Retrieval of greenhouse gases from GOSAT and GOSAT-2 using the FOCAL algorithm

Stefan Noël , Maximilian Reuter , Michael Buchwitz , Jakob Borchardt , Michael Hilker , Oliver Schneising , Heinrich Bovensmann , John P. Burrows , Antonio Di Noia , Robert J. Parker , Hiroshi Suto , Yukio Yoshida , Matthias Buschmann , Nicholas M. Deutscher , Dietrich G. Feist , David W. T. Griffith , Frank Hase , Rigel Kivi , Cheng Liu , Isamu Morino , Justus Notholt , Young-Suk Oh , Hirofumi Ohyama , Christof Petri , David F. Pollard , Markus Rettinger , Coleen Roehl , Constantina Rousogenous , Mahesh Kumar Sha , Kei Shiomi , Kimberly Strong , Ralf Sussmann , Yao Té (1) , Voltaire A. Velazco , Mihalis Vrekoussis , Thorsten Warneke

1 LERMA - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique

We show new results from an updated version of the Fast atmOspheric traCe gAs retrievaL (FOCAL) retrieval method applied to measurements of the Greenhouse gases Observing SATellite (GOSAT) and its successor GOSAT-2. FOCAL was originally developed for estimating the total column carbon dioxide mixing ratio (XCO₂) from spectral measurements made by the Orbiting Carbon Observatory-2 (OCO-2). However, depending on the available spectral windows, FOCAL also successfully retrieves total column amounts for other atmospheric species and their uncertainties within one single retrieval. The main focus of the current paper is on methane (XCH₄; full-physics and proxy product), water vapour (XH₂O) and the relative ratio of semi-heavy water (HDO) to water vapour (δD). Due to the extended spectral range of GOSAT-2, it is also possible to derive information on carbon monoxide (XCO) and nitrous oxide (XN₂O) for which we also show first results. We also present an update on XCO₂ from both instruments.

For XCO₂, the new FOCAL retrieval (v3.0) significantly increases the number of valid data compared with the previous FOCAL retrieval version (v1) by 50 % for GOSAT and about a factor of 2 for GOSAT-2 due to relaxed pre-screening and improved post-processing. All v3.0 FOCAL data products show reasonable spatial distribution and temporal variations. Comparisons with the Total Carbon Column Observing Network (TCCON) result in station-to-station biases which are generally in line with the reported TCCON uncertainties.

With this updated version of the GOSAT-2 FOCAL data, we provide a first total column average XN₂O product. Global XN₂O maps show a gradient from the tropics to higher latitudes on the order of 15 ppb, which can be explained by variations in tropopause height. The new GOSAT-2 XN₂O product compares well with TCCON. Its station-to-station variability is lower than 2 ppb, which is about the magnitude of the typical N₂O variations close to the surface. However, both GOSAT-2 and TCCON measurements show that the seasonal variations in the total column average XN₂O are on the order of 8 ppb peak-to-peak, which can be easily resolved by the GOSAT-2 FOCAL data. Noting that only few XN₂O measurements from satellites exist so far, the GOSAT-2 FOCAL product will be a valuable contribution in this context.

Domains: Sciences of the Universe [physics]

Stefan Noël, Maximilian Reuter, Michael Buchwitz, Jakob Borchardt, Michael Hilker, et al.. Retrieval of greenhouse gases from GOSAT and GOSAT-2 using the FOCAL algorithm. Atmospheric Measurement Techniques, 2022, 15, pp.3401-3437. ⟨10.5194/amt-15-3401-2022⟩. ⟨insu-03717107⟩