Retrieval of methane and carbon monoxide using near infrared spectra recorded by SCIAMACHY onboard ENVISAT : Algorithm Development and data analysis

On the night of 28 February/1 March 2002 the European Space Agency (ESA) launched the environmental satellite ENVISAT on an Ariane 5 rocket from the European spaceport in Kourou. Onboard is, amongst others, the instrument SCIAMACHY that, for the first time in the history of space-borne remote sensing, features near infrared spectrometers measuring sunlight reflected and scattered back from the earth’s surface and atmosphere. In contrast to thermal emission sounders, the near infrared spectrometers enable the global retrieval of methane, carbon dioxide and carbon monoxide for the first time with high sensitivity to nearground atmospheric layers. In this thesis, an algorithm was developed based on the principles of differential optical absorption spectroscopy (DOAS), that deals with the peculiarities of retrieving strong absorbers in the near infrared, thus allowing a precise retrieval of the respective trace gases. It was shown that nonlinear iterative schemes are necessary to account for saturation effects and to avoid interdependencies of spectrally overlapping strong absorbers. The resulting nonlinear maximum a Posteriori algorithm (IMAP-DOAS) was applied to spectra recorded by SCIAMACHY and the retrieved total columns of methane and carbon monoxide were analyzed and interpreted. Since SCIAMACHY cannot be considered a dedicated greenhouse gas mission but a prototype with respect to near infrared spectroscopy, several instrumental shortcomings had to be solved. An extensive analysis of these effects enabled the retrieval of methane and carbon monoxide. Enhancements of carbon monoxide in biomass-burning regions could be clearly identified. In addition, the seasonal and geographical patterns of typical biomass burning regions could be observed. Only in the industrial regions of China, high carbon monoxide abundances were found to be persistent over the year. The worldwide first global measurements of the total columns of methane with high precision were made possible by use of concurrent retrievals of the relatively homogenously distributed carbon dioxide as proxy for the light path of the recorded photons. It was shown that the retrieved methane abundances show a very good agreement with modeled abundances from the chemistry-transport model TM3. The highest abundances were found over areas of rice cultivation in South-East Asia and can be considered a direct proof of large scale methane emissions in Asia. In the time-period from August through November 2003, large discrepancies between measurements and model were discovered over tropical rainforest areas. Measured abundances showed persistently higher abundances than those predicted by the model. This led to the conclusion that the tropical regions as methane source have been hitherto underestimated in current emissions inventories. Whether the discrepancies are attributable to a so far unknown methane source or to simply an underestimation of already known sources remains unclear. Recent studies suggest that there might indeed be a yet unknown source, namely the in-situ formation of methane in plants. If these findings can be supported by validation studies, a reassessment of the global methane budget should be indispensable. An extension of the analysis to the years 2003 and 2004 showed that the discrepancies are highest during the months of August through October. In addition, the highest methane abundances averaged over 2 years are found in the red basin in China. The precision of these measurements now allows their use in inversion models to quantify the temporal and geographical distribution of methane sources. Since methane, as second strongest anthropogenic greenhouse gas, is subject to the Kyoto-Protocol, a precise determination of its source strengths is indispensable. Thus, this work can be considered an important step towards greenhouse gas source inversions using space based retrievals.