XCO2 retrieval for GOSAT and GOSAT-2 based on the FOCAL algorithm
Authors: Noël Stefan , Reuter Maximilian , Buchwitz Michael , Borchardt Jakob , Hilker Michael , Bovensmann Heinrich , Burrows John P. , Di Noia Antonio , Suto Hiroshi , Yoshida Yukio , Buschmann Matthias , Deutscher Nicholas M. , Feist Dietrich G. , Griffith David W. T. , Hase Frank , Kivi Rigel , Morino Isamu , Notholt Justus , Ohyama Hirofumi , Petri Christof , Podolske James R. , Pollard David F. , Sha Mahesh Kumar , Shiomi Kei , Sussmann Ralf , Té Yao , Velazco Voltaire A. , Warneke Thorsten 
Published: May 2021

Abstract. Since 2009, the Greenhouse gases Observing SATellite (GOSAT) has performed
radiance measurements in the near-infrared (NIR) and
shortwave infrared (SWIR) spectral region.
From February 2019 onward, data from GOSAT-2 have also been available. We present the first results from the application of the Fast atmOspheric
traCe gAs retrievaL (FOCAL) algorithm to derive
column-averaged dry-air mole fractions of carbon dioxide (XCO2) from GOSAT and
GOSAT-2 radiances and their validation.
FOCAL was initially developed for OCO-2 XCO2 retrievals and
allows simultaneous retrievals of several gases over both land and
ocean.
Because FOCAL is accurate and numerically very fast, it is currently
being considered as a candidate algorithm for the forthcoming European
anthropogenic CO2 Monitoring (CO2M) mission to be launched
in 2025. We present the adaptation of FOCAL to GOSAT and discuss the changes
made and GOSAT specific additions.
This particularly includes modifications in pre-processing (e.g. cloud
detection) and post-processing (bias correction and filtering). A feature of the new application of FOCAL to GOSAT and GOSAT-2 is the
independent use of both S- and P-polarisation spectra in the retrieval.
This is not possible for OCO-2, which measures only one polarisation
direction.
Additionally, we make use of GOSAT's wider spectral coverage compared
to OCO-2 and derive not only XCO2, water vapour
(H2O), and solar-induced fluorescence (SIF) but also methane
(XCH4), with the potential for further atmospheric
constituents and parameters like semi-heavy water vapour (HDO). In the case of GOSAT-2, the retrieval of nitrous oxide (XN2O) and carbon monoxide (CO) may also be possible. Here, we concentrate on the new FOCAL XCO2 data products.
We describe the generation of the products as well as applied
filtering and bias correction procedures.
GOSAT-FOCAL XCO2 data have been produced for the time interval 2009 to 2019. Comparisons with other independent GOSAT data sets reveal
agreement of long-term temporal variations within about
1 ppm over 1 decade; differences in seasonal variations of
about 0.5 ppm are observed.
Furthermore, we obtain a station-to-station bias of the new GOSAT-FOCAL
product to the ground-based Total Carbon Column Observing Network
(TCCON) of 0.56 ppm with a mean scatter of 1.89 ppm. The GOSAT-2-FOCAL XCO2 product is generated in a similar way as the
GOSAT-FOCAL product, but with adapted settings. All GOSAT-2 data until the end of 2019 have been processed. Because of this limited time interval, the GOSAT-2 results are considered to be preliminary only, but first comparisons show that these data compare well with the GOSAT-FOCAL results and also TCCON.