MOPITT spectral characterisation and forward modelling

The importance of understanding the MOPITT instrument transmittance is discussed. Measurements were made of the MOPITT Flight Model instrument transmittance using a Difference Frequency Laser System. The data suffered from alignment problems, which were largely corrected in later tests on the Engineering Qualification Model. Despite these problems, the Flight Model data indicated that the methane channel filters were found to differ significantly from piece-part measurements of the MOPITT narrow band-pass filter transmittances. The Flight Model data motivated adjustments to the piece-part data for all channels to produce nominal filter profiles used for current MOPITT retrievals. These profiles are shown to be consistent with laboratory and in-flight data.; A radiative transfer model of MOPITT and the Earth's atmosphere was developed and validated. The model was used to establish sensitivity levels of MOPITT to uncertainty in calculation parameters, the filter profile and other instrument parameters. The calculation parameters employed by earlier radiative transfer models are shown to be of insufficient accuracy for MOPITT retrieval purposes. MOPITT calibration events are shown to reduce the impact of instrument parameter uncertainties on target gas retrievals. The model was also used to retrieve filter shift information for the methane channels.