On the zero-level offset in the GOSAT TANSO-FTS O 2 A-band and the quality of solar-induced chlorophyll fluorescence (SIF): Comparison of SIF between GOSAT and OCO-2

Abstract. Satellite remote sensing of solar-induced chlorophyll fluorescence (SIF) has attracted attention as a method for improving the estimation accuracy of the photosynthetic production of terrestrial vegetation in recent years. The Greenhouse gases Observing SATellite (GOSAT) has the ability to observe both SIF and the concentrations of CO2 and CH4 and thus is expected to contribute to understanding of the global carbon budget. Evaluating artifact signals (e.g., zero-level offset caused by non-linearity in the analogue circuit in the case of GOSAT) is effective for inferring the instrument status and important for retrieving SIF from satellite measurements. Here we investigate the criteria for identifying vegetation-free areas to evaluate the zero-level offset and the offset correction method, while comparing the derived SIF with the Orbiting Carbon Observatory-2 (OCO-2) SIF at multiple spatial scales (footprint to global). The criteria were determined as a small variation in the radiance within the GOSAT instantaneous field of view for cloudy ocean scenes and a higher albedo in the 2.0 μm band than in the 1.6 μm band for bare soil scenes, which were slightly different from the previously used criteria. The GOSAT SIF that was most consistent with the OCO-2 SIF was obtained when the zero-level offset was evaluated from bare soil over the globe, with a bias of about 0.1 mW m−2 nm−1 sr−1. Our results agree with the previous comparisons and support the consistency among the present satellite SIF data, which is important for the utilization of those data. An analysis of the temporal variation of the zero-level offset over a period of 9 years suggests that the radiometric sensitivity of the GOSAT spectrometer changed after switching the optics path selector in January 2015.