Environmental Engineering Reference
In-Depth Information
Interferometer) polar orbiting sensors and is useful for studies of climate-related
effects due to volcanic SO
2
(see Schmidt and Robock, this volume) and also for
aviation. The weaker bands at 4 and 8.6
μ
m are also in much more transparent parts
of the electromagnetic spectrum and consequently can be used down to the surface.
The 4
μ
m band is generally too weak to be useful for diffuse SO
2
emissions but for
large emissions, close to the surface, or for systems utilising solar absorption
spectroscopy, the band can be used. The 8.6
m band has been used from space
and from the surface to measure SO
2
(Realmuto
et al
.,
1994
; Prata and Bernardo,
There are also absorption lines in the microwave region, principally around
204.25, 346.52, 624.34, 624.89, 625.84, 626.17 and 649.24 GHz.
μ
7.4.2 Satellite retrievals
Methods
Retrieval of SO
2
from remote-sensing instruments falls into three main categories:
UV, IR and microwave methods. The UV methods essentially adopt the DOAS
(differential optical absorption spectroscopy) technique (see Platt and Bobrowski,
this volume) to determine the slant-path molecular column density of SO
2
. Good
explanations of the current methods used can be found in Yang
et al
.(
2010
) and
Krotkov
et al
.(
2010
), including a description of the effects of the SO
2
height in the
atmosphere and ways to ameliorate these.
Examples
To illustrate the capability of satellite instruments to measure SO
2
the example of
the Nabro eruption in June 2011 is used. This stratovolcano in Eritrea, longitude
41.69
E, latitude 13.36
N, elevation 2219 m (above sea level), erupted on 12 June
2011. Very little was known about the eruptive history of Nabro so the eruption
came as a surprise. There are many satellite images of the eruption including high-
resolution thermal imagery from the ASTER (Advanced Space-borne Thermal
Emission and Re
ection Radiometer) instrument, 15-minute geostationary data
from the MSG SEVIRI (Meteosat Second Generation Spin-stabilised Enhanced
Visible and Infrared Imager) instrument as well as research satellite data from
Aura/OMI, MODIS/Terra and Aqua, the AIRS instrument, IASI and GOME-2,
among others. The AIRS sensor provided a good assessment of the SO
2
in the
dispersing emissions from Nabro. Prata and Bernardo (
2007
) have described the
retrieval of SO
2
using AIRS and noted that because there is limited sensitivity
within the waveband (
c
. 7.3
m) to SO
2
below
c
. 3 km, largely UTLS SO
2
is
measured. In the cases of large emissions of SO
2
into the UTLS,
3
μ
signi
cant
3
For Nabro this has been estimated as
c
.1.5 Tg
