Update of GEMS AEH algorithm V2.1 to V3.0 based on O4 VCD correction

Optical path length reaching to satellite sensor is affected by changes of aerosol layer altitude. Based on this principle, Aerosol Effective Height (AEH) from the GEMS is retrieved by using the O₄ Slant Column Density (SCD) variation. AEH retrieval algorithm is being improved up to V3.0. This algorithm uses a Look-up Table (LUT) to convert O₄ SCD, which is calculated directly from GEMS L1C data, into AEH. O₄ SCD is retrieved by DOAS fitting and AEH is inversely estimated by comparing O₄ SCD, aerosol properties, and geometry to LUT.

In this study, we discuss changes resulting from the update of AEH algorithm V2.1 to V3.0. This update is mainly about correcting atmospheric profile, which is based on the US Standard Atmosphere. O₄ Vertical Column Density (VCD) shows seasonal and spatial differences and temperature-dependent (Choi et al., 2019). Using variable O₄ VCD can be suitable for realistic atmospheric condition. O₄ VCD can be derived from surface pressure, temperature, and relative humidity (Beirle et al., 2022). We corrected the O₄ VCD by applying the ratio of monthly mean temperature and pressure from ECMWF ERA5 from 2019 to 2023 relative to the US Standard value.

After update, AEH decreased in India and Southeast Asia, while it slightly increased in Northeastern China and Korea. Compared to TROPOMI Aerosol Layer Height (ALH), March 2021, the bias decreased by approximately 0.5 km compared to V2.1. Although algorithm update results retrievals in stable range, there are changes in retrieval coverage and regional differences. Therefore, further research is required to investigate these regional differences.