Environmental Engineering Reference
In-Depth Information
T
s
T
b
T
s
T
c
τ ¼
ln 1
,
ð
7
:
4
Þ
where
T
c
is the cloud-top temperature,
T
s
is the surface temperature and
T
b
is the
brightness temperature in one of the channels. The term
T
s
-T
c
is often referred to as
the thermal contrast and it can be seen that this should be large, while
T
s
should be
larger than the cloud-top temperature. These conditions may be interpreted as implying
that retrieval of
is best for semi-transparent clouds where the ash cloud has a
temperature different to that of the surface below. For very transparent clouds
(i.e. dilute ash clouds),
T
b
τ
!
T
s
and retrieval becomes problematic. Likewise, as
T
b
!
T
c
the cloud becomes opaque and
τ!
∞
. An IR optical depth of
4 corresponds
to
T
b
0.5 K and may be considered a limiting value for most IR satellite sensors.
For an optically thick ash cloud with
-
T
c
10
6
gm
3
,
Q
ext
ρ
2.5
2.5,
r
e
5
μ
m, the
20 g m
2
, while for an optically thin ash cloud, with
T
s
mass loading
m
i
-
T
b
0.5 K,
0.15 g m
2
. In practice, the lower
limit has been found to have the slightly higher value of
m
i
τ
0.01, and with the same values as before,
m
i
0.2gm
2
(Prata and
Prata,
2012
). An important point to note here is that it is the IR opacity that matters;
ash clouds with large particles outside the sensitivity range are easily detected.
In summary, these simple calculations show that satellite IR remote sensing of
volcanic ash clouds can be used to determine mass loadings within the broad range
of 0.2 to 20 g m
2
and, for a 1-km-thick ash cloud, this covers the entire range of
concentrations of concern to aviation (viz. 0.2, 2 and 4 mg m
3
and higher).
7.4 Remote sensing of SO
2
7.4.1 Absorption properties of SO
2
The SO
2
molecule exhibits a wide range of absorption properties from the UV to
the microwave part of the electromagnetic spectrum. In the UV, SO
2
exhibits
signi
cant absorption features within the region between 240 and 338 nm. There
are several satellite-borne sensors that exploit these UV features for SO
2
retrieval,
including TOMS, GOME (Global Ozone Monitoring Experiment), GOME-2,
SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric
CHartographY), OMI (Ozone Monitoring Instrument) and, more recently, OMPS
(Ozone Mapping Pro
ler Suite).
In the IR region, SO
2
has three important absorption bands centred near 4,
7.3 and 8.6
m band (1362 cm
1
) is the very strong anti-symmetric
stretch absorption feature (
v
3
-band) of the SO
2
molecule, but the band lies in a
region of strong water-vapour absorption, which limits its use in the lower
troposphere. This band is used to determine upper
μ
m. The 7.3
μ
lower
stratosphere (UTLS) SO
2
partial column abundance using data from the AIRS
(Atmospheric Infrared Sounder) and IASI
troposphere
-
(Infrared Atmospheric Sounding
