A new SIF (solar‐induced chlorophyll fluorescence) product derived from TROPOMI onboard Sentinel-5 Precursor

The total amount of CO₂ absorbed and turned into organic matter through photosynthesis can be quantified as Gross Primary Productivity (GPP). Terrestrial vegetation GPP is responsible for offsetting about 30% of carbon dioxide emissions released by anthropogenic activities into the atmosphere. This corresponds to approximately 120 to 170 PgC/year, making terrestrial vegetation both the largest and most uncertain component of the global carbon cycle. Over the last few years, solar- induced chlorophyll fluorescence (SIF) observations from space have emerged as a promising resource for evaluating the spatio-temporal distribution of GPP by terrestrial ecosystems. SIF is an electromagnetic signal emitted by the chlorophyll a of green plants: part of the absorbed energy not used for photosynthesis is emitted at longer wavelengths as a two-peak spectrum roughly covering the 650–850 nm spectral range. The SIF signal responds instantaneously to perturbations in  environmental conditions such as light and water stress, which makes it a direct proxy for photosynthetic activity.

SIF has been estimated at the global scale from several space-borne spectrometers originally intended for atmospheric research because they provide the necessary spectral  resolution and radiometric sensitivity. However, the exploitation of SIF estimates has remained limited by their coarse spatial resolution (GOSAT, GOME-2) or low number of observations (OCO-2). This has been strongly alleviated by the advent of the TROPOspheric Monitoring Instrument (TROPOMI) onboard Sentinel-5 Precursor which combines a global continuous spatial sampling with a 5.5 km x 3.5 km pixel size at nadir, a daily  revisit time, a wide spectral coverage and an enormous increase in the number of clear-sky measurements per day in comparison to previous missions, as demonstrated by the first SIF retrievals from TROPOMI derived by Caltech (Köhler et al. 2018).

In this study, we present a new independent SIF product derived from TROPOMI observations using two fitting windows at 743-758 nm (all sky SIF) and 735-758 nm (clear sky SIF). The first one is very robust against atmospheric effects (especially cloud contamination) whereas the second one is less sensitive to estimation errors (due to the greater number of spectral points in the fitting window). The retrieval scheme is not much different from the one from Caltech (in 743-758 nm) but relies on different cloud fraction inputs. We will present the principles of the retrieval scheme, the format of the output TROPOSIF product, and the results of the comparison of our SIF estimates against the Caltech ones as well as SIF data derived from OCO-2 which show a very good consistency between all SIF products.