Abstract
Abstract. We present a new method to obtain accurate SCIAMACHY CO columns over clouded ocean scenes. Based on an improved version of the Iterative Maximum Likelihood Method (IMLM) retrieval algorithm, we now have retrieved five years of data over both land and clouded ocean scenes between 2003 and 2007. The ocean-cloud method uses the CH4 columns retrieved simultaneously with the CO columns to determine the cloud top height. The CH4 cloud top height is in good agreement with the FRESCO+ cloud top height determined from UV-VIS oxygen-A band measurements, providing confidence that the CH4 cloud top height is a good diagnostic of the cloud top height over (partially) clouded ocean scenes. The CO measurements over clouded ocean scenes have been compared with collocated modeled CO columns over the same clouds and agree well. Using clouded ocean scenes quadruples the number of useful CO measurements compared to land-only measurements. The five-year CO data set over land and clouded ocean scenes presented here is based on an improved version of the IMLM algorithm which includes a more accurate determination of the random instrument-noise error for CO. This leads to a smaller spread in the differences between single CO measurements and the corresponding model values. The new version, IMLM version 7.4, also uses updated spectroscopic parameters for H2O and CH4 but this has only a minor impact on the retrieved CO columns. The five-year data set shows significant interannual variability over land and over clouded ocean scenes. Three examples are highlighted: the Asian outflow of pollution over the northern Pacific, the biomass-burning outflow over the Indian Ocean originating from Indonesia, and biomass burning in Brazil. In general there is good agreement between observed and modeled seasonal cycles and interannual variability.
Highlights
Carbon monoxide (CO) is an important pollutant and the major sink of OH
Outflow regions over the equatorial Atlantic Ocean and the northern Pacific can be discerned in both data sets, but the observed CO columns tend to be higher than the corresponding model values
The gradient between the clean Southern Hemisphere and the polluted Northern Hemisphere is clearly visible in both observed and modeled CO above clouded ocean scenes, the observed CO seems to be lower than modeled CO at high southern latitudes
Summary
Carbon monoxide (CO) is an important pollutant and the major sink of OH. Its life time of typically weeks to months (Cicerone, 1988) makes CO a suitable tracer for long range atmospheric transport and the global distribution of air pollution (e.g. Shindell et al, 2006). Significant interannual variability in CO has been reported both near emission sources and over areas affected by longrange transport of pollution (e.g. Edwards et al, 2004, 2006b; Gloudemans et al, 2006; Yurganov et al, 2008). These variations are mostly due to variations in biomass burning which is one of the major sources of CO. Satellite measurements from MOPITT and AIRS show significant enhancements of CO in the free troposphere over the oceans due to long-range transport Our previous SCIAMACHY studies presented only CO measurements over land
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