| Summary: | With the help of various polar-orbiting environment observing platforms, the atmospheric concentration of carbon dioxide (CO<sub>2</sub>) has been well established on a global scale. However, the spatial and temporal pattern of the CO<sub>2</sub> emission and its flux dependence on daily human activity processes are not yet well understood. One of the limiting factors could be attributed to the low revisit time frequency of the polar orbiting satellites. With high revisiting frequency and CO<sub>2</sub>-sensitive spectrum, the Geostationary Interferometric Infrared Sounder (GIIRS) onboard the Chinese FY-4A and FY-4B satellites have the potential to measure the CO<sub>2</sub> concentration at a higher temporal frequency than polar-orbiting satellites. To provide a prototypical demonstration on the CO<sub>2</sub> monitoring capability using GIIRS observations, a hybrid-3D variational data assimilation system is established in this research and a one-month-long experiment is conducted. The evaluations against the Goddard Earth Observing System version 5 (GEOS-5) analysis field and Orbiting Carbon Observatory -2/-3 (OCO-2/-3) CO<sub>2</sub> retrieval products reveal that assimilating GIIRS observations can reduce the first guess’s CO<sub>2</sub> concentration mean bias and standard deviation, especially over the lower troposphere (975–750 hPa) and improve the diurnal variation of near surface CO<sub>2</sub> concentration.
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