP backscatter signal suggests that
these cloud layers have heights of ∼ 1–6 km. Between ∼ 68.6
and ∼ 72.0
◦
N (right-most white-coloured ellipse), a strato-
spheric cloud layer of SO
2
is detected in the AIRS data. The
CALIOP instrument is insensitive to SO
2
but does scatter
light from ash and SO
2
4−
aerosols as well as meteorological
clouds of ice and water droplets. The height of this layer in
the CALIOP curtain is between 10 and 12 km and above the
tropopause. Panels (d) and (e) in Fig. 6 show vertical profiles
of the backscatter for these two layers, averaged over the two
latitude sections identified. These data suggest that the up-
per layer is most likely to be an SO
4
2−
layer coincident with
the SO
2
gas. Low volume / depolarization ratios (δ
v
∼
0.1–
0.2), indicative of spherical particles, within the stratospheric
layer are also consistent with an SO
4
2−
layer rather than ash
or ice clouds. For the eruption of Sarychev Peak, Prata et al.
(2017) found a mean δ
v
of 0.05 ± 0.04 and for Kasatochi
δ
v
was 0.08 ± 0.03. However, these were based on nighttime
measurements and daytime data are noisier in the backscatter
signal and can contribute to higher-than-expected δ
v
values.
For Puyehue-Córdon Caulle (dominated by ash particles), δ
v
was ∼ 0.28 ± 0.03. The threshold between sulfates and ash
used was δ
v
∼
0.2. This makes the Grímsvötn observation
somewhat ambiguous. There are three potential interpreta-
tions:
1. The layer was sulfate and the δ
v
∼
0.2 was due to day-
time noise in the backscatter signal.
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Atmos. Chem. Phys., 17, 10709–10732, 2017