GEMS ozone profile retrieval: impact and validation of version 3.0 improvements

Abstract

This study presents the first comprehensive description of the operational GEMS (Geostationary Environment Monitoring Spectrometer) ozone profile retrieval algorithm and evaluates the performance of the reprocessed version 3.0 dataset. The retrieval operates in the 310–330 nm spectral range and yields total degrees of freedom for ozone ranging from 1.5 to 3. Although the vertical sensitivity is limited, GEMS achieves an effective vertical resolution of 5–10 km and is capable of separating tropospheric and stratospheric ozone layers. This work highlights significant algorithmic and calibration improvements in version 3.0. Radiometric offsets in irradiance measurements are corrected using a scaling factor derived from the average ratio to a solar reference, while residual wavelength-dependent biases in the normalized radiance are further mitigated through soft calibration. In addition, shift corrections are applied separately to irradiance and radiance wavelengths. As a result, version 3.0 significantly reduces spectral fitting residuals, lowering them from 0.8 % in version 2.0 to 0.2 % under nominal conditions. This improvement also mitigates altitude-dependent oscillating biases observed in the previous version (+40 DU in the troposphere, −20 DU in the stratosphere). The version 3 ozone profiles show agreements within ±10 DU of ozonesonde observations, with a mean bias of −7.7 % in tropospheric ozone columns and within 5 % in the stratosphere. Furthermore, the retrievals capture day-to-day vertical ozone variability, as demonstrated by comparisons with daily ozonesonde launches in February and March 2024. Integrated ozone columns derived from the profiles also show improved consistency with ground-based total ozone measurements, yielding a mean bias of −3.6 DU and outperforming the GEMS operational total column ozone product.