Kohler, P; Behrenfeld, MJ; Landgraf, J; Joiner, J; Magney, TS; Frankenberg, C (2020). Global Retrievals of Solar-Induced Chlorophyll Fluorescence at Red Wavelengths With TROPOMI. GEOPHYSICAL RESEARCH LETTERS, 47(15), e2020GL087541.

Observations of solar-induced chlorophyll a fluorescence (SIF) from spaceborne spectrometers can advance our understanding of terrestrial and aquatic carbon cycles. Here we present the first global retrievals of SIF at red wavelengths from the TROPOspheric Monitoring Instrument (TROPOMI). Despite the weak signal level, considerable uncertainties, and subtle measurement artifacts, spatial patterns and magnitudes agree with independent data sets. Over land, spatial patterns of our red SIF estimates covary with the far-red SIF data. Red SIF over the ocean is highly consistent with the normalized fluorescence line height (nFLH) inferred from measurements of the MODerate resolution Imaging Spectroradiometer (MODIS), even when comparing single days and fine spatial scales. Major advantages of our Fraunhofer line-based SIF retrievals include the capability to sense SIF through optically thin cloud/aerosol layers and an insensitivity to ocean color. This opens up new avenues for studying ocean biogeochemistry from space. Plain Language Summary Plants absorb sunlight to power photosynthesis, but a small fraction of the energy is always re-emitted as a faint glow termed solar-induced chlorophyll a fluorescence (SIF). This weak electromagnetic signal can be measured with spectrometers that are sensitive enough and cover the wavelengths where SIF occurs (red to far red). Even though SIF is not a direct measure of the carbon uptake by photosynthesis, strong linear relationships between far-red SIF and independent estimates of photosynthesis have been observed over land. ESA's Sentinel 5 Precursor satellite hosts the TROPOspheric Monitoring Instrument (TROPOMI), a highly sensitive sensor designed to monitor atmospheric trace gases and air pollutants from space. The band setting of TROPOMI permits us also to infer SIF in both the red and far-red wavelength domain. Here, we present global estimates of red SIF over both marine and terrestrial surfaces as seen from TROPOMI.