Instrument Line Shape Calibration and Comparison with TCCON Measurements for Greenhouse Gas Monitoring at a New COCCON Site in Korea

Abstract

A new COllaborative Carbon Column Observing Network (COCCON) site has been established in South Korea to monitor greenhouse gases (GHGs) effectively. This study focuses on the calibration of the COCCON observing instrument through precise measurements of the instrumental line shape (ILS) parameters and comparison with TCCON measurements. The COCCON network employs Bruker's EM27/SUN mobile Fourier transform infrared spectrometer (mFTIR), requiring validation against data from high-resolution (HR-FTIR) instruments. To achieve this, we compared data from the mFTIR with Bruker's IFS 125HR HR-FTIR to derive an instrument-specific calibration factor. Two ILS values were obtained by analyzing the water vapor spectrum using the open path (OP) method and the C₂H₂ Gas-filled cell (GFC) spectrum. The ILS measurements indicated a modulation efficiency amplitude (MEA) difference of 0.712% between the GFC and OP methods, with values of (0.982 ± 0.005) and (0.975 ± 0.008), respectively. Notably, a comparison of the XGas values derived from both sets of ILS parameters revealed a high degree of consistency, as indicated by slopes close to 1, suggesting that the choice of ILS parameters had minimal impact on the accuracy of XGas retrieval. In this study, a 0.82% increase in the MEA value led to increases of 0.17% in XCO₂, 0.09% in XCH₄, 0.13% in XCH₄_S5P, and 0.14% in XCO concentrations. Furthermore, seasonal variations were observed in the mFTIR measurements for XCO₂, XCH₄, and XCO. The highest XCO₂ value recorded was 427.82 ± 0.56 ppmv in April 2024, while the lowest monthly average of 415.23 ± 0.58 ppmv was observed in September 2023. The highest XCH₄ concentration was recorded in September 2024 at 1.966 ± 0.021 ppmv, while the lowest occurred in March 2024, with a value of 1.917 ± 0.016 ppmv. The highest XCO concentration was observed in spring, reaching 0.114 ± 0.014 ppmv in March 2024, while the lowest was recorded in fall at 0.102 ± 0.011 ppmv in September 2024. Additionally, the mFTIR measurements were compared to those obtained from HR-FTIR to evaluate the compatibility between the COCCON and TCCON measurement methods. Results indicated a strong correlation between the two measurement techniques, with instrument-specific calibration factors ranging from 0.9884 for XCO to 1.0017 for XH₂O. These results demonstrate that the mFTIR is ready for use in measurements at the COCCON site and for measurement campaigns in South Korea.