New methods for the retrieval of chlorophyll red fluorescence from hyperspectral satellite instruments: simulations andapplication to GOME-2 and SCIAMACHY

摘要

Global satellite measurements of solar-induced fluorescence (SIF) fromchlorophyll over land and ocean have proven useful for a number of differentapplications related to physiology, phenology, and productivity of plants andphytoplankton. Terrestrial chlorophyll fluorescence is emitted throughout thered and far-red spectrum, producing two broad peaks near 683 and 736 nm.From ocean surfaces, phytoplankton fluorescence emissions are entirely fromthe red region (683 nm peak). Studies using satellite-derived SIF over landhave focused almost exclusively on measurements in the far red(wavelengths    712 nm), since those are the most easily obtained withexisting instrumentation. Here, we examine new ways to use existinghyperspectral satellite data sets to retrieve red SIF(wavelengths    712 nm) over both land and ocean. Red SIF is thought toprovide complementary information to that from the far red for terrestrialvegetation. The satellite instruments that we use were designed to makeatmospheric trace-gas measurements and are therefore not optimal forobserving SIF; they have coarse spatial resolution and only moderate spectralresolution (0.5 nm). Nevertheless, these instruments, the Global OzoneMonitoring Instrument 2 (GOME-2) and the SCanning Imaging AbsorptionspectroMeter for Atmospheric CHartographY (SCIAMACHY), offer a uniqueopportunity to compare red and far-red terrestrial SIF at regional spatialscales. Terrestrial SIF has been estimated with ground-, aircraft-, orsatellite-based instruments by measuring the filling-in of atmospheric and/orsolar absorption spectral features by SIF. Our approach makes use of theoxygen (O) band that is not affected by SIF. The SIF-freeO band helps to estimate absorption within the spectrallyvariable O B band, which is filled in by red SIF. SIF also fills inthe spectrally stable solar Fraunhofer lines (SFLs) at wavelengths bothinside and just outside the O B band, which further helps to estimate redSIF emission. Our approach is then an extension of previous approachesapplied to satellite data that utilized only the filling-in of SFLs by redSIF. We conducted retrievals of red SIF using an extensive database ofsimulated radiances covering a wide range of conditions. Our new algorithmproduces good agreement between the simulated truth and retrievals and showsthe potential of the O bands for noise reduction in red SIF retrievals ascompared with approaches that rely solely on SFL filling. Biases seen withexisting satellite data, most likely due to instrumental artifacts that varyin time, space, and with instrument, must be addressed in order to obtainreasonable results. Our 8-year record of red SIF observations over land withthe GOME-2 allows for the first time reliable global mapping of monthlyanomalies. These anomalies are shown to have similar spatiotemporal structureas those in the far red, particularly for drought-prone regions. There is asomewhat larger percentage response in the red as compared with the far redfor these areas that are drought sensitive. We also demonstrate thatgood-quality ocean fluorescence line height retrievals can be achieved withGOME-2, SCIAMACHY, and similar instruments by utilizing the full complementof radiance measurements that span the red SIF emission feature.