Ground-Based FTIR Observations of XCO2 and XCH4 during the ASIA-AQ Campaign in Seoul, South Korea

During the ASIA-AQ international field campaign in February 2024, ground-based greenhouse gas observations were conducted at urban and background sites in South Korea. Portable Fourier Transform Infrared (FTIR) spectrometers, EM27/SUN, as a component of the COllaborative Carbon Column Observing Network (COCCON), were deployed at Ewha Womans University and Olympic Park in Seoul to measure column-averaged dry-air mole fractions of carbon dioxide (XCO2) and methane (XCH4). In parallel, a high-resolution stationary FTIR spectrometer, the IFS125HR, operated as a reference instrument of the Total Carbon Column Observing Network (TCCON), conducted background observations at Anmyeondo. Prior to the campaign, a side-by-side intercomparison involving two EM27/SUN instruments and the IFS125HR was carried out at Anmyeondo to assess instrument consistency. The results showed that temporal concentration variability and short-term enhancement events were consistently captured by all three instruments, although systematic biases were identified among them. To align the EM27/SUN measurements with the TCCON reference scale, calibration factors for both EM27/SUN instruments were derived based on comparisons with the IFS125HR and subsequently applied to the measurements acquired during the campaign period. During the campaign, three days of coincident observations among the three instruments were obtained. Analysis of these data revealed that both XCO2 and XCH4 exhibited the highest concentrations at Ewha Womans University, followed by Olympic Park and Anmyeondo. In addition, daily variability of greenhouse gas concentrations differed by site and varied from day to day. Following the campaign, extended EM27/SUN observations were conducted at Ewha Womans University through May 2024 and were used to evaluate consistency with satellite retrievals. Comparisons with S5P TROPOMI showed that XCH4 agreed within approximately 2% and exhibited a correlation coefficient of 0.68. This study shows that ground-based XCO2 and XCH4 observations obtained during the study period are meaningful for intercomparison among ground-based sites and comparison with satellite observations.