Retrievals of plant fluorescence from spectrally resolved dark lines in the solar backscatter spectra near 690nm

Solar-induced plant fluorescence (ChlF) provides a means to study the vegetation and phytoplankton physiological status and their links to global land/ocean biology and climate-driven fluctuation in the Earth's ecosystem. These emissions exhibit two broad peaks in the red and the far-red at ~690 nm and ~740 nm. The ChlF signal is very weak compared to the bright solar radiation reflected by plants in the same spectral region making the fluorescence intensity measurement a challenging problem. One technique, which has been employed extensively over last 2-3 decades is to extract the ChlF signal from the filling-in of the narrow dark lines of telluric and solar origins present in the solar backscatter signals. Deducing ChlF from the fill-in of Fraunhofer lines has long been considered the most reliable method to quantify ChlF. In this paper, we will present the retrieval technique used to extract ChlF from the spectrally-resolved solar backscatter signals. We will focus on solar Fraunhofer line features near the ChlF red peak (~ 690 nm), a spectral region that is dominated by O2 and H2O absorption lines and has large reflectance spectral variability. A detailed analysis of the retrieval uncertainties (statistical and systematic) associated with the instrument and the radiative transfer properties will be presented and discussed. These uncertainties will also be examined under various solar illumination and ChlF radiance conditions.