Vertical Resolution and Information Content of MOPITT CO Profiles

The MOPITT (Measurements of Pollution in the Troposphere) remote sensing instrument was designed to monitor global distributions of carbon monoxide and methane from a space-based platform using gas correlation radiometry. MOPITT became operational in March, 2000 after being launched in late December, 1999. Current MOPITT retrievals of CO are based on a subset of the instrument's eight 4.6 μ m-band channels. The sensitivity of the retrieved CO profile to the `true' profile is expressed mathematically by the retrieval averaging kernels. Analysis of the averaging kernels reveals valuable information about (1) vertical resolution and (2) inclusion of a priori information. Calculated averaging kernels for operational retrieved CO profiles indicate the capability of resolving tropospheric CO into approximately two layers. Calculation of the averaging kernels' eigenvectors and eigenvalues provides additional insights into issues of vertical resolution and information content. Instruments such as MOPITT that exploit thermal infrared radiation for trace gas profile retrievals are sensitive to temperature contrasts within the atmosphere and at the surface/atmosphere interface. Over land, retrieval vertical resolution (and information content) varies considerably between daytime and nighttime observations due to the diurnal variability of surface/atmosphere temperature contrasts. Over deserts, for example, resolution is usually much greater for daytime observations than for nighttime observations. Over the oceans, retrieval vertical resolution also follows patterns associated with skin temperature (with the highest resolution observed typically in tropical regions). Comparisons of the vertical gradient of MOPITT retrieved CO profiles with in-situ CO profiles measured using aircraft-based instruments are consistent with the vertical resolution indicated by the calculated averaging kernels. Similar findings apply to comparisons of MOPITT retrievals with output from the chemical transport model `MOZART.' Thus, we find that from (1) averaging kernel calculations using operational retrievals, (2) comparisons with validation results based on in-situ measurements, and (3) comparisons with a chemical transport model that MOPITT retrievals contain quite useful information regarding the vertical distribution of carbon monoxide (in addition to its total column value).