Abstract
The Greenhouse gases Observing SATellite (GOSAT) launched in January 2009 has provided radiance spectra with a Fourier Transform Spectrometer for more than eight years. The Orbiting Carbon Observatory 2 (OCO-2) launched in July 2014, collects radiance spectra using an imaging grating spectrometer. Both sensors observe sunlight reflected from Earth’s surface and retrieve atmospheric carbon dioxide (CO2) concentrations, but use different spectrometer technologies, observing geometries, and ground track repeat cycles. To demonstrate the effectiveness of satellite remote sensing for CO2 monitoring, the GOSAT and OCO-2 teams have worked together pre- and post-launch to cross-calibrate the instruments and cross-validate their retrieval algorithms and products. In this work, we first compare observed radiance spectra within three narrow bands centered at 0.76, 1.60 and 2.06 µm, at temporally coincident and spatially collocated points from September 2014 to March 2017. We reconciled the differences in observation footprints size, viewing geometry and associated differences in surface bidirectional reflectance distribution function (BRDF). We conclude that the spectral radiances measured by the two instruments agree within 5% for all bands. Second, we estimated mean bias and standard deviation of column-averaged CO2 dry air mole fraction (XCO2) retrieved from GOSAT and OCO-2 from September 2014 to May 2016. GOSAT retrievals used Build 7.3 (V7.3) of the Atmospheric CO2 Observations from Space (ACOS) algorithm while OCO-2 retrievals used Version 7 of the OCO-2 retrieval algorithm. The mean biases and standard deviations are −0.57 ± 3.33 ppm over land with high gain, −0.17 ± 1.48 ppm over ocean with high gain and −0.19 ± 2.79 ppm over land with medium gain. Finally, our study is complemented with an analysis of error sources: retrieved surface pressure (Psurf), aerosol optical depth (AOD), BRDF and surface albedo inhomogeneity. We found no change in XCO2 bias or standard deviation with time, demonstrating that both instruments are well calibrated.
Highlights
Spectral radiances are created by taking an inverse Fourier transform of the raw interferograms, using the radiance conversion table prepared from the gases Observing SATellite (GOSAT) pre-launch calibration data shown in Kuze et al [8], and radiance degradation factors (RDF) described in detail in Kuze et al [1,17]
GOSAT and Orbiting Carbon Observatory 2 (OCO-2) are the first two satellite programs dedicated to monitoring the global CO2 distribution
We demonstrated the reliability of CO2 remote sensing from space by comparing radiances and retrieved XCO2 and Psurf values from different types of spectrometers
Summary
The Orbiting Carbon Observatory 2 (OCO-2) was launched on 2 July 2014 and has been returning data since September 2014 Both instruments are designed to measure the global distribution of atmospheric carbon dioxide (CO2) from space. The column-averaged dry air mole fraction of CO2 (XCO2) in the atmosphere can be measured remotely using differential optical absorption spectroscopy. Both GOSAT and OCO-2 measure sunlight reflected by the Earth, including contributions from the surface, clouds and aerosols, at 0.76, 1.60 and 2.06 μm. To investigate the uncertainties and biases between the respective data products, the OCO-2 and GOSAT teams have executed a number of cross-comparison studies both prior to launch and on-orbit. These exercises have facilitated the interpretation of the data from both missions
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