Solar-induced chlorophyll fluorescence from the Orbiting Carbon Observatory-2: Overview of the retrieval and biophysical performance

The Orbiting Carbon Observatory-2 (OCO-2) satellite was successfully launched in July 2014, with a primary objective to measure the atmospheric column CO2. Fortuitously, the high resolution spectra provided by the OCO-2 spectrometer also allow for retrieval of the solar-induced chlorophyll fluorescence (SIF), a direct proxy for the terrestrial gross primary productivity (GPP). Compared to existing spaceborne SIF-related instruments, the data density of OCO-2 is substantially higher due to a combination of higher temporal sampling frequency and finer spatial resolution of footprint. This increased data density represents a large improvement for the SIF retrieval from space. We evaluate the initial two years of SIF products retrieved from OCO-2 (September 2014-present). This retrieval is achieved by solely in-filling the solar Fraunhofer lines at 757nm and 771nm in the vicinity of the O2 A band. Comparison with the Global Ozone Monitoring Experiment-2 (GOME-2) shows that the OCO-2 SIF retrieval has much better precision because of its higher data acquisition density. The OCO-2 SIF closely resembles the average spatial pattern of Max Planck Institute (MPI) GPP globally, and depicts a more realistic spatial contrast between tropics and extratropics than GOME-2 does. The relationship between OCO-2 SIF and GPP diverges across biomes and also exhibits considerable seasonal variation. Based on our analyses, we make recommendations on potential applications of the OCO-2 SIF data products and offer our insights for future satellite missions optimized for SIF retrievals. This study demonstrates that the OCO-2 SIF products are robust and valuable for monitoring global terrestrial carbon cycle and for constraining the carbon source/sink of the Earth system.