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| Titel |
Remote sensing of near-infrared chlorophyll fluorescence from space in scattering atmospheres: implications for its retrieval and interferences with atmospheric CO2 retrievals |
| VerfasserIn |
C. Frankenberg, C. O'Dell, L. Guanter, J. McDuffie |
| 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 ; 5, no. 8 ; Nr. 5, no. 8 (2012-08-28), S.2081-2094 |
| Datensatznummer |
250003058
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| Publikation (Nr.) |
 copernicus.org/amt-5-2081-2012.pdf |
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| Zusammenfassung |
With the advent of dedicated greenhouse gas space-borne spectrometers sporting high resolution
spectra in the O2 A-band spectral region (755–774 nm), the retrieval of
chlorophyll fluorescence has become feasible on a global scale. If unaccounted for, however,
fluorescence can indirectly perturb the greenhouse gas retrievals as it perturbs the oxygen
absorption features. As atmospheric CO2 measurements are used to invert net fluxes at the
land–atmosphere interface, a bias caused by fluorescence can be crucial as it will spatially
correlate with the fluxes to be inverted. Avoiding a bias and retrieving fluorescence accurately
will provide additional constraints on both the net and gross fluxes in the global carbon
cycle. We show that chlorophyll fluorescence, if neglected, systematically interferes with
full-physics multi-band XCO2 retrievals using the O2 A-band. Systematic
biases in XCO2 can amount to +1 ppm if fluorescence constitutes 1% to
the continuum level radiance. We show that this bias can be largely eliminated by simultaneously
fitting fluorescence in a full-physics based retrieval.
If fluorescence is the primary target, a dedicated but very simple retrieval based purely on
Fraunhofer lines is shown to be more accurate and very robust even in the presence of large
scattering optical depths. We find that about 80% of the surface fluorescence is retained at
the top-of-atmosphere, even for cloud optical thicknesses around 2–5. We further show that small
instrument modifications to future O2 A-band spectrometer spectral ranges can result in
largely reduced random errors in chlorophyll fluorescence, paving the way towards a more dedicated
instrument exploiting solar absorption features only. |
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