Environmental Engineering Reference
In-Depth Information
chemical conversion of the gas into sulfate aerosols can cause radiative heating
of the UTLS and cooling at the surface. A composite of AIRS retrievals on 18 June
2011 for the Nabro eruption, 6 days after the start, shows (
Figure 7.4
, upper panel)
that the cloud had dispersed widely, reaching China. The plot shows
'
plumes
'
or
'
streamers
'
of SO
2
winding across the Asian subcontinent, giving a comprehensive
overview of the horizontal dispersion. Little height information is available from this
retrieval scheme, but coincident CALIOP data can be used to obtain vertical height
information. The lower panel of
Figure 7.4
shows a backscatter
curtain
'
obtained
from the CALIOP 532 nm lidar between 18:13 and 18:27 UT on 18 June 2011.
The satellite UV sensors (e.g. TOMS/OMI and GOME/GOME-2) have proven to
be extremely valuable for assessing SO
2
emissions from both explosive and passively
degassing volcanoes. The work by Carn
et al
.(
2003
,
2008
) describe how the
TOMS and OMI sensors can be used to study passively degassing volcanoes and
more vigorous explosive activity. A useful aspect of the OMI sensor is that it has been
operating for many years (9 years by the end of 2013), so that regional composites
can be made providing insights into trends in temporal increases or decreases
of SO
2
emissions. Such information is helping climate scientists to better constrain
SO
2
effects on the chemistry of the atmosphere and possible radiative impacts.
'
7.4.3 Ground-based retrievals
Methods
The COSPEC instrument (Moffat and Millan,
1971
) has been widely used to
measure and quantify SO
2
in volcanic emissions but it has been largely superseded
by compact scanning spectrometers utilising the DOAS method (see Platt and
Bobrowski, this volume) for retrieving SO
2
.SO
2
cameras (Kern,
2009
) are begin-
ning to replace the spectrometers and they are undergoing rapid development. As
both spectrometers and cameras seem to be the future technologies for remote
sensing of SO
2
, we describe these in more detail below.
Compact UV spectrometers
The development of UV spectrometers for SO
2
measurement occurred in the late
1990s when it became economically feasible to use small
fibre-optic spectrometers
with quite high resolution (0.5
-
1 nm), and good signal-to-noise ratios operating
between about 300
360 nm within the UV part of the electromagnetic spectrum.
At the heart of these systems is a small, compact spectrometer utilising a
-
fibre-optic
probe and focusing optics, and a mirror to allow scanning.
Emission rates can be obtained from these systems by scanning under the plume
from either a stationary instrument or by traversing with a mobile unit, and then
estimating the wind speed at plume height. The integral of the path concentration
