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This paper presents the first global set of observations of CO2 in themesosphere and lower thermosphere (MLT) obtained by the ACE-FTS instrumenton SCISAT-I, a small Canadian satellite launched in 2003. The observationsuse the solar occultation technique and document the fall-off in the mixingratio of CO2 in the MLT region. The beginning of the fall-off of theCO2, or "knee" occurs at about 78 km and lies higher than in theCRISTA-1 measurements (~70 km) but lower than in the SABER 1.06(~80 km) and much lower than in rocket measurements. We also present themeasurements of CO obtained concurrently which provide important constraintsfor analysis. We have compared the ACE measurements with simulations of theCO2 and CO distributions in the vertically extended version of theCanadian Middle Atmosphere Model (CMAM). Applying standard chemistry we findthat we cannot get agreement between the model and ACE CO2 observationsalthough the CO observations are adequately reproduced. There appears to beabout a 10 km offset compared to the observed ACE CO2, with the model"knee" occurring too high. In analyzing the disagreement, we have investigatedthe variation of several parameters of interest (photolysis rates, formationrate for CO2, and the impact of uncertainty in turbulent eddydiffusion) in order to explore parameter space for this problem. Ourconclusions are that there must be a loss process for CO2, about 2–4~timesfaster than photolysis that will sequester the carbon in some formother than CO and we have speculated on the role of meteoritic dust as apossible candidate. In addition, from this study we have highlighted apossible important role for unresolved vertical eddy diffusion in 3-D modelsin determining the distribution of candidate species in the mesosphere whichrequires further study

atmospheric chemistry and physics

First multi-year occultation observations of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in the MLT by ACE satellite: observations and analysis using the extended CMAM