Retrieval of carbon dioxide using cross-track infrared sounder (Cris) on s-npp

The Cross-track Infrared Sounder (CrIS) aboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite is a spaceborne Fourier transform infrared spectrometer. The study aims to retrieve carbon dioxide (CO₂ ) information (the CO₂ profile and column-averaged dry-air mole fraction of XCO₂ ) from June 2018 to December 2019 based on the The National Oceanic and Atmospheric Administration (NOAA)-Unique Combined Atmospheric Processing System (NUCAPS) Cloud-Cleared Radiances (CCRs) via the CrIS. The CCRs products for the CrIS with 2223 channels have been available since 22 May 2018. Characteristics of the CO₂ weighting functions inform the choice of multiple channels that are around 15 µm in size that differ by latitude and season to maximize retrieval sensitivity to CO₂ and minimize sensitivity to other interfering atmospheric parameters. CO₂ was retrieved from these channels using an adopted nonlinear optimization algorithm. The temperature, water vapor, and ozone profiles used in the inversion process were gathered from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5). Validations of CO₂ concentrations as retrieved from CrIS showed the following conclusions: (1) The relative error of the retrieved CO₂ concentrations, as compared to Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) in situ aircraft measurements, was less than 0.5%, and the root mean square errors (RMSE) were less than 0.7 ppmv (with correlation coefficients of 0.56–0.86); (2) the retrieved XCO₂ from June 2018 to December 2019 correlated well with the ground-based Total Carbon Column Observing Network (TCCON) observations, and the differences were within ±0.2 ppmv. Further analysis of the temporal and spatial distribution of the retrieved CO₂ at 300 hPa demonstrated a strong seasonal variation of CO₂ in 0–60^◦ N in the Northern Hemisphere with the maximum values in June–August and larger amplitudes of seasonal variation in the northeast of Asia and northeastern part of North America. The variations likely occurred due to larger sinks of atmospheric CO₂ that are dominated by CO₂ uptake in the summer. In the Southern Hemisphere, the CO₂ displayed high concentration anomalies in the latitudinal range of 30–60^◦ S in September–November and December–February, which probably occurred due to the lofted smoke plumes from the strong fire seasons in South America and Southern Africa.