10716
F. Prata et al.: Separation of ash and SO
2
SEVIRI
11 µ m temperature, date: 21 May 2010, time: 19:00 UT
-19.0
-18.5
-18.0
-17.5
-17.0
-16.5
-16.0
Longitude (
o
)
63.0
63.5
64.0
64.5
65.0
65.5
66.0
Latitide (
o
)
Temperature (K)
250
252
254
256
258
260
262
264
266
268
270
SEVIRI 11 µm temperature, date: 21 May 2010, time: 19:15 UT
-19.0
-18.5
-18.0
-17.5
-17.0
-16.5
-16.0
Longitude (
o
)
63.0
63.5
64.0
64.5
65.0
65.5
66.0
Latitide (
o
)
228
232
236
240
Temperature (K)
220
224
228
232
236
240
244
248
252
256
260
SEVIRI 11 µm temperature, date: 21 May 2010, time: 19:30 UT
-19.0
-18.5
-18.0
-17.5
-17.0
-16.5
-16.0
Longitude (
o
)
63.0
63.5
64.0
64.5
65.0
65.5
66.0
Latitide (
o
)
228
232
236
240
240
Temperature (K)
220
224
228
232
236
240
244
248
252
256
260
SEVIRI 11 µm temperature, date: 21 May 2010, time: 19:45 UT
-19.0
-18.5
-18.0
-17.5
-17.0
-16.5
-16.0
Longitude (
o
)
63.0
63.5
64.0
64.5
65.0
65.5
66.0
Latitide (
o
)
226
228
228
228
232
232
236
236
240
240
Temperature (K)
220
224
228
232
236
240
244
248
252
256
260
Figure 4. Close-up views of the 11 µm brightness temperatures from the SEVIRI instrument at 15 min intervals, starting at 19:00 UTC on
21 May 2011. Isolines (contours) of brightness temperatures are shown in white to highlight the location and expansion of the top of the
column. The rapid plume rise may be interpreted by the change in extent of the cloud-top temperatures. There is no evidence of an eruption
in the image at 19:00 UTC. The location of Grímsvötn is shown as a black triangle.
2. The layer was sulfate with some depolarization from ice
particles (not very likely in the stratosphere).
3. The layer was sulfate with some depolarization from
small ash particles (below the detection limit of AIRS).
The last interpretation is certainly consistent with obser-
vations of some small amounts of ash in the northern part of
the plume (Prata and Rose, 2015).
The colour ratios (χ ; see https://eosweb.larc.nasa.
gov/PRODOCS/calipso/Quality_Summaries/CALIOP_
L2LayerProducts_3.01.html for more details of these
parameters) are χ ∼ 1 for ash and χ ∼ 0.5 for SO
4
2−
,
showing a clear difference between these two aerosol layers.
The ash layer shows considerable vertical structure, with
thin layering evident below 3 km and a broader feature
peaking ∼ 4.5 km. These data provide compelling evi-
dence for ash and SO
2
(and SO
4
2−
) separation from the
Grímsvötn eruption and also provide quantitative estimates
of the height separation with a lower-troposphere ash cloud
and a stratospheric gas and aerosol cloud.
4.3
SO
2
gas
The satellite instruments used to retrieve SO
2
are also shown
in Table 1. Details of the retrieval algorithms may be found
in the papers by Prata and Bernardo (2007) for the AIRS,
Atmos. Chem. Phys., 17, 10709–10732, 2017
www.atmos-chem-phys.net/17/10709/2017/