Detailed Validation of MOPITT Instrument Radiances Using In-situ Profile Data

Operational retrievals of carbon monoxide profiles using data from the MOPITT (Measurements of Pollution in the Troposphere) instrument are based on a total of 12 calibrated satellite radiances in two spectral bands. To a large extent, the quantitative agreement of the satellite radiances with values calculated by the MOPITT operational forward radiative transfer model determines the ultimate accuracy of the MOPITT CO retrieval results. For example, a radiance bias as small as 1% in one particular signal may produce a retrieval bias in the CO profile larger than 10%. Validation of both MOPITT Level 1 (calibrated satellite radiances) and Level 2 (CO profile and total column retrievals) products is primarily based on comparisons of MOPITT results with in-situ profiles acquired as part of a program conducted by NOAA's Climate Monitoring and Diagnostics Laboratory. Regular sampling flights have been made at five globally-distributed sites. The goal of the work presented here is to identify and quantify MOPITT radiance biases using the CMDL in-situ data in a manner that accounts for all significant sources of error. These potential error sources include local surface characteristics (e.g. surface emissivity), proximity to CO emission sources, in-situ vertical sampling characteristics, and MOPITT cloud-clearing performance. By minimizing errors associated with each of these effects, we show that the goal of quantifying MOPITT radiance biases to within 1% is realistic.