Abstract
Abstract. The first satellite-based global retrievals of terrestrial sun-induced chlorophyll fluorescence (SIF) were achieved in 2011. Since then, a number of global SIF datasets with different spectral, spatial, and temporal sampling characteristics have become available to the scientific community. These datasets have been useful to monitor the dynamics and productivity of a range of vegetated areas worldwide, but the coarse spatiotemporal sampling and low signal-to-noise ratio of the data hamper their application over small or fragmented ecosystems. The recent advent of the Copernicus Sentinel-5P TROPOMI mission and the high quality of its data products promise to alleviate this situation, as TROPOMI provides daily global measurements at a much denser spatial and temporal sampling than earlier satellite instruments. In this work, we present a global SIF dataset produced from TROPOMI measurements within the TROPOSIF project funded by the European Space Agency. The current version of the TROPOSIF dataset covers the time period between May 2018 and April 2021. Baseline SIF retrievals are derived from the 743–758 nm window. A secondary SIF dataset derived from an extended fitting window (735–758 nm window) is included. This provides an enhanced signal-to-noise ratio at the expense of a higher sensitivity to atmospheric effects. Spectral reflectance spectra at seven 3 nm windows devoid of atmospheric absorption within the 665–785 nm range are also included in the TROPOSIF dataset as an important ancillary variable to be used in combination with SIF. The methodology to derive SIF and ancillary data as well as results from an initial data quality assessment are presented in this work. The TROPOSIF dataset is available through the following digital object identifier (DOI): https://doi.org/10.5270/esa-s5p_innovation-sif-20180501_20210320-v2.1-202104 (Guanter et al., 2021).
Highlights
The sun-induced fluorescence (SIF) signal emitted by the chlorophyll a of terrestrial vegetation has been shown to be a closer indicator of vegetation functioning than other variables traditionally derived from optical remote sensing data (Mohammed et al, 2019)
The different panels show the spatial distribution of TOA radiance at 743 nm, the cloud fraction in TROPOspheric Monitoring Instrument (TROPOMI)’s L2 Cloud product, qa_value, the TOA reflectance at 665 and 781 nm, the normalized difference vegetation index (NDVI) derived from those two channels, and the retrieved SIF, 1σ error, and the χr2 of the fit for both 743–758 and 735–758 nm fitting windows. χr2 values larger than 1 indicate an underestimation of the model–data error variance
This paper has described the first version of a processing chain for the generation of far-red SIF product from the S5PTROPOMI mission which has been developed within the framework of the European Space Agency (ESA) TROPOSIF project
Summary
The sun-induced fluorescence (SIF) signal emitted by the chlorophyll a of terrestrial vegetation has been shown to be a closer indicator of vegetation functioning than other variables traditionally derived from optical remote sensing data (Mohammed et al, 2019). TROPOMI combines a global continuous spatial sampling with a 3.5 × 7.5 km pixel size at nadir in the near infrared (3.5 × 5.5 km since August 2019) with a daily revisit time, which leads to a large increase in the number of clear-sky measurements per day in comparison to earlier missions. It measures with a high signal-to-noise ratio, a spectral resolution of 0.37 nm, and a wide spectral coverage in the near-infrared window. The first publications exploiting TROPOMI SIF data for scientific applications confirm those high expectations on the use of TROPOMI for vegetation monitoring (Turner et al, 2020; Doughty et al, 2019; Zhang et al, 2019; Yin et al, 2020; He et al, 2020)
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