Retrieving the Vertical Profile of Greenhouse Gas Using Fourier Transform Spectrometer (FTS) - Part I: Carbon Dioxide (CO₂)

The Total Carbon Column Observing Network (TCCON) utilizes Fourier Transform Spectrometers (FTS) to provide the atmospheric column-average concentrations of greenhouse gases. While the retrieval process scales a priori gas profiles as input parameters, the FTS data itself enables the derivation of vertical profiles for these gases. This study focused on deriving and validating the vertical profile of carbon dioxide (CO₂) from FTS measurements at TCCON’s Anmyeondo observation site. Accurate CO₂ profiles require improving the precision of a priori profiles, as their vertical structure persists through the retrieval algorithm. To achieve this, a new a priori profile simulated by the Goddard Earth Observing System-Chemistry (GEOS-Chem) model was adopted instead of the standard TCCON-provided profiles. Although the GEOS-Chem a priori profile exhibited greater differences from the TCCON profile in the comparison with airborne observations, its overall vertical shape appeared more representative of actual conditions. When the GEOS-Chem a priori profiles were employed, the retrieved CO₂ profiles achieved much better agreement with both surface in-situ measurements and airborne observations, particularly in the lowermost atmospheric layers near the ground. Notably, the discrepancy between the derived surface CO₂ values and in-situ readings was significantly reduced to 6.4±8.7 ppm using the GEOS-Chem a priori profile, compared to 12.6±9.5 ppm with the standard TCCON a priori profile. This study highlights the importance of using representative a priori profiles to precisely retrieve greenhouse gas vertical distributions from ground-based remote sensing measurements.