A precise and atmospherically-robust method for ground-based retrieval of red and far-red sun-induced chlorophyll fluorescence

Authors: NAETHE, PAUL - Jb Hyperspectral Devices Gmbh; JULITTA, TOMMASO - Jb Hyperspectral Devices Gmbh; Chang, Christine; BURKART, ANDREAS - Jb Hyperspectral Devices Gmbh; MIGLIAVACCA, MIRCO - Max Planck Institute For Biogeochemistry; GUANTER, LUIS - Universitat Politècnica De Catalunya (UPC); RASCHER, UWE - Forschungszentrum Juelich Gmbh

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2022
Publication Date: 9/8/2022
Citation: Naethe, P., Tommaso, J., Chang, C.Y., Burkart, A., Migliavacca, M., Guanter, L., Rascher, U. 2022. A precise and atmospherically-robust method for ground-based retrieval of red and far-red sun-induced chlorophyll fluorescence. Agricultural and Forest Meteorology. 325:109152. https://doi.org/10.1016/j.agrformet.2022.109152.
DOI: https://doi.org/10.1016/j.agrformet.2022.109152

Interpretive Summary: Solar induced chlorophyll fluorescence (SIF) is a novel remote sensing tool which may enable tracking of photosynthesis from field to airborne and satellite scales. However, the actual retrieval of the SIF signal is still challenging. This study demonstrates the performance of a new SIF retrieval method based on partial-least-squares which performs as well or better than existing retrieval methods based on precision and robustness against atmospheric distortion, and furthermore requires less computation time. This finding offers an improved analytical method to retrieve SIF from up- and down-welling solar radiance.

Technical Abstract: In remote sensing, solar induced chlorophyll fluorescence (SIF) is employed as a proxy for photosynthesis from field to airborne and satellite scales. The actual retrieval of the SIF signal is still challenging. However, the investigation of SIF offers a unique way for investigating vegetation functioning from the local to the global scale. Common retrieval approaches extract the SIF infilling directly from atmospheric oxygen bands in down-welling and up-welling radiance. They require a complex signal correction to compensate for atmospheric reabsorption and, thus, often require long computing time. In contrast, the exploitation of solar Fraunhofer lines is devoid of atmospheric disturbances. We propose a new retrieval method for red and far-red SIF directly from up-welling radiance spectra, using the entire range between 650 nm and 810 nm by applying Partial Least Squares (PLS) regression models. Solar Fraunhofer lines are exploited for SIF retrieval with the PLS approach excluding the telluric absorption features. The PLS models are trained and tested on synthetic reflectance data modelled with SCOPE for which the SIF signal is simulated. We identify a logarithmic relationship of the performance with respect to signal-to-noise ratio of the instrument. The approach has been transferred to real-world measured data from Fluorescence Box (FloX) monitoring field spectrometers and evaluated against two well-established retrieval methods: SFM and SVD. PLS regression models exploiting solar Fraunhofer lines were able to retrieve meaningful SIF values with high precision and robustness against atmospheric distortion, including from a 100m tall tower, with a much shorter running time (37 times faster than SFM). Hence, PLS retrieval allows the exploitation of SIF from solar Fraunhofer lines under conditions in which other retrieval approaches require complex correction of atmospheric disturbances with high precision compared with SVD methods.