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| Titel |
Improved determination of volcanic SO2 emission rates from SO2 camera images |
| VerfasserIn |
Angelika Klein, Peter Lübcke, Nicole Bobrowski, Ulrich Platt |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105818
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| Publikation (Nr.) |
 EGU/EGU2015-5401.pdf |
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| Zusammenfassung |
| SO2 cameras determine the SO2 emissions of volcanoes with a high temporal and spatial
resolution. They thus visualize the plume morphology and give information about turbulence
and plume dispersion. Moreover, from SO2 camera image series emission rates can
be determined with high time resolution (as will be explained below), these data
can help to improve our understanding of variations in the degassing regime of
volcanoes.
The first step to obtain emission rates is to integrate the column amount of SO2 along two
different plume cross sections (ideally perpendicular to the direction of plume propagation);
combined with wind speed information this allows the determination of SO2 fluxes. A
popular method to determine the mean wind speed relies on estimating the time lag of the
SO2 signal derived for two cross sections of the plume at different distances downwind of the
source. This can be done by searching the maximum cross-correlation coefficient of the two
signals.
Another, more sophisticated method to obtain the wind speed is to use the optical flow
technique to obtain a more detailed wind field in the plume from a series of SO2 camera
images. While the cross correlation method only gives the mean wind speed between the two
cross sections of the plume, the optical flow technique allows to determine the wind speed
and direction for each pixel individually (in other words, a two-dimensional projection of the
entire wind field in the plume is obtained).
While optical flow algorithms in general give a more detailed information about the wind
velocities in the volcanic plume, they may fail to determine wind speeds in homogeneous
regions (i.e. regions with no spatial variation in SO2 column densities) of the plume.
Usually the wind speed is automatically set to zero in those regions, which leads to an
underestimation of the total SO2 emission flux. This behavior was observed more than
once on a data set of SO2 camera images taken at Etna, Italy in July, 2014. For
those data the cross-correlation method leads to a more realistic result, which was
close to simultaneously measured SO2 fluxes calculated from spectra taken by a
zenith looking differential optical absorption spectroscopy (DOAS) instrument
traversing underneath the plume. In the analyzed data the flux determined with
the cross-correlation method was twice the flux determined with the optical flow
algorithm. We further investigated the potential error in the SO2 flux determination
caused by a slant view on the plume. This is a situation commonly encountered when
observing volcanic SO2-fluxes by remote sensing techniques. Frequently it is difficult to
determine the precise angle between wind direction (i.e. plume propagation direction)
and observation direction. We find that in volcanic plumes with an inclination that
differs more than 20 degree from the assumed wind direction, can cause an error in
the determined SO2 flux can deviate from the true value by more than 10 percent. |
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