Reflectance-based cross-calibration of GOME and SCIAMACHY spectrometers enhanced by polarization monitoring devices data

Kurzfassung

Spectrometer instruments have significantly contributed to monitoring atmospheric composition and climate change for decades. Among them, the Global Ozone Monitoring Experiment (GOME, 1995–2011) and the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY, 2002–2012) were two well-known sensors whose missions overlapped by nearly a decade. Both instruments provided valuable data for atmospheric applications. However, to ensure data consistency and extend long-term time series, cross-calibration between the two instruments was important. The Fundamental Data Record for Atmospheric Composition (FDR4ATMOS) project, initiated by the European Space Agency (ESA), aims at harmonizing GOME and SCIMACHY Level 1 data, i.e., irradiance and reflectance measurements.

This paper presents, for the first time, the cross-calibration methodology for spectrometers used in the FDR4ATMOS project. Several challenges, such as differing spatial resolutions, lack of exact spatiotemporal overlap, and the need to preserve spectral structure, were addressed using targeted strategies. This process involved selecting scenes with minimal acquisition time differences over Pseudo-Invariant Calibration Sites (PICS) characterized by stable meteorological and atmospheric conditions. A key step involves spatially weighted averaging of SCIAMACHY pixels within each GOME footprint and computing spectral channel-wise ratios over Bands 2B, 3, and 4, which represent ultraviolet, visible, and near-infrared (UV/VIS/NIR) wavelengths. Additionally, the paper presented an analysis approach based on Polarization Monitoring Devices (PMDs) data to investigate the spatial homogeneity of pixels used in the cross-calibration and its influence on the performance of the cross-calibration.

Observations under near-clear-sky conditions from 2003 were collocated over PICS and used to derive transfer functions (TFs). Polynomial TFs were fitted for Bands 2B and 3, while a constant TF was used for Band 4. The TFs showed dependence on viewing zenith angle (VZA), degradation, and wavelength. The uncertainty of TFs increased with wavelength, corresponding to reduced homogeneity in PMD measurements. Using PMD measurements from cross-calibrated pixels as an indicator to filter out non-homogeneous pixels of the main spectral channels resulted in an uncertainty reduction up to 70 % in the TFs.

Overall, the presented cross-calibration approach and PMD-based analysis provide a pathway toward generating consistent and long-term spectrometer records. This work highlights the potential for expanding future TFs derivation beyond ideally suited scenes, increasing robustness across the varied surface and atmospheric conditions.