Optimizing the Atmospheric CO₂ Retrieval Based on the NDACC-Type FTIR Mid-Infrared Spectra at Xianghe, China

Carbon dioxide (CO2) is the most important long-lived greenhouse gas and can be retrieved using solar absorption spectra recorded by a ground-based Fourier-transform infrared spectrometer (FTIR). In this study, we investigate the CO2 retrieval strategy using the Network for the Detection of Atmospheric Composition Change-Infrared Working Group (NDACC-IRWG) type spectra between August 2018 and April 2022 (similar to 4 years) at Xianghe, China, aiming to find the optimal observed spectra, retrieval window, and spectroscopy. Two spectral regions, near 2600 and 4800 cm(-1), are analyzed. The differences in column-averaged dry-air mole fraction of CO2 (XCO2) derived from spectroscopies (ATM18, ATM20, HITRAN2016, and HITRAN2020) can be up to 1.65 +/- 0.95 ppm and 7.96 +/- 2.02 ppm for NDACC-type 2600 cm(-1) and 4800 cm(-1) retrievals, respectively, which is mainly due to the CO2 differences in air-broadened Lorentzian HWHM coefficient (gamma air) and line intensity (S). HITRAN2020 provides the best fitting, and the retrieved CO2 columns and profiles from both 2600 and 4800 cm(-1) are compared to the co-located Total Column Carbon Observing Network (TCCON) measurements and the greenhouse gas reanalysis dataset from the Copernicus Atmosphere Monitoring Service (CAMS). The amplitude of XCO2 seasonal variation derived from the NDACC-type (4800 cm(-1)) is closer to the TCCON measurements than that from the NDACC-type (2600 cm(-1)). Moreover, the NDACC-type (2600 cm(-1)) retrievals are strongly affected by the a priori profile. For tropospheric XCO2, the correlation coefficient between NDACC-type (4800 cm(-1)) and CAMS model is 0.73, which is higher than that between NDACC-type (2600 cm(-1)) and CAMS model (R = 0.56).