Filling-in of near-infrared solar lines by terrestrial fluorescence and other geophysical effects: simulations and space-based observations from SCIAMACHY and GOSAT

[摘要] Global mapping of terrestrial vegetation fluorescence from space has recentlybeen accomplished with high spectral resolution(ν/Δν > 35 000) measurements from the Japanese Greenhousegases Observing SATellite (GOSAT). These data are of interest because theycan potentially provide global information on the functional status ofvegetation including light-use efficiency and global primary productivitythat can be used for global carbon cycle modeling. Quantifying the impact offluorescence on the O₂-A band is important as this band is used forphoton pathlengthcharacterization in cloud- and aerosol-contaminated pixels for trace-gas retrievals includingCO₂. Here, we examine whether fluorescence information can be derivedfrom space using potentially lower-cost hyperspectral instrumentation, i.e.,more than an order of magnitude less spectral resolution(ν/Δν ~ 1600) than GOSAT, with a relatively simplealgorithm. We discuss laboratory measurements of fluorescence near one of thefew wide and deep solar Fraunhofer lines in the long-wave tail of thefluorescence emission region, the calcium (Ca) II line at 866 nm that isobservable with a spectral resolution of ~0.5 nm. The filling-in of theCa II line due to additive signals from various atmospheric and terrestrialeffects, including fluorescence, is simulated. We then examine filling-in ofthis line using the SCanning Imaging Absorption spectroMeter for AtmosphericCHartographY (SCIAMACHY) satellite instrument. In order to interpret thesatellite measurements, we developed a general approach to correct forvarious instrumental artifacts that produce false filling-in of solar linesin satellite measurements. The approach is applied to SCIAMACHY at the866 nm Ca II line and to GOSAT at 758 and 770 nm on the shoulders of theO₂-A feature where there are several strong solar Fraunhofer lines thatare filled in primarily by vegetation fluorescence. Finally, we comparetemporal and spatial variations of SCIAMACHY additive signals with those ofGOSAT and the Enhanced Vegetation Index (EVI) from the MODerate-resolutionImaging Spectroradiometer (MODIS). Although the derived additive signals fromSCIAMACHY are extremely weak at 866 nm, their spatial and temporalvariations are consistent with chlorophyll a fluorescence or anothervegetation-related source. We also show that filling-in occurs at 866 nmover some barren areas, possibly originating from luminescent minerals inrock and soil.

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