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Observations of atmospheric carbon monoxide (CO) can only bemade on continental and global scales by remote sensing instrumentssituated in space. One such instrument is the Infrared AtmosphericSounding Interferometer (IASI), producing spectrally resolved,top-of-atmosphere radiance measurements from which CO verticallayers and total columns can be retrieved. This paper presents atechnique for intercomparisons of satellite data with low verticalresolution. The example in the paper also generates the firstintercomparison between an IASI CO data set, in this case thatproduced by the University of Leicester IASI Retrieval Scheme(ULIRS), and the V3 and V4 operationally retrieved CO productsfrom the Measurements Of Pollution In The Troposphere (MOPITT)instrument. The comparison is performed for a localised region ofAfrica, primarily for an ocean day-time configuration, in order todevelop the technique for instrument intercomparison in a region withwell defined a priori.  By comparing both the standard data and a special version of MOPITTdata retrieved using the ULIRS a priori for CO, it is shown thatstandard intercomparisons of CO are strongly affected by thediffering a priori data of the retrievals, and by the differingsensitivities of the two instruments. In particular, the differing apriori profiles for MOPITT V3 and V4 data result in systematicretrieved profile changes as expected. An application of averagingkernels is used to derive a difference quantity which is much lessaffected by smoothing error, and hence more sensitive to systematicerror. These conclusions are confirmed by simulations with modelprofiles for the same region. This technique is used to show that forthe data that has been processed the systematic bias between MOPITTV4 and ULIRS IASI data, at MOPITT vertical resolution, is less than7 % for the comparison data set, and on average appearsto be less than 4 %. The results of this study indicatethat intercomparisons of satellite data sets with low verticalresolution should ideally be performed with: retrievals using acommon a priori appropriate to the geographic region studied; theapplication of averaging kernels to compute difference quantitieswith reduced a priori influence; and a comparison with simulateddifferences using model profiles for the target gas in the region

A comparison of OEM CO retrievals from the IASI and MOPITT instruments