XCO₂ retrieval for GOSAT and GOSAT-2 based on the FOCAL algorithm

<p>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.</p> <p>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 (<span class="inline-formula">XCO₂</span>) from GOSAT and GOSAT-2 radiances and their validation. FOCAL was initially developed for OCO-2 <span class="inline-formula">XCO₂</span> 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 <span class="inline-formula">CO₂</span> Monitoring (CO2M) mission to be launched in 2025.</p> <p>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).</p> <p><span id="page3838"/>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 <span class="inline-formula">XCO₂</span>, water vapour (<span class="inline-formula">H₂O</span>), and solar-induced fluorescence (SIF) but also methane (<span class="inline-formula">XCH₄)</span>, with the potential for further atmospheric constituents and parameters like semi-heavy water vapour (<span class="inline-formula">HDO</span>). In the case of GOSAT-2, the retrieval of nitrous oxide (<span class="inline-formula">XN₂O</span>) and carbon monoxide (<span class="inline-formula">CO</span>) may also be possible.</p> <p>Here, we concentrate on the new FOCAL <span class="inline-formula">XCO₂</span> data products. We describe the generation of the products as well as applied filtering and bias correction procedures. GOSAT-FOCAL <span class="inline-formula">XCO₂</span> 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 <span class="inline-formula">ppm</span> over 1 decade; differences in seasonal variations of about 0.5 <span class="inline-formula">ppm</span> 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 <span class="inline-formula">ppm</span> with a mean scatter of 1.89 <span class="inline-formula">ppm</span>.</p> <p>The GOSAT-2-FOCAL <span class="inline-formula">XCO₂</span> 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.</p>