Water vapour continuum absorption in near- and middle-IR: Recent investigations

1. Water vapour continuum absorption in near- and middle-IR: Recent investigations Department of Meteorology, University of Reading: Igor Ptashnik, David Paynter, Keith Shine MSF, Rutherford Appleton Laboratory: Robert McPheat, Kevin Smith, Gary Williams Annual CAVIAR meeting, 16.12.2008, Imperial College London

2.   + 2 Recent CAVIAR measurements at MSF RAL:1600 - 7200 cm-1 Simulated spectra for 15 mbar pure water vapour and water dimers According to the S&K ab initio calculations and pressurised steam measurementsmost pronounced water dimer features should be expected within strongest vibrational water vapour bands Annual CAVIAR meeting, 16.12.2008, Imperial College London

3. Transmittance Optical depth Recent CAVIAR measurements at MSF RAL: 1600 cm-1 H2O band Annual CAVIAR meeting, 16.12.2008, Imperial College London

4. Recent CAVIAR measurements at MSF RAL: 1600 cm-1 H2O band (RAL) Annual CAVIAR meeting, 16.12.2008, Imperial College London

5. MSF RAL Line-by-line (Hitran+CKD) MT_CKD continuum model 17.3 mb H2O (150.33 mb total) Recent CAVIAR measurements at MSF RAL: 1600 cm-1 H2O band Annual CAVIAR meeting, 16.12.2008, Imperial College London

6. Threshold absorption Threshold gradient Recent CAVIAR measurements at MSF RAL Water vapour continuum retrieval from experiment: < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 Annual CAVIAR meeting, 16.12.2008, Imperial College London

7. Recent CAVIAR measurements at MSF RAL(1600 cm-1 band) Water vapour continuum retrieval: < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 First high-resolution measurements of the continuum in this spectral region Annual CAVIAR meeting, 16.12.2008, Imperial College London

8. Recent CAVIAR measurements at MSF RAL(3700 cm-1 band) Water vapour continuum retrieval: < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 Annual CAVIAR meeting, 16.12.2008, Imperial College London

9. Recent CAVIAR measurements at MSF RAL(3700 cm-1 band) Water vapour continuum retrieval: < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 Y. Bouteiller and J.P. Perchard (Chem. Phys., 2004) S. Kuma, M. Slipchenko, T. Momose, A. Vilesov (Chem. Phys. Lett., 2007) Annual CAVIAR meeting, 16.12.2008, Imperial College London

10. Recent CAVIAR measurements at MSF RAL(5300 cm-1 band) Water vapour continuum retrieval: < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 1(PD)+2(PD)2(PA)+3(PA) 1(PA)+2(PA)2(PD)+3(PD) Annual CAVIAR meeting, 16.12.2008, Imperial College London

11. Recent CAVIAR measurements at MSF RAL(7200 cm-1 band) Water vapour continuum retrieval: < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 21(PA) 1+3(PD) 21(PD)1+3(PA) 23(PD) T. Salmi, V. Hanninen, A. Garden, H. Kjaergaard, J. Tennyson, L. Halonen,J. Phys. Chem. (2008) Annual CAVIAR meeting, 16.12.2008, Imperial College London

12. Recent CAVIAR measurements at MSF RAL: Water continuum in band wings First estimation of the continuum absorption was made in the far band wings. However, uncertainty is still too big and further work is required. Annual CAVIAR meeting, 16.12.2008, Imperial College London

13. Far wings of water monomer lines OR water dimers: 40 years anniversary… Ma, Tipping, Leforestier, J. Chem. Phys. (2008) "...In practice, a measured continuum value results from a subtraction between two very large quantities i.e., a raw measurement of the absorption coefficient and a calculated local line contribution ...... In our opinion, the inapplicability of the Lorentzian line shapes in the near-wing region (~25-30 cm-1) could cause large uncertainties in the measured continuum..." "...Therefore one should not consider this inapplicability within the bands as strong a priori evidence to support either the dimer theory or collision-induced absorption mechanism..." "... In any case, before one can claim that the continuum is due mainly to dimers or to collision-induced absorption, one must make accurate theoretical calculations of both the magnitudes and the T-dependencies of these mechanisms." Annual CAVIAR meeting, 16.12.2008, Imperial College London

14. Far wings of water monomer lines OR water dimers: Water monomer lines contribution Lorentzian contribution from water monomer lines within investigated microwindows is much less then detected continuum features. Inside the band these microwindows are often situated within few cm-1 from the strongest WM lines, that is, too close to the line centres to expect such huge deviation from Lorentz profile. Annual CAVIAR meeting, 16.12.2008, Imperial College London

15. Water dimer:Equilibrium constant Keq(T) PWD = PH2O .Keq(T) Annual CAVIAR meeting, 16.12.2008, Imperial College London

16. Temperature dependence of WD absorption within bands The temperature dependence of the equilibrium constantKeq(T) seems enough to setup reasonable quantitative relationship between the magnitude of the in-band continuum (dimer) features within very broad temperature region (300 - 650K)... ...which implies relatively weak temperature dependence of the other terms describing intensity of the strongest water dimer spectral lines. Annual CAVIAR meeting, 16.12.2008, Imperial College London

17. 296 K 311 K 326 K 363 K Band wings: Ma & Tipping ab initio continuum model 296 K (Ma&Tipping) 311 K 326 K 363 K Both MT_CKD and M&T continuum model deviates up to 30-40% from experimental continuum in middle-IR at temperatures 296 - 363 K. Annual CAVIAR meeting, 16.12.2008, Imperial College London

18. Band wings: Ma & Tipping ab initio continuum model Ma, Tipping, Leforestier (J. Chem. Phys. 2008) Good agreement is shown with experimental T-dependence at 944 cm-1… however… Cormier, Hodges, Drummond (J. Chem. Phys. 2005) Have shown markedly weaker T-dependence of Ma&Tipping continuum model as compared with the experiment at 944.2 cm-1.No any comments are given… Annual CAVIAR meeting, 16.12.2008, Imperial College London

19. Band wings: Ma & Tipping and CKD continuum models k(,T) = ko() eTo(1/To - 1/296) (Ma & Tipping) Both MT_CKD and M&T continuum model can not explain spectral variation of T-dependence observed in experiment. Annual CAVIAR meeting, 16.12.2008, Imperial College London

20. Recent ab initio prediction for WD in middle and far-IR • Y. Scribano and C. Leforestier, J. Chem. Phys. (2007) • "... the role of water dimers decreases with frequency, to become minimal (3%) at the highest frequency considered 944 cm−1..." "...apart from the millimeter range, the far wing hypothesis seems to prevail as the origin of the water continuum..." • M. Lee, F. Baletto, D. Kanhere, and S. Scandolo, J. Chem. Phys. (2008) • "...the calculated far-IR absorption due to dimers is significantly smallerthan the observed water vapor continuum..." "...Hence, it is highly likely that collisional broadening is the predominant mechanism responsible for the water vapor continuum..." Annual CAVIAR meeting, 16.12.2008, Imperial College London

21. Conclusion • There are obtained firm experimental evidence to assert significant water dimer contribution to the water vapour self-continuum within near-IR water vapour bands. • The question about the band wings is still open and is one of the main priorities for the current CAVIAR investigations. • Neither modern ab initio theory (WM far wings nor dimer) can fully explain experimental continuum absorption, especially in band wings. However, dimer theory seems better in explaining T-dependence of the continuum and its spectral variation. • Sophisticated ab initio calculations of water dimer spectra at room-temperatures are necessary and should be performed within CAVIAR grant. Annual CAVIAR meeting, 16.12.2008, Imperial College London