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| Titel |
On the absolute calibration of SO2 cameras |
| VerfasserIn |
P. Lübcke, N. Bobrowski, S. Illing, C. Kern, J. M. Alvarez Nieves, L. Vogel, J. Zielcke, H. Delgado Granados, U. Platt |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 6, no. 3 ; Nr. 6, no. 3 (2013-03-14), S.677-696 |
| Datensatznummer |
250017840
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| Publikation (Nr.) |
 copernicus.org/amt-6-677-2013.pdf |
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| Zusammenfassung |
Sulphur dioxide emission rate measurements are an important tool for volcanic
monitoring and eruption risk assessment. The SO2 camera technique
remotely measures volcanic emissions by analysing the ultraviolet absorption
of SO2 in a narrow spectral window between 300 and 320 nm using solar
radiation scattered in the atmosphere. The SO2 absorption is selectively
detected by mounting band-pass interference filters in front of a
two-dimensional, UV-sensitive CCD detector. One important step for correct
SO2 emission rate measurements that can be compared with other
measurement techniques is a correct calibration. This requires conversion
from the measured optical density to the desired SO2 column density
(CD). The conversion factor is most commonly determined by inserting quartz
cells (cuvettes) with known amounts of SO2 into the light path. Another
calibration method uses an additional narrow field-of-view Differential
Optical Absorption Spectroscopy system (NFOV-DOAS), which measures the column
density simultaneously in a small area of the camera's field-of-view. This
procedure combines the very good spatial and temporal resolution of the
SO2 camera technique with the more accurate column densities obtainable
from DOAS measurements.
This work investigates the uncertainty of results gained through the two
commonly used, but quite different, calibration methods (DOAS and calibration
cells). Measurements with three different instruments, an SO2 camera, a
NFOV-DOAS system and an Imaging DOAS (I-DOAS), are presented. We compare the
calibration-cell approach with the calibration from the NFOV-DOAS system. The
respective results are compared with measurements from an I-DOAS to verify the
calibration curve over the spatial extent of the image.
The results show that calibration cells, while working fine in some cases,
can lead to an overestimation of the SO2 CD by up to 60% compared
with CDs from the DOAS measurements. Besides these errors of calibration,
radiative transfer effects (e.g. light dilution, multiple scattering) can
significantly influence the results of both instrument types. The
measurements presented in this work were taken at Popocatépetl, Mexico,
between 1 March 2011 and 4 March 2011. Average SO2 emission rates
between 4.00 and 14.34 kg s−1 were observed. |
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