Water vapour self-continuum within bands: Arguments for dimers

1. Water vapour self-continuum within bands: Arguments for dimers Igor Ptashnik1,2, David Paynter1, Keith Shine1 Robert McPheat3, Kevin Smith3, Gary Williams3 1 Department of Meteorology, University of Reading (UK) 2 Institute of Atmospheric Optics, RAS, Russia 3 MSF, Rutherford Appleton Laboratory (UK) Special thanks to Andrey Vigasin for useful discussion Funded by National Environment Research Council (NERC) UK, Engineering and Physical Sciences Research Council, and in part by Russian Fund for Basic Research (RFBR)

2. Laboratory FTS measurements at RAL (2003-2009) • High spectral resolution Bruker IFS 125HR spectrometer covers from UV to microwave, and captures several water vapour bands simultaneously • Absorption cell allows wide range of conditions (100-400 K, 0-5 atm) with good characterization of temperature and vapour amount

3. Our debut in continuum studies: 5000-5600 cm-1 / NERC Grant on water vapour (2001-2003) / Simulated spectra for 15 mbar pure water vapour and water dimers According to the S&K ab initio calculations most pronounced water dimer features could be expected withinstrongest vibrational water vapour bands (not in band wings investigated most thoroughly so far!)

4. I. Ptashnik, K. Smith, K. Shine, D. Newnham,Q. J. R. Meteorol. Soc., v. 130, 2391-2408 (2004) Continuum measurements at MSF RAL (2003) Measurements: 10 m, 98 mbar H2O, 342 K (heated cell) 128 m, 20 mbar H2O, 299 K (long path) Data processing: Measurement – HITRAN_&_M&T-continuum PDimer=Keq(T)PH2O

5. Recent CAVIAR measurements at MSF RAL:1200 - 8000 cm-1 Kevin Smith, Robert McPheat, David Paynter IFS 120HR, IFS 125HR Short-path cell (up to 20m) Long-path cell (from 32 to 512m),   + 2 Pressures: 20 - 280 mbar Temperatures: 293 - 350K Spectral resolution: 0.3 – 0.001cm-1

6. Transmittance Optical depth Retrieval of the water vapour continuum (1600 cm-1 H2O band)

7. Threshold absorption Threshold gradient Retrieval of the water vapour continuum < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 cm-1 1) Continuum is derived only in microwindows, where it is comparable with the contribution of the local water vapour lines. So, it is not “a small difference between two big values (Ma et al., 2008)! 2) Filtering of the final data is very important procedure.

8. 2) Strong negative T-dependence: Roberts, Selby, Biberman(Appl.Opt., 1976): MT_CKD continuum model (1989): Vigasin (JQSRT, 2000), water dimer model: Under investigation… Recent CAVIAR measurements in RAL Retrieval of the water vapour self-continuum 1) Quadratic PH2O-dependence:Cs (PH2O)2 Converting to the "continuum" units 3) Well pronounced spectral features:

9. Water continuum and water dimers(1600-8000 cm-1) 2(PA), 2(PD) 3(PD), 1(PD)3(PA) 21(PA) 1+3(PD) 21(PD)1+3(PA)23(PD) 1(PD)+2(PD)2(PA)+3(PA) 1(PA)+2(PA)2(PD)+3(PD) First high spectral resolution measurement

10. Water continuum and water dimers(1600-8000 cm-1) < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 Vigasin, Jin & Ikawa (Molec. Phys., 2008) 22(PD) 1(PD)3(PD), 3(PA) inter + intra Bouteiller & Perchard(2004) intensities are scaled (factor 0.52) using Slipchenko, Kuyanov, Sartakov, Vilesov(2006) absolute intensity measurement of WD trapped in He droplets.

11. Water continuum and water dimers(1600-8000 cm-1) What is the possible impact of the error in line parameters? From 100 to 300% error is “required” to explain deviation from MTCKD  50-100% error is “required” to explain this deviation from MTCKD

12. Possible impact of the error in line parameters < RAL_measurement – Calculated_H2O_lines_contribution >1cm-1 Ptashnik, Smith, Shine, JMS-2005 Jenouvrier et al., JQSRT-2007 From 20 to >100% error in the intensities and self-widths of the strongest H2O lines in HITRAN-2008 is required to explain deviation from MT_CKD in the most spectral intervals within bands.

13. Possible impact of the error in line parameters Line parameters fitting from 5mm MSF RAL measurements 1300 - 2000 cm-1 3400 - 4000 cm-1

14. Temperature dependence of the self-continuum

15. Temperature dependence of the self-continuum

16. Recent CAVIAR measurements in MSF RAL Temperature dependence of the continuum Paynter, Ptashnik, Shine, Smith, McPheat, Williams, JGR-2009

17. Water dimer and monomer's spectral features Collisionally and predissociatively broadened lines of water dimers overlap, producing broad continuum-like sub-bands, replicating to some degree smoothed spectrum of water monomers. Positions of WD sub-bands caused by quasi-free oscillations in acceptor H2O unit are very close to the respected WM bands (shift < 10 - 15 cm-1), while oscillations in the donor H2O unit in WD may have quite large shift (up to 100 cm-1) from the respected smoothed water monomer spectrum. Contribution from the metastable dimers must form broad and unstructured underlying "basement" under the true dimer's sub-band structure due to very short lifetime (lifetime broadening).

18. cm-1 Stable or metastable dimers and free-pair CIA Vigasin, Kluwer (2003): Fig. 6. "CIA spectrum in the region of the Fermi doublet of low temperature CO2" (from Vigasin et al., J.Mol.Spectr., 2002). Total CIA(1); base profile, caused by metastable and free-pairCIA(2); and true dimer profile (3). The MT_CKD model: In-band continuum is caused by Collision-Induced Absorption. (But which component? Free-pairs collisions or stable or/and metastable dimers?) A. Vigasin(Kluwer, 2003): Fig. 4. Partitioning of the normalized CIA intensity in the phase space of CO2 pairs.

19. Stable or metastable dimers and free-pair CIA A. Vigasin(Kluwer, 2003): "Preliminary partitioning of the pair states in water vapor showed that the role of free pair statesis almost negligible at near room temperatures; metastable and true bound states should dominate instead. This is not surprising since the interaction between water molecules is at least three times stronger than that between carbon dioxide molecules."

20. Continuum in the near-IR windows from the recent MSF RAL measurements Self-continuum in all near-IR windows exceeds MTCKD by order of magnitude!

21. Conclusion • Derived continuum features are too largeto be explained by possible uncertainties/errors in parameters of the water vapour lines in HITRAN (especially in 3600 cm-1 band) or by any known deviation from Voigt profile (Dicke narrowing, Line mixing etc.). • The main spectral features of the derived continuum coincide reasonably well with known ab initio predictions and low-T matrix measurements of water dimers fundamental and combinational bands, and are not described by any existing continuum model. • Observed spectral features are often shifted towards smoothed water monomer spectra within bands. • Observed continuum has strong negative T-dependence, which is stronger within observed spectral features, and in a wide T-region is in a good agreement with that expected from dimer theory. • Partitioning of the pair states in phase space (Epifanov & Vigasin, 1997; Schenter et al., 2002): Therole of free pair states is almost negligible at near room temperatures; metastable and true bound states should dominate instead. • Water vapour self-continuum in near-IR windows is about an order of magnitude stronger than is predicted by MT_CKD model.