Global carbon monoxide as retrieved from SCIAMACHY by WFM-DOAS

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Abstract
Vertical columns of CO have been retrieved from SCIAMACHY/ENVISAT short wave/near infrared (~2.3µm) nadir spectra using the Weighting Function Modified (WFM) DOAS retrieval algorithm. WFM-DOAS has been applied to a small spectral fitting window located in SCIAMACHY's channel 8 (~2365 nm) covering four CO absorption lines. The focus of this paper is to demonstrate that quantitative information on carbon monoxide (CO) on a global scale can be retrieved from SCIAMACHY. It is shown that plumes of CO resulting from, e.g. biomass burning in Africa, are clearly detectable with SCIAMACHY. The SCIAMACHY CO columns are in good agreement with the CO column data product of MOPITT (V3). For example, the correlation between SCIAMACHY and MOPITT CO columns for cloud free pixels over land is typically in the range r=0.4–0.7, where r is the correlation coefficient. In order to retrieve good CO columns it was necessary to improve the calibration of the SCIAMACHY nadir spectra. Nevertheless, there is still room for significant improvement. The fit residuals, for example, are dominated by stable and systematic spectral artifacts on the order of the weak CO lines. These artifacts are most pronounced in spectral regions of strong overlapping methane and water vapour absorption bands. They might result from spectrometer slit function uncertainties. The CO columns of the WFM-DOAS Version 0.4 CO column data product presented in this paper have been multiplied by a constant and ground scene independent scaling factor of 0.5 to quantitatively adjust the WFM-DOAS retrieved CO columns to the MOPITT CO data. If and how this scaling factor is influenced by SCIAMACHYs much higher sensitivity to the lower troposphere and boundary layer CO and/or by the currently not perfect spectral fitting needs further investigation.
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