Remote sensing of near-infrared chlorophyll fluorescence from space in scattering atmospheres: implications for its retrieval and interferences with atmospheric CO2 retrievals

[摘要] With the advent of dedicated greenhouse gas space-borne spectrometers sporting high resolutionspectra in the O₂ A-band spectral region (755–774 nm), the retrieval ofchlorophyll fluorescence has become feasible on a global scale.If unaccounted for, however,fluorescence can indirectly perturb the greenhouse gas retrievals as it perturbs the oxygenabsorption features.As atmospheric CO₂ measurements are used to invert net fluxes at theland–atmosphere interface, a bias caused by fluorescence can be crucial as it will spatiallycorrelate with the fluxes to be inverted. Avoiding a bias and retrieving fluorescence accuratelywill provide additional constraints on both the net and gross fluxes in the global carboncycle. We show that chlorophyll fluorescence, if neglected, systematically interferes withfull-physics multi-band X_CO2 retrievals using the O₂ A-band. Systematicbiases in X_CO2 can amount to +1 ppm if fluorescence constitutes 1% tothe continuum level radiance. We show that this bias can be largely eliminated by simultaneouslyfitting fluorescence in a full-physics based retrieval.

If fluorescence is the primary target, a dedicated but very simple retrieval based purely onFraunhofer lines is shown to be more accurate and very robust even in the presence of largescattering optical depths. We find that about 80% of the surface fluorescence is retained atthe top-of-atmosphere, even for cloud optical thicknesses around 2–5. We further show that smallinstrument modifications to future O₂ A-band spectrometer spectral ranges can result inlargely reduced random errors in chlorophyll fluorescence, paving the way towards a more dedicatedinstrument exploiting solar absorption features only.

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