10718
F. Prata et al.: Separation of ash and SO
2
Figure 6. (a) AIRS brightness temperature difference for volcanic ash (yellow/orange/red) and AIRS brightness temperature difference for
SO
2
(shades of blue). The ascending trace (travelling from south to north) of the CALIPSO satellite is indicated by the black and green line,
in which the green-coloured portion of the line indicates the region of overlap between the CALIOP and AIRS image data. (b) MODIS/Aqua
true-colour image showing the low-level ash cloud (brown). (c) CALIOP backscatter curtain for 532 nm light with the ash and sulfate layers
indicated by the white-coloured ellipses. The lower strips show temperature differences based on the AIRS data indicating regions affected
by ash and SO
2
gas. Panels (d) and (e) show vertical profiles of backscatter for ash and SO
4
2−
, respectively. δ
v
is the volume depolarization
ratio and χ is the colour ratio. The horizontal black lines in panels (d) and (e) show the height range over which the parameters have been
calculated. Date and time of overpass: 23 May 2011, 14:01–14:06 UTC.
pixels (using a sequence of cloud tests), multiplying these by
the area of the pixel (which varies with scan position) and
summing them to arrive at a total mass. For this case, the
zenith viewing angle decreased from ∼ 60 to ∼ 10
◦
as the
ash cloud progressed eastwards.
The maximum mass of very fine ash was estimated to be
0.19 ± 0.03 Tg late on 23 May. This is about 0.05 % of the to-
tal mass of magma erupted, suggesting that the very fine ash
fraction is
1 wt % of the overall mass of tephra produced
by the eruption. Four MODIS overpasses were also used to
estimate very fine ash mass, shown in Fig. 8 together with
estimates from IASI (L. Clarisse, personal communication,
2015; also see Moxnes et al., 2014) that are sampled twice
per day. The MODIS retrievals are shown in Fig. S4 (Sup-
plement).
The MODIS data give slightly higher estimates than SE-
VIRI, decreasing from ∼ 0.23 Tg at 12:05 UTC on 23 May
to ∼ 0.15 Tg at 03:25 UTC on 24 May. The low SEVIRI es-
timates at the start of the series are a consequence of the
inability of the SEVIRI retrieval scheme to quantify ash at
these high zenith viewing angles and the confounding effects
of meteorological cloud that sometimes overlaid the ash (the
ash layer was mostly confined to heights below ∼ 3 km; see
also Fig. 9).
IASI retrievals are consistently higher than SEVIRI and
MODIS until late on 24 May when there is closer agreement.
Prata and Prata (2012) showed that the SEVIRI mass loading
retrievals were consistent with ground-level PM
10
concentra-
tion measurements at several stations in Scandinavia (e.g. at
Bergen and Oslo), if the ash cloud was assumed to be con-
fined to a layer no deeper than 3 km. The reason for the large
differences between IASI and SEVIRI retrievals is under in-
vestigation, but IASI has a greater sensitivity to ash due to
the higher spectral resolution, and the assumptions used in
the retrievals are different (Clarisse and Prata, 2016). It is
also clear from some of the MODIS images that meteorolog-
ical cloud overlaid the ash cloud and it is difficult to retrieve
ash from these broadband low-spectral-resolution data under
these circumstances.
Atmos. Chem. Phys., 17, 10709–10732, 2017
www.atmos-chem-phys.net/17/10709/2017/
F. Prata et al.: Separation of ash and SO
2
10719
22
23
24
25
26
27
28
29
Day in May 2011
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Mass of SO
2
(Tg)
AIRS SO retrievals for Gr msvötn, May 2011.
2
Peak mass = 0.24 Tg at 13:59–14:05 UTC on 23 May 2011
'
Figure 7. AIRS UTLS SO
2
mass loading (Tg) as a function of time
for 22–29 May 2011. The dashed line shows the locus of maximum
mass loadings – since AIRS sometimes has incomplete coverage
of the whole plume, a true estimate of the maximum SO
2
mass is
difficult.
22
23
24
25
Day in May 2011
0.0
0.1
0.2
0.3
0.4
0.5
Mass (Tg)
SEVIRI
MODIS
IASI
Figure 8. Very fine ash mass (Tg) estimated from SEVIRI, MODIS,
and IASI data for the period 22–24 May 2011.
4.5
Error in ash retrievals
The error (precision) in estimating very fine ash mass from
infrared retrievals has been investigated by Wen and Rose
(1994) and Prata and Prata (2012), who suggest errors of 40–
50 %. Stevenson et al. (2015) discuss potential errors in satel-
lite retrievals by using cryptotephra data to speculate that
larger particles exist in dispersing ash clouds (although no
atmospheric observations are presented) and claim through
modelling studies that current retrieval schemes (all of them)
underestimate mass loadings because of the dense sphere as-
Figure 9. MODIS true-colour image showing the low-level ash
cloud coming off Iceland and spreading southwards. Notice that
meteorological cloud is clearly evident above the ash layer and is
either obscuring the ash layer over Iceland or the ash layer ends
near the coast. Grímsvötn is just off the image to the north. (Image:
MODIS/Terra, 23 May 2011, 12:05 UTC.)
sumption and lack of sensitivity to particles with diameters >
10 µm. Estimating precision in retrievals is difficult because
of the uncertainties in the input parameters, such as the com-
plex index of refraction, the size distribution, and the shapes
of the particles, although shape is generally found to result
in the smallest discrepancy of the input parameters, with the-
oretical simulations showing differences in the range of 10–
40 % (Yang et al., 2007b; Kylling et al., 2014). An additional
problem with estimating precision due to shape is that apart
from having no observations, the effect of their statistical
orientation in space and the distribution of the shapes as a
function of particle size is unknown and potentially large.
Attempting to model these uncertainties in the absence of
any observational constraints is unproductive. An alternate
strategy, and one that is adopted widely, is to use what few
data there are and compare retrievals with independent ob-
servations to estimate the accuracy with respect to the inde-
pendent estimate. It is acknowledged that this approach may
www.atmos-chem-phys.net/17/10709/2017/
Atmos. Chem. Phys., 17, 10709–10732, 2017