Retrieving the Vertical Profile of Greenhouse Gas Using Fourier Transform Spectrometer (FTS) - Part II: Methane (CH₄)

This study derived the vertical profile of methane (CH₄) concentrations using Fourier Transform Spectrometer (FTS) measurements from the Anmyeondo site, and validated with ground-based and airborne measurements. The a priori methane profiles currently employed by the Total Carbon Column Observing Network (TCCON) were found to inadequately represent local emissions, exhibiting weak diurnal variations, especially within the boundary layer. In contrast, methane profiles simulated by the Goddard Earth Observing System-Chemistry (GEOS-Chem) model displayed distinct diurnal fluctuations near the surface and demonstrated better agreement with surface in-situ measurements. Interestingly, the column-averaged methane concentrations (XCH₄) retrieved using the TCCON and GEOS-Chem a priori profiles differed only slightly, within 1~2 ppb, suggesting negligible influence of the a priori’s vertical distribution on XCH₄. However, employing the GEOS-Chem a priori profiles markedly improved the retrieved vertical profiles of methane compared to using the standard TCCON a priori. When the GEOS-Chem profiles were utilized, the discrepancy between the derived surface methane values and in-situ measurements was halved. Moreover, the GEOS-Chem retrieved profiles exhibited relatively better agreement with airborne measurements, particularly within the boundary layer. These findings highlight the importance of using representative a priori profiles that adequately capture local emissions and boundary layer dynamics to accurately retrieve greenhouse gas vertical distributions from ground-based remote sensing data.