From Canopy‐Leaving to Total Canopy Far‐Red Fluorescence Emission for Remote Sensing of Photosynthesis: First Results From TROPOMI

Abstract Solar‐induced chlorophyll fluorescence (SIF) from the TROPOspheric Monitoring Instrument (TROPOMI), which has substantially improved spatial and temporal resolutions, will improve the global estimations of gross primary production (GPP) than previous satellite SIF data. However, the canopy‐leaving SIF observed by sensors (SIF obs ) represents only a portion of the total canopy SIF emission (SIF total ). This portion is sensitive to the canopy structure and observation direction, resulting in uncertainties in GPP estimations using SIF obs . Here we used the spectral invariant theory to derive global soil‐resistant SIF total (SIF total‐SR ) from TROPOMI SIF obs and evaluated the SIF total‐SR performance in estimating GPP. We found that the clear differences in SIF obs between needleleaf forest and crops diminished for SIF total‐SR . SIF total‐SR increased the coefficient of determination ( R 2 ) by 0.09 and 0.11 against the flux tower instantaneous and daily GPP, respectively. This derived SIF total‐SR can be used to develop more robust GPP models and better constrain carbon cycle models. Plain Language Summary Photosynthesis is an essential process by which plants convert carbon dioxide (CO 2 ) and water into sugars and provides the primary source of energy for life on Earth. Variations in photosynthesis also affect the CO 2 concentration in the atmosphere, the water‐heat balance, and the climate. It is therefore important to estimate the photosynthesis rate, which is generally quantified as the gross primary production (GPP) at the large spatial and temporal scales. Solar‐induced chlorophyll fluorescence (SIF) retrieved from the recently launched TROPOspheric Monitoring Instrument, which has substantially improved spatial and temporal resolutions, has greater potential than previous satellite SIF data to be used as a proxy of photosynthesis. However, SIF observed by sensors (SIF obs ) represents only a portion of the total canopy SIF emission from all leaves within a canopy (SIF total ). This portion depends on the canopy structure and observation direction, hindering the application of SIF obs for GPP estimation. In this study, a correction method is proposed to convert directional SIF obs retrievals into SIF total by considering the soil effect in the conversion. The derived soil‐resistant SIF total could help to better estimate the terrestrial GPP compared with using SIF obs . Key Points Total canopy solar‐induced chlorophyll fluorescence (SIF) emission (SIF total ) is derived from TROPOMI SIF with reflectance data SIF total better correlates to gross primary production (GPP) than observed SIF (SIF obs ) The soil background effect on SIF total is minimized by estimating the probability of observing a sunlit background under vegetation