The Measurements Of Pollution In The Troposphere (MOPITT) instrument on Terra platform measures the tropospheric CO and CH4 at a nadir-viewing geometry. The measurements are taken at 4.7um in the thermal region, and 2.3um and 2.2um in the solar region for CO mixing ratio retrieval, CO total column amount and CH4 column amount retrieval, respectively. To ensure the required measurement accuracy, it is critical to identify and remove any cloud contamination to the channel signals. MOPITT cloud removing algorithm includes detecting the cloudy pixels, reconstructing clear column radiance for pixels with partial cloud covers, and estimating equivalent cloud top positions under overcast conditions to enable CO profile retrievals above clouds.

This paper describes the techniques used in the MOPITT cloud algorithm to reconstruct clear column radiances for pixels partially covered by cloud. One technique is generally referred as N* method (Smith, 1976). N*s, the ratios of the cloud covers between two adjacent pixels, are calculated by use of the MODIS cloud mask collocated with MOPITT pixels. N* technique requires a certain amount of contrast on the cloud covers between two adjacent pixels, which limits the application of the method to relatively a small number of cases for MOPITT spacial resolution. An alternative method is to use MOPITT forward model to calculate the radiance contributed by the cloudy portion of the pixel and the clear contribution can be derived once the cloud coverage is known. This technique requires the knowledge of the cloud top pressures that can be obtained from MODIS cloud products. Both techniques will be tested and the results will be discussed.