Retrieval of global terrestrial solar-induced chlorophyll fluorescence from TanSat satellite

Shanshan Du,Liangyun Liu,Xinjie Liu,Xiao Zhang,Xingying Zhang,Yanmeng Bi,Lianchong Zhang

doi:10.1016/j.scib.2018.10.003

Abstract

The first Chinese Carbon Dioxide Observation Satellite Mission (TanSat), which was launched on December 21, 2016, is intended to measure atmospheric CO2 concentration. The high spectral resolution (0.044 nm) and high SNR (360 at 15.2 mW m−1 sr−1 nm−1) measurements in the region of the O2-A band of the Atmospheric Carbon dioxide Grating Spectroradiometer (AGCS) module onboard TanSat make it possible to retrieve solar-induced chlorophyll fluorescence (SIF) from TanSat observations at the global scale. This paper aims to explore the potential of the TanSat data for global SIF retrieval. A singular vector decomposition (SVD) statistical method was employed to retrieve SIF using radiance over a micro spectral window (∼2 nm) around the Fe Fraunhofer lines (centered at 758.8 nm). The global SIF at 758.8 nm was successfully retrieved with a low residual error of 0.03 mW m−1 sr−1 nm−1. The results show that the spatial and temporal patterns of the retrieved SIF agree well with the global terrestrial vegetation pattern. The monthly SIF products retrieved from the TanSat data were compared with other remote sensing datasets, including OCO-2 SIF, MODIS NDVI, EVI and GPP. The overall consistency between TanSat and OCO-2 SIF products (R2 = 0.86) and the consistency of the spatial patterns and temporal variations between the TanSat SIF and MODIS vegetation indices and GPP enhance our confidence in the potential and feasibility of TanSat data for SIF retrieval. TanSat, therefore, provides a new opportunity for global sampling of SIF at fine spatial resolution (2 km × 2 km), thus improving photosynthesis observations from space.

Highlights

Photosynthesis supplies all of the organic compounds and most of the energy necessary for life on Earth, and is an essential part of the global carbon cycle [1]
The second and third vectors mainly contain information about the Doppler shifts and spectral slope. These two vectors show an opposite trend to the spectral variance, whereas the spectral symmetry of the TanSat-Atmospheric Carbon dioxide Grating Spectroradiometer (ACGS) measurements is clear from the fourth vector
We have demonstrated that the Solar-induced chlorophyll fluorescence (SIF) can be reliably retrieved from TanSat observations

Summary

Introduction

Photosynthesis supplies all of the organic compounds and most of the energy necessary for life on Earth, and is an essential part of the global carbon cycle [1]. After the Japanese Greenhouse gases Observing SATellite (GOSAT) was successfully launched, the first global SIF map was produced using high resolution spectra from the Thermal And Near-infrared Sensor for CARBON Observation-Fourier Transform Spectrometer (TANSOFTS) onboard GOSAT [12]; this was followed by many studies focused on GOSAT fluorescence retrieval [13,14,15,16]. The Orbiting Carbon Observatory-2 (OCO-2) can provide similar SIF retrievals but with a 100-fold increase in the amount of sounding compared to GOSAT [17,18]. It still cannot generate continuous global coverage which is same with GOSAT due to the sparse spatial sampling mode.

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Conclusion