The CHOCOLATE (CH₄, H₂O and CO from Limb middle ATmosphere Emissions) project aims at broadening our understanding and helping to quantify the natural variability and the human-induced changes in the Earth’s climate system. It mainly focuses on the study of the distribution and the changes on water vapour, methane and carbon monoxide in the middle atmosphere. The work is based on the analysis of the high spectral resolution measurements made by the Envisat MIPAS instrument since its launch in 2002.

Water vapour is an important constituent in the middle atmosphere. It is involved in the ozone chemistry, since its oxidation and photolysis produce hydrogen radicals, well-known efficient destroyers of odd oxygen. It is also the precursor of polar mesospheric clouds, involved in important heterogeneous reactions and thought to be an indicator of the climate change. Additionally, water vapour is an important infrared cooler in the stratosphere, with the subsequent effect on its thermal structure. Its source in the stratosphere is methane, being oxidized by OH and O(¹D) to produce both water vapour and carbon monoxide. H₂O and CH₄ are vertically transported up to the mesosphere where their photolysis occurs. Carbon monoxide is produced in the mesosphere and thermosphere by photolysis of CO₂. Its stratospheric sink is its reaction with the hydroxyl radical.

These three species are excellent middle-atmosphere tracers because their lifetimes, as compared to the typical times of vertical transport, are long. The concentration of methane has increased in the last decades due to human-activity and there is a growing interest on the impact of such increase on the H₂O and CO middle-atmosphere abundances.

Few simultaneous global measurements of H₂O, CH₄ and CO are currently available in the middle atmosphere for an extended period of time. Some of the main questions involving the chemical and dynamical behaviour of these components in the middle atmosphere that need to be addressed are:
1) Is the concentration of water vapour increasing in the stratosphere and mesosphere as a consequence of the increase in methane due to human activity?;
2) How does the human-induced increase of methane affect the concentration of carbon monoxide?;
3) How much water vapour is formed at the lower limit of the polar mesospheric clouds (PMCs) after sedimentation of ice particles and what is the connection, if any, between PMCs patterns and atmospheric change?;
4) Is there a second maximum of water vapour located at 65 km during the polar summer and, if so, is it due to meridional circulation and autocatalytic processes?; and
5) What is the magnitude of natural modulations, such as the semi-annual oscillation, on the water vapour, methane and carbon monoxide distribution in the stratosphere and the mesosphere and what is their dependence on latitude?

Within the CHOCOLATE project, the H₂O, CH₄ and CO concentrations from the lower stratosphere to the mesopause are being derived from MIPAS spectra around 6.3, 7.6 and 4.7 µm, respectively, in order to answer those questions. One of the main difficulties of those retrievals is the non-LTE (non-Local Thermodynamic Equilibrium) nature of the emissions, which needs an accurate knowledge of the excitation mechanisms affecting the populations of the emitting vibrational states.

In a first step, the analysis of MIPAS observations has helped to constrain certain poorly-known non-LTE processes affecting the measured emissions (O₂-O quenching, vibrationally-excited O₂ production after ozone photolysis and CH₄–O₂ and H₂O-O₂ vibrational energy exchange). Then, the IMK-IAA processor, able to cope with non-LTE processes, was used to perform the retrievals from MIPAS spectra taken from 2005 in its Middle Atmosphere, Upper Atmosphere and NoctiLucent Cloud modes. The analysis of the retrieved global abundances has shown:
1) The presence of a dry region in the area where the clouds form and an enhanced H₂O abundance below the PMCs;
2) The lack of a second maximum at the mesosphere in the polar summers;
3) The characteristics of the H2O SAO along 5 years;
4) The magnitude of the strong H₂O descent after major sudden stratospheric warmings; and
5) Polar inter-hemispheric differences due to different strengths in the transport mechanisms.

Future work will be focused on:

• The understanding of the polar vertical distributions;
• The study of the correlation, if any, between H₂O abundance and PMCs occurrence;
• The characterization of the SAO and AO of CH₄ and CO; and,
• The impact of the methane increase on the CO and H₂O abundances.

Complementary to MIPAS, data from HALOE will be used for extension of temporal coverage.