Validation of Carbon Monoxide Total Columns from SCIAMACHY Near Infrared Nadir Spectra with NDACC/TCCON Ground-Based Measurements

Kurzfassung

The major objective of this validation study is to examine the accuracy of SCIA- MACHY carbon monoxide (CO) total columns derived from near infrared nadir radiometric measurements from 2003 – 2012. For this purpose, an intercomparison of the CO columns estimated from SCIAMACHY measurements with coincident and colocated retrievals provided by ground-based (g-b) observations is performed. Therefore, data from several NDACC (Network for the Detection of Atmospheric Composition Change) and TCCON (Total Carbon Column Observing Network) Fourier transform (FT) infrared spectrometers have been used. Some fundamen- tal aspects of the assessment of space-borne measurements and particularities in the validation with respect to these g-b networks are pointed out and strategies for dealing with them have been developed. Like SCIAMACHY’s channel-8, the TCCON instruments utilize the 2.3 μm carbon monoxide band, whereas NDACC instruments observe CO in mid infrared spectral region. The outline of this master thesis follows. First, an introduction to important aspects of the radiative transfer theory in the Earth’s atmosphere and elements of short wave infrared spectroscopy are presented. In addition, an overview on atmo- spheric CO is given. For the analysis of short wave infrared spectra measured by SCIAMACHY the Beer InfraRed Retrieval Algorithm (BIRRA) is utilized. BIRRA simulates the radiative transfer by means of the Beer law and performs a separable least squares fit of the measured radiance with respect to molecular column densi- ties and auxiliary parameters. The validation study reported here is based on the BIRRA prototype version giving more flexibility than the operational version which is currently implemented in routine operation. The basic mathematical aspects of least squares are presented along with their application to trace gas retrieval. This study demonstrates that the SCIAMACHY CO total column validation results depend on many aspects. The results indicate particularly 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 devia- tions of the space-based (s-b) and g-b columns in the later years of the mission clearly demonstrate the impact of the degradation of the channel-8 detector. Beside these investigations, which primarily intended to examine new approaches/alternatives to deliver more accurate SCIAMACHY CO total column products retrieved using BIRRA, a so far ’best known’ configuration is used for most comparisons. It turns out that this configuration, comprising settings from retrieval to postprocessing, is an adequate choice in most cases. Using this settings, the s-b retrievals agree with the g-b estimates, particularly in the early years of the mission. It has been found that in most cases monthly mean SCIAMACHY CO total columns agree within the standard deviation compared to the g-b measurements. Finally the results are discussed, related to findings made by other authors, and recommendations for the direction of further analysis are given.