1. CAVIAR meeting April, 30, 2010 – UCL, London 1/16 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. CAVIAR meeting April, 30, 2010 – UCL, London Summary An optimized Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS) experimental set-up has been developed and utilized to measure water vapour optical depth profiles at room temperature and pressure over a representative range of monomer concentrations between 1.31 × 10 17 and 5.42 × 10 17 molecules cm -3. These measurements were made between 1120 and 1380 nm a region of weak water monomer absorption, where a large uncertainty is currently associated with the semi-empirical MT_CKD continuum model, on account of 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 were systematically inconsistent with those derived theoretically – about three time larger. The feature work is aimed to break down this inconsistency and to expand experiment towards higher temperatures. 2/16

3. CAVIAR meeting April, 30, 2010 – UCL, London Experimental set-up 3/16

4. CAVIAR meeting April, 30, 2010 – UCL, London Advancement of the experimental set-up selecting proper mirror reflectivity wavelength and FWHM calibration – neon spectral lamp minimization of the stray light – adjustment of the f-matcher light tightening of the whole system – now we can estimate the real dark current accounting for time variations of the dark current accounting for time variations of the light source accounting for detuned cavity signal measurement of mirror reflectivity 4/16

5. CAVIAR meeting April, 30, 2010 – UCL, London Advancement of the experimental set-up 5/16

6. CAVIAR meeting April, 30, 2010 – UCL, London Advancement of the experimental set-up 6/16

7. CAVIAR meeting April, 30, 2010 – UCL, London Advancement of the experimental set-up 7/16

8. CAVIAR meeting April, 30, 2010 – UCL, London Experimental procedure A sequence of consecutive measurements were used aiming to collect data Neon 1 – Neon Dark 1 - Detuned F1 - D1 – F1 – I1 – I2 – D2 – I3 – I4 – D3 – I5 – I6 – D4 – I7 – I8 – D5…(n-1) – F2 – Dn – Detuned F2 - Neon 2 – Neon Dark 2 where the water signals I1,8, I2,7, I3,6 and I4,5 were recorded with a wet air flow rate of 1.50 slm and dry air flow rates of 4.50, 1.50, 0.50, and 0.00 slm respectively. These four dry air flow rates were used to generate a range of RHs within the absorption cell, and hence a representative range of H2O concentrations. A number of dark spectra were recorded between D5 and D(n-1) in order to allow sufficient time for the absorption cell to re-stabilize at RH = 0.0% before recording F2 and Det.F2. 8/12

9. CAVIAR meeting April, 30, 2010 – UCL, London Experimental procedure Exp. R u n RH/ % T/ °C I1I1 I2I2 I3I3 I4I4 I5I5 I6I6 I7I7 I8I8 121.645.770.290.292.673.145.922.220.5 221.744.769.989.292.374.946.422.419.4 321.443.969.088.391.170.744.521.622.7 424.947.573.590.492.073.146.723.022.0 522.246.271.489.591.872.745.021.520.2 621.645.371.889.191.976.045.822.420.1 Mean22.245.671.089.592.073.445.722.220.8 Relative humidity, mean H 2 O concentration and temperature measured inside the absorption cell during each experimental run. Mean Water ContentI1I1 I2I2 I3I3 I4I4 I5I5 I6I6 I7I7 I8I8 [H 2 O] × 10 -17 / molecules cm -3 1.312.694.185.275.424.332.691.31 9/12

10. CAVIAR meeting April, 30, 2010 – UCL, London Analysis results 10/16

11. CAVIAR meeting April, 30, 2010 – UCL, London 11/16

12. CAVIAR meeting April, 30, 2010 – UCL, London Analysis results 12/16

13. CAVIAR meeting April, 30, 2010 – UCL, London Analysis results I2I2 I1I1 13/16

14. CAVIAR meeting April, 30, 2010 – UCL, London Analysis results I4I4 I5I5 14/16

15. CAVIAR meeting April, 30, 2010 – UCL, London Analysis results 15/16

16. CAVIAR meeting April, 30, 2010 – UCL, London Further work 16/16 Experiments at higher temperatures Experiment within spectral range 930-1200 nm Measurement of mirror relativities by means PS CRDS Accuracy of the retrieved monomer concentrations Understanding the nature of detuned signal

17. CAVIAR meeting April, 30, 2010 – UCL, London Further work

18. CAVIAR meeting April, 30, 2010 – UCL, London