Retrieval of Solar-Induced Chlorophyll Fluorescence at Red Spectral Peak with Tropomi on Sentinel-5 Precursor

Abstract

Solar-Induced chlorophyll Fluorescence (SIF) is a signal emitted by chlorophyll molecules after the absorption of solar radiation. SIF is closely related with the photosynthetic activity of vegetation, and it could be used as an indicator of photosynthetic status. SIF retrieved from spaceborne measurements can provide a synoptic perspective of the surface targets. The recently launched TROPOspheric Monitoring Instrument (TROPOMI) on board of Sentinel-5 precursor provides dataset for the retrieval of SIF with an unprecedented resolution in spatial and temporal dimensions. In this study, we implemented a data-driven approach to retrieval of SIF at red spectral peak measured from TROPOMI. We chose a retrieval window ranging from 669 to 686 nm, a subset of TROPOMI's band 5 because this wavelength range is free from atmospheric absorption and covers several solar Fraunhofer lines. Preliminary retrieval results indicated that the modeled spectra through the forward model accurately fit the measured spectra within the fitting window of TROPOMI. Global composites of the red SIF at 685 nm extracted from TROPOMI measurement on 3–13 June, 2019, show reasonably high and low values as expected in different parts of the world. Comparisons with independent far-red TROPOMI SIF product, Normalized difference vegetation index calculated from TROPOMI's apparent reflectance factors, and Moderate Resolution Imaging Spectroradiometer gross primary productivity product show similar spatial patterns, which confirms that the red TROPOMI SIF is able to provide a reasonable tool to map plant productivity across different regions of the globe.