The data assimilation of 2000-2004 carbon monoxide (CO) retrievals by the MOPITT (Measurements Of Pollution In The Troposphere) instrument onboard the NASA Terra satellite provide an opportunity for the first time to study the transport and sources of pollution including their year-to-year variations. Based on the different representations of assimilated CO in the chemistry transport model (CTM) space and at the MOPITT retrieval grid this study advocates for direct mapping of CO-sensitive radiances or characterized CO retrievals by the chemical data assimilation schemes. The comprehensive CO forecast provides a great deal of information on the vertical scales that cannot be constrained by the measured radiances. It also provides comprehensive a priori specifications for the inverse problems especially for the vertical levels and geographical regions where the radiometer begins to misplace its high sensitivity to the CO loading. Evaluation of the multi-year MOPITT retrievals and assimilated CO against in situ CO statistics showed how the data assimilation helps to diminish a priori effects in the reprocessed CO retrievals. Data analysis of the multi-year data reveal substantial inter-annual variations of CO loading in the free troposphere and call for the unbiased tracer assimilation schemes in the CTM with optimized CO surface emissions.