Carbon Monoxide from SCIAMACHY Near Infrared Nadir Spectra: Impact of Retrieval Settings

Kurzfassung

The SCIAMACHY instrument was mounted on the ENVISAT satellite which was one of ESA's largest Earth observation missions in orbit from 2002 to 2012 with the major objective of measuring radiances to retrieve trace gas concentrations. The SCIAMACHY channel 8 detector observes radiances in the shortwave infrared (SWIR) spectral range from 4191 to 4426 cm-1 which is used to retrieve carbon monoxide vertical column densities. Due to low concentrations and weak absorption the signal to noise ratio for CO is low and the CO absorption lines interfere with strong CH4 and H2O lines. Moreover, a growing ice-layer and detector degradation caused by solar insolation induces large uncertainties to the retrieval. Data for the years 2006 and beyond is currently not usable for CO retrieval. In the framework of this Master Thesis the impact of retrieval settings on carbon monoxide retrieval using SCIAMACHY channel 8 near infrared spectra and the BIRRA software package that was developed at the Remote Sensing Technology Institute at DLR is assessed. The spectroscopic line parameter input, the fit of the wavenumber shift and the climatological a-priori information are investigated in order to recommend the optimal parameter settings for a high quality CO product. For all three settings an orbit analysis of the ENVISAT orbit 8663 is performed which shows the latitudinal dependency of the retrieved results. For the spectroscopic input analysis, the Sahara region (2003 - 2005) is examined as well which includes temporal/seasonal variations. The results achieved by the use of the spectroscopic databases HITRAN 2012, HITRAN 2008 and GEISA 2011 are compared before analyzing the impact of the wavenumber shift fit on CO results. Climatological information is given by the CIRA and NCEP databases which exhibit substantial differences in spatial and temporal resolution influencing the CO final product. The retrieved parameters that are analyzed are the CO VCD, the scale factors for carbon monoxide, methane and water vapor as well as the respective retrieval errors for the VCD, CO, CH4 and H2O scale factors. Moreover, the ratio of CO to CH4 is investigated due to the definition of a proxy-normalized final CO product. For the spectroscopic, wavenumber shift and climatological investigations the results are far from conclusive. The retrieved parameters are extremely similar for all tested retrieval settings and it is difficult to provide recommendations as to which settings yield a final product of improved quality. After careful consideration of the investigated quantities and the results, we arrive at the conclusion that other factors, such as level 1b calibration, and other retrieval settings in BIRRA are the predominant influence on the retrieval quality of a final CO product.