1. CAVIAR meeting May, 13, 2010 – UCL, London 1/11 Cambridge Centre for Atmospheric Science Broad Band Cavity Enhanced Absorption Spectroscopy of the Water Vapour Continuum in the Near-IR Prof. R. Jones, Dr. A. Shillings, A. Wilson, Dr. V. Tsanev,

2. Experimental set-up The Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS) experimental set-up for the 1230-1380 nm region has been advanced by: measuring directly the mirror reflectivity wavelength and FWHM calibration using neon spectral lamp minimization of the stray light by selection and adjustment of f-matching of optical fibres minimization of light leaks in the system to improve measurement of pure dark current accounting for time variations of the dark current accounting for time variations of the light source intensity accounting for a detuned cavity signal (origin unclear) 2/11 CAVIAR meeting May, 13, 2010 – UCL, London

3. Experimental set-up 3/11 CAVIAR meeting May, 13, 2010 – UCL, London

4. Experimental results 4/11 CAVIAR meeting May, 13, 2010 – UCL, London

5. 5/11 CAVIAR meeting May, 13, 2010 – UCL, London Experimental results

6. Analysis 6/11 CAVIAR meeting May, 13, 2010 – UCL, London

7. 7/11 CAVIAR meeting May, 13, 2010 – UCL, London Analysis

8. I2I2 I1I1 8/11 CAVIAR meeting May, 13, 2010 – UCL, London Analysis

9. 9/11 CAVIAR meeting May, 13, 2010 – UCL, London

10. CAVIAR meeting May, 13, 2010 – UCL, London The system has been utilized to measure with resolution 1.6 nm the water vapour optical depth profiles at room temperature and pressure and monomer concentra- tions between 1.31×10 17 and 5.42×10 17 molecules cm -3 (RH 20-95%). Measurements were made between 1120 and 1380 nm a region of weak water monomer absorption, where the semi-empirical MT_CKD model uncertainty is associated with the limited experimental data available. The comparison of the experimental optical depth profiles with corresponding theoretical models, calculated using spectral data from HITRAN and the MT_CKD continuum model, revealed the presence of a weak, smoothly-varying continuum absorption profile superposed upon the dominant monomer absorption features. The measured optical depth profiles are reproducible but they are systematically inconsistent with those derived theoretically. Summary of results 10/11

11. CAVIAR meeting May, 13, 2010 – UCL, London The feature work is aimed to Rationalise the inconsistency between experiment and model Expand experiment towards higher temperatures. Extend measurements to 1000 to 1100 nm region Future work 11/11

12. Further work CAVIAR meeting May, 13, 2010 – UCL, London