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

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 (SIFobs) represents only a portion of the total canopy SIF emission (SIFtotal). This portion is sensitive to the canopy structure and observation direction, resulting in uncertainties in GPP estimations using SIFobs. Here we used the spectral invariant theory to derive global soil-resistant SIFtotal (SIFtotal-SR) from TROPOMI SIFobs and evaluated the SIFtotal-SR performance in estimating GPP. We found that the clear differences in SIFobs between needleleaf forest and crops diminished for SIFtotal-SR. SIFtotal-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 SIFtotal-SR can be used to develop more robust GPP models and better constrain carbon cycle models.