Fast CO₂ Retrieval Using a Semi-Physical Statistical Model for the High-Resolution Spectrometer on the Fengyun-3D Satellite

China’s Fengyun-3D meteorological satellite launched in December 2016 carries the high-resolution greenhouse-gases absorption spectrometer (GAS) aimed at providing global observations of carbon dioxide (CO₂). To date, GAS is one of the few instruments measuring CO₂ from the near-infrared spectrum. On orbit, the oxygen (O₂) A band suffers a disturbance, and the signal-to-noise ratio (SNR) is significantly lower than the nominal specification. This leads to difficulties in the retrieval of surface pressure and hence a degradation of the retrieval of the column-averaged CO₂ dry air mole fraction (XCO₂) if a full physics retrieval algorithm is used. Thus, a fast CO₂ inverse method, named semi-physical statistical algorithm, was developed to overcome this deficiency. The instrument characteristics, the semi-physical statistical algorithm, and the results of comparison with ground-based measurements over land were introduced in this paper. XCO₂ can be obtained from three bands, namely, the O₂ A, weak CO₂, and strong CO₂ bands, with compensation from the Medium Resolution Spectral Imager-2 (MERSI-2) products, ECMWF Reanaly-sis v5 (ERA-5) data, and Total Carbon Column Observing Network (TCCON) data. The eigenvectors of covariance matrices and the least square fits were used to derive retrieval coefficients and yield cloud-free solutions. In addition to the GAS radiance, some key factors necessary for the accurate estimations of XCO₂ were also taken as input information (e.g., air mass, surface pressure, and a priori XCO₂). The global GAS XCO₂ restricted over land was compared against the simultaneously collocated observations from TCCON. The retrieval algorithm can mitigate the issue caused by the low SNR of the O₂ A band to a certain extent. Overall, through site-by-site comparisons, GAS XCO₂ agreed well with the average precision (1σ) of 1.52 ppm and bias of −0.007 ppm. The seasonal variation trends of GAS XCO₂ can be clearly seen at TCCON sites on the 1-yr timescale.