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Titel |
Space-borne detection of volcanic carbon dioxide anomalies: The importance of ground-based validation networks |
VerfasserIn |
F. M. Schwandner, S. A. Carn, S. Corradini, L. Merucci, G. Salerno, A. La Spina |
Konferenz |
EGU General Assembly 2012
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 14 (2012) |
Datensatznummer |
250064664
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Zusammenfassung |
We have investigated the feasibility of space-borne detection of volcanic carbon dioxide
(CO2) anomalies, and their integration with ground-based observations. Three goals provide
motivation to their integration: (a) development of new volcano monitoring techniques, with
better spatial and temporal coverage, because pre-eruptive volcanic CO2 emissions are
potentially the earliest available indicators of volcanic unrest; (b) improvement the
currently very poor global CO2 source strength inventory for volcanoes, and (c) use of
volcanic CO2 emissions for high altitude strong point source emission and dispersion
studies.
(1) Feasibility of space-borne detection of volcanic CO2 anomalies.
Volcanoes are highly variable but continuous CO2 emitters, distributed globally, and
emissions often occur at high altitudes. To detect strong point sources of CO2 from
space, several hurdles have to be overcome: orographic clouds, unknown dispersion
behavior, a high CO2 background in the troposphere, and sparse data coverage from
existing satellite sensors. These obstacles can be overcome by a small field of view,
enhanced spectral resolving power, and by employing repeat target mode observation
strategies.
The Japanese GOSAT instrument has been operational since January 2009, producing
CO2 total column measurements with a repeat cycle of 3 days and a field of view of 10km.
GOSAT thus has the potential to provide spatially integrated data for entire volcanic edifices,
especially in target mode. Since summer 2010 we have conducted repeated target
mode observations of over 20 persistently active global volcanoes including Etna
(Italy), Erta Ale (Ethiopia), and Ambrym (Vanuatu), using L2 GOSAT FTS SWIR
data.
One of our best-studied test cases is Mt. Etna on Sicily (Italy), which reawakened in 2011
after a period of quiescence and produced a sequence of eruptive activities including lava
fountaining events, coinciding with target-mode GOSAT observations conducted there since
2010. For the 2011 activity we compare GOSAT custom re-processed target mode
observation CO2 data to SO2 data from the Ozone Monitoring Instrument (OMI), the
Moderate-Resolution Imaging Spectroradiometer (MODIS), and ground-based SO2
measurements obtained by the FLAME ultraviolet scanning DOAS network, as well as
ground-based multi-species measurements obtained by FTIR technique. GOSAT CO2 data
show an expected seasonal pattern, because the signal is dominated by ambient atmospheric
CO2. However, some possible significant variations do appear to exist before and during
eruptive events. Besides cloud and aerosol effects and volcanic emission pulses, two further
factors seem to also strongly affect the signal beyond seasonal variability: different
altitudes ranges of sensitivity for OMI and GOSAT appear to cause inverse signal
correlations when the presence of clouds allows for multiple scattering effects. The
second effect is wintertime high-altitude snow cover, which enhances the reflected
light yield in the suspected high-concentration column portions near the ground.
The latter two effects may dominate between emission pulses and their inverse
correlations stand in contrast to magmatic events, which we suspect to give rise to positive
correlations.
(2) Integration of space-borne and ground-based observations of volcanic CO2
emissions.
Monitoring of remote terrestrial volcanic point sources of CO2 from space and using
ground-based observations have advantages and disadvantages. Advantages of satellite
methods include homogenous coverage potential, a single data format, and a largely
unbiased, mostly global coverage potential. Advantages of ground-based observations include
easier calibration and targeting, validation and spatial resolution capacity. While cost plays
a strong role in either approach, ground-based methods are often hampered by
available personnel to expand observations to global coverage, by a patchwork of
instrumentation types, coverage, availability, quality, and disparity of data formats and
frequencies.
The solution to achieving high quality regional or global coverage may lie in
the combination of the two methods, and satellite campaigns do already rely on
ground-based validation methods for their data products. To optimize a combination of
space-borne and ground-based techniques, two aspects need to be addressed: (a)
database and data format compatibility, and (b) questions of instrumentation network
design and compatibility. For regional spatio-temporal atmospheric variances, no
homogenous data formats and databases exist to date. In volcanology, such an approach
already exists in the emerging WOVOdat database and its data format convention
(http://www.wovodat.org). For ground-based CO2 network designs in volcanology and in
greenhouse gas monitoring in general, no common approach has yet found common use that
besides integration of existing networks would also enable nesting, scaling and fractal
growth.
In summary, promising first results from the GOSAT instrument indicate that a
space-based detection of volcanic CO2 pulses may indeed be possible. Once newer
generations of satellite sensors become operational (e.g., OCO-2, OCO-3, and GOSAT-2),
better ground resolution and scanning capabilities might enable detection and imaging of
volcanic CO2 plumes. To derive the best quality product from such measurements, we
anticipate an urgent need to design new approaches to ground-based validation networks and
their data products. Such networks would ideally become more compatible, with a higher
degree of instrumentation compatibility, and a common data exchange format to connect
between distributed databases. Community-based efforts will be necessary to progress toward
these goals. |
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