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| Titel |
Global remote sensing of chlorophyll fluorescence using high-resolution O2 A-band spectra recorded by the GOSAT satellite |
| VerfasserIn |
Christian Frankenberg, André Butz, Joshua B. Fisher, Geoffrey C. Toon, Akihiko Kuze, Tatsuya Yokota, Grayson Badgley, John Worden |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056575
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| Zusammenfassung |
| Solar-induced chlorophyll fluorescence is a powerful proxy for assessing biomass
photosynthetic activity since part of the energy absorbed by Chlorophyll is dissipated by
fluorescence. This gives rise to re-emission of light between approximately 670 and 780 nm.
Passive methods to quantify the fluorescence signal are mainly based on the filling-in of
highly saturated O2 absorption structures. This method, however, was mostly applied in
above-canopy measurements and is not directly applicable to space-borne retrievals.
We show that variability of aerosols in the atmosphere load and surface pressure
cannot be unequivocally be disentangled from fluorescence since all these factor
impact the absorption depths of O2 lines. This gives rise to biases in the retrieved
scattering properties in typical multi-spectral XCO2 retrievals when using the O2 A
band.
In the vicinity of the O2 A-band, however, several narrow and strong absorption features
in the solar spectrum (so called Fraunhofer lines) exist whose absorption depth is not
affected by atmospheric scattering but only by fluorescence. The Japanese GOSAT
satellite is the first to provide high spectral resolution in the O2 A-band and thereby
allows for quantification of fluorescence using Fraunhofer lines in the 755-775 nm
range. We will a) present our retrieval method based on an iterative, non-linear
least-squares fitting of Fraunhofer lines and b) discuss the potential impact on XCO2
retrievals.
The retrieval method has been applied to a full year of GOSAT data, providing global
maps of chlorophyll fluorescence, clearly correlating with photosynthetic activity.
On the global scale, tropical regions are most pronounced but show little seasonal
variability. The seasonal cycle of biomass activity in the northern hemisphere can be
clearly detected. We will discuss the annual average and seasonal variability of
retrieved fluorescence and compare with other methods to derive photosynthetic
activity (such as NDVI or model GPP). While the prime objective of dedicated
greenhouse-gas satellites such as GOSAT (and OCO-2) is to deliver top-down constraints
on carbon-fluxes, quantification of chlorophyll fluorescence now even provides a
unique and unexpected bottom-up constraint as a proxy for photosynthetic activity. |
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