Improving Time-Dependent Bias Patterns in GEMS Aerosol Retrievals: A Model-Enforced Post-correction Approach

The Geostationary Environment Monitoring Spectrometer (GEMS) is a cutting-edge air quality monitoring instrument onboard the GEO-KOMPSAT-2B (GK-2B) satellite, launched in 2020. Operating in geostationary Earth orbit, GEMS captures hyperspectral radiances with 0.6 nm spectral resolution in the ultraviolet and visible ranges over the Asian region during daytime, providing hourly air quality information. The aerosol retrieval algorithm utilized by GEMS makes use of six channels in ultraviolet and visible wavelengths, offering the advantage of measuring aerosol absorption and height information. However, the diurnal variation of satellite aerosol optical properties (AOPs) may not precisely mirror the actual diurnal variation of AOPs. Bias patterns of satellite AOPs can differ among various geostationary satellites and algorithms due to several factors, including errors in surface reflectance assumptions, L1 calibration, short light paths during noon etc. In the case of GEMS, the current aerosol optical depth (AOD) exhibits a bias throughout the day. To address this issue, a model-enforced post-process correction approach based on machine-learning models (Lipponen et al., 2021) is adopted. This correction method does not necessitate modifications to the original retrieval algorithm. The objective is to minimize the bias patterns of GEMS AOPs over time by incorporating the post-processing results using the random forest-based correction model into the near-real-time AOD retrievals with AERONET as reference. In addition, this study aims to assess the hourly accuracy of GEMS AOD and single-scattering albedo (SSA) using measurements from AERONET sites and conduct uncertainty analyses across the entire GEMS domain. Additionally, GEMS aerosol layer heights (ALHs) are quantitatively compared to CALIOP total attenuated backscatter profiles at 532 nm, enabling a comprehensive evaluation of the GEMS aerosol algorithm's performance and limitations over time. To ensure the reliable estimation of radiative forcing and human health impacts, comprehensive evaluations of satellite aerosol products are vital.