Global Retrievals of Solar-Induced Chlorophyll Fluorescence With TROPOMI

Abstract:

Spaceborne observations of solar-induced chlorophyll a fluorescence (SIF) hold promise to advance our understanding of terrestrial and aquatic carbon cycles. The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite was launched in October 2017 and enabled the development of a truly unique SIF data set with respect to current and previous satellite instruments capable of retrieving SIF. TROPOMI’s large swath of 2600 km permits global mapping at both fine temporal and spatial scales (up to 5 km x 3.5 km pixels with daily revisit). The high data rate and the quality of TROPOMI’s spectra paved the way for studies covering a variety of topics, including tropical dynamics, agricultural productivity, studies of directional effects, and the seasonality of photosynthesis in diverse ecosystems. Most recently, estimates of SIF at red wavelength have become available in addition to the widely used far-red SIF retrievals, extending the coverage of TROPOMI SIF observations to the oceans and other water bodies. We will show that the new red SIF estimates are highly consistent with MODIS normalized chlorophyll fluorescence line height (nFLH) retrievals and illustrate major advantages of our Fraunhofer line based SIF retrievals, which include the capability to sense SIF through optically thin cloud/aerosol layers, and an insensitivity to ocean color, opening up new avenues for studying ocean biogeochemistry from space. We will further clarify common misconceptions about the measurement properties, discuss the need to consider directional effects, and provide guidance on how SIF data may be analyzed on regional scales.