CO2 Measurements from Space: Lessons Learned from the Collaboration between the ACOS/OCO-2 and GOSAT Teams

The NASA Orbiting Carbon Observatory (OCO) and the Japanese Greenhouse gases Observing SATellite (GOSAT) were the first two missions designed to collect space-based observations of the column-averaged CO2 dry air mole fraction, XCO2, with the sensitivity, coverage, and resolution needed to quantify CO2 fluxes on regional scales over the globe. The OCO and GOSAT teams formed a close collaboration during the development phases of these missions. After the loss of OCO, the GOSAT project team invited the OCO team to contribute to the analysis of measurements collected by the GOSAT Thermal And Near infrared Sensor for carbon Observations-Fourier Transform Spectrometer (TANSO-FTS). NASA responded by reformulating the OCO science team under the Atmospheric CO2 Observations from Space (ACOS) task to exploit this opportunity. This collaboration is providing an independent GOSAT XCO2 product, and valuable insights into the retrieval algorithms, calibration methods, and validation techniques that are being developed to analyze data anticipated the NASA Orbiting Carbon Observatory-2 (OCO-2). The ACOS/OCO-2 and GOSAT teams have conducted four, joint, vicarious calibration campaigns at Railroad Valley, Nevada to track the long-term radiometric performance of the TANSO-FTS instrument. The methods used in these campaigns evolved from those used to characterize the radiometric performance of high spatial resolution, imaging spectroradiometers. For TANSO-FTS, the conventional, surface based radiometric measurements have been augmented with surface and aircraft measurements of atmospheric temperature and trace gas profiles, as well as surface observations from MODIS and ASTER to characterize spatial variations of the surface reflectance within the (relatively large) sounding footprints. Similar methods will be needed for OCO-2. The ACOS/OCO-2 retrieval algorithm and associated data screening methods have been modified to estimate XCO2 from TANSO-FTS observations. Comparisons of TANSO-FTS XCO2 retrievals with surface-based XCO2 estimates from the Total Carbon Column Observing Network (TCCON) have contributed to significant improvements in the accuracy of the OCO-2 retrieval algorithm. This approach has helped to identify and correct subtle biases associated with air mass, surface pressure, optically-thick aerosols, ice-covered surfaces, and other environmental factors. Persistent spectral residuals common to TCCON and TANSO-FTS retrievals have revealed limitations in the spectroscopy of CO2 and O2, which are being addressed with new laboratory measurements. These calibration, retrieval algorithm development, and validation activities are now yielding XCO2 estimates with regional-scale errors < 2 ppm over much of the globe. The XCO2 products from this effort are being archived at the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC). Preliminary efforts to assimilate these XCO2 estimates in CO2 flux inversion models are providing new insights into the strengths and weaknesses of space based measurements for this application. Lessons learned from the ACOS experience are expected to substantially accelerate the delivery of high quality products from the OCO-2 mission, which is currently scheduled for launch in July 2014.