Global evaluation of SCIAMACHY and MOPITT carbon monoxide column differences for 2004-2005

This paper presents a detailed global comparison of Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) and Measurement of Pollution in the Troposphere (MOPITT) carbon monoxide (CO) column measurements for the years 2004 and 2005. Qualitatively, spatiotemporal variations of SCIAMACHY and MOPITT are similar. Quantitative comparisons have been performed taking the effects of instrument noise errors, vertical sensitivities via the averaging kernel and a priori, different spatiotemporal sampling and clouds into account using simulated CO profiles from the TM4 model. SCIAMACHY and MOPITT CO columns are similar over tropical, subtropical, and Northern Hemisphere oceans as well as over boreal regions where SCIAMACHY and MOPITT agree to within 10% or 2 x 10(17) molecules/cm(2). The short-wave infrared SCIAMACHY observations also provide information about lower tropospheric CO in Arctic and subarctic regions north of 60 degrees N, where the MOPITT sensitivity is strongly reduced. South of 45 degrees S, SCIAMACHY CO columns are 3-5 x 10(17) molecules/cm(2) smaller than MOPITT CO columns. Approximately 1.5 x 10(17) molecules/cm(2) (similar to 10%) of this difference is attributed to a bias in the SCIAMACHY CO columns, which is currently under investigation. The remaining difference is possibly related to MOPITT biases in this region. In the transition from oceans to dry desert regions, MOPITT CO total columns show a rapid increase of approximately 3 x 10(17) molecules/cm(2) (similar to 15%). While MOPITT and SCIAMACHY agree over oceans, MOPITT is approximately 5 x 10(17) molecules/cm(2) (similar to 25%) larger than SCIAMACHY results over dry land regions. The origin of this bias needs further investigation.