Validation of Carbon Monoxide Vertical Column Densities Retrieved from SCIAMACHY Infrared Nadir Observations

Abstract

Carbon monoxide and methane are key species of Earth's atmosphere. Accordingly, a large number of spaceborne sensors are observing these species in the microwave, thermal and near infrared. For the analysis of short wave infrared spectra measured by SCIAMACHY and similar instrument(s) we had developed the Beer InfraRed Retrieval Algorithm: BIRRA is a separable least squares fit of the measured radiance with respect to molecular column densities and auxiliary Parameters optional: surface albedo, baseline, slit function width, and wavenumber shift). BIRRA has been implemented in the operational SCIAMACHY L1 to 2 processor for the retrieval of CO and CH4 from channel 8 and 6, respectively; the validation study reported here is based on the BIRRA prototype version giving more flexibility. Verification and validation are critical elements of any code development, and mandatory for the assessment of spaceborne remote sensing products. In this poster we report on intercomparisons of CO columns estimated from SCIAMACHY with coincident and co-located retrievals provided by ground-based Fourier transform infrared spectroscopy. More specifically, we have used data from several NDACC (Network for the Detection of Atmospheric Composition Change) and TCCON (Total Carbon Column Observing Network) stations. Like SCIAMACHY's channel 8 the TCCON instruments utilize the 2.3 µm band of carbon monoxide, whereas NDACC observes the CO mid infrared absorption. The results indicate the importance of appropriate sampling areas around the station, calibration of level 1 data, and post-processing of the BIRRA retrievals (esp. filtering). Furthermore, the increased deviations of the spaceborne and ground-based columns in the later years of the mission clearly demonstrate the impact of the degrading channel 8 detector. Overall, the CO columns derived from SCIAMACHY and NDACC/TCCON largely agree within the error bars.