Influence of surface reflectivity variability on MOPITT 2.2-2.3 μm channel radiances and the retrieval of CO and CH4

The MOPITT (Measurement of Pollution in the Troposphere) instrument uses gas-correlation spectroscopy to retrieve the tropospheric profile of CO and the total column of CO and CH₄. The instrument´s 2.2-2.3 μm channel signals can be used to determine the CH₄ and CO columns. At these wavelengths, surface effects are important since the channel radiances are determined by reflected solar radiation. Small changes in scene during data acquisition for a given pixel can introduce important variations in surface reflectivity, even when averaged over the instrument field-of-view. These variations must be carefully accounted for to ensure a quality column retrieval. MOPITT simulations based on reflectivity measurements from the MODIS Airborne Simulator are used to construct examples illustrating these effects, along with a method for their mitigation.