Impact of Level 1 quality on BIRRA CO retrieval

Abstract

The SCIAMACHY near infrared (NIR) channels 6-8 are suitable to retrieve vertical column densities (VCDs) of atmospheric gases such as CO, CH4, CO2 or H2O. BIRRA will be used in the coming version 5 of the offline Level 1b to 2 SCIAMACHY processor to retrieve CO values operationally. The quality of VCD retrievals depends on theoretical, computational and observational issues. Realistic atmospheric data as well as efficient yet accurate, robust radiation transfer models and inversion methods have to be used. However, especially operational applications require to keep computational time within strict limits and the best possible models are chosen according to this restriction. The quality of the measured spectra greatly affects the accuracy of the retrievals. Since model parameters are optimally varied during the inversion process to mimic the measured values, errors in the input spectra will lead to wrong retrievals. SCIAMACHY spectra are spectrally and radiometrically calibrated and corrected for several effects, namely: Leakage current, pixel-to-pixel gain, non-linear response, straylight, polarisation. Reflectances are calculated using inflight sun diffuser spectra. Additionally, the degradation of the instrument is being monitored and the quality of the individual spectral pixels assessed. It turned out that the effect of the bad/dead pixel mask (BDPM) has a major impact on the retrievals. Focussing on carbon monoxide, this work aims at studing the effect of different calibration steps on the retrieval of atmospheric gas VCDs.