This project was funded by the Natural Environment Research Council (NERC) with the grant reference - NE/J010316/1 - led by Professor Jonathan Tennyson (University College London).

All CO2 remote sensing activity, from both the ground and space, relies on monitoring how CO2 absorbs light. All this monitoring is therefore heavily dependent on understanding the absorption properties of the CO2 molecule which is usually obtained by measurements performed in the laboratory. In particular the accurate knowledge of the strength of individual absorption lines is crucial to determining how much CO2 is present and allowing the atmospheric data to be interpreted. Without high accuracy values for line intensities, reliable CO2 retrievals are simply not possible. Particularly with their emphasis on variation of CO2 concentrations with time, current missions and proposed missions require CO2 line intensities to be determined to significantly better than 1% accuracy if they are to fulfill their stated goals: intensities accurate to better than 0.5% are really required. Current line intensities measured in the laboratory simply do not gives this level of accuracy: most are accurate to about 5% with a few high quality measurements being good to 1 - 3%. Hence current CO2 retrievals values are limited by the available laboratory data.

The aim of this project was to provide an accurate theoretical solution to the problem of CO2 line intensities based on the application of high accuracy, first principles quantum mechanical calculations for the intensities and experimental data for the line positions. The resulting new lists of CO2 transition intensities were made widely available and, in particular, used to inform atmospheric databases which are used for the majority of atmospheric applications of molecular spectroscopy.

The main objectives of the project were to determine a complete set of high accuracy line intensities for the CO2 molecule for use in atmospheric studies. With the following specific objectives:

1. Develop a theoretical model to compute a high accuracy dipole moments for asymmetric geometries of CO2.

2. Compute dipole moments as a function of geometry and fit them to give a high accuracy dipole moment surface.

3. Develop a model for and calculate high accuracy vibration-rotation wavefunctions and hence transition intensities
for CO2.

4. Compare our results with laboratory data, particularly ultrahigh accuracy studies currently being performed in NIST.

5. Generate a comprehensive linelist of CO2 transitions.

6. Distribute our CO2 linelist widely via the web and via data compilations such as HITRAN, GEISA and BADC.