Update Plan for GEMS Total Column Ozone: L1C irradiance and Cloud Parameter Improvements

The Geostationary Environment Monitoring Spectrometer (GEMS) onboard the GEO-KOMPSAT-2B satellite provides high spatiotemporal resolution observations of various atmospheric constituents over East Asia, including ozone, its precursors (NO2, HCHO), SO2, and aerosols. The GEMS total ozone retrieval algorithm (O3T), based on the TOMS Version 9 look-up table approach, was recently updated to Version 2.2 in December 2024. However, the current GEMS total ozone product has shown a systematic underestimation compared to other satellite observations, such as TROPOMI and OMI, as well as ground-based measurements, mainly due to limitations in the Level 1C (L1C) irradiance data.

To address these issues, we evaluate the O3T algorithm using two types of updated L1C datasets: one applying only the bidirectional transmittance distribution function (BTDF) correction (A1), and another applying both the BTDF and degradation corrections (A2). The results show that the use of both A1 and A2 substantially reduces the underestimation of total ozone, with particularly pronounced improvements at high ozone levels. Furthermore, when using A2, the retrieved total ozone exhibits improved long-term stability. Using these improved GEMS total ozone products, we analyze the characteristics and variability of total ozone over East Asia. In addition, we investigate the impact of cloud information, another critical input to the O3T algorithm, on the total ozone retrieval. These results enhance the reliability of GEMS ozone retrievals and provide a foundation for further algorithm optimization.