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
I’m Igor Ptashnik from Met. Dept Univ. of Reading. I would like to tell you today about the main results that were obtained during recent 6 years of our collaboration with MSF RAL.
The main point of the investigations, that were funded by NERC, concerned water vapour and water vapour continuum. Though, I think there is no especial need in this auditory to explain why water vapour is so important in the earth atmosphere, still I will show a few visual pictures just to remind you the main points.
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 ( K, 0-5 atm) with good characterization of temperature and vapour amount
The MSF facility at RAL give as access to state-of –the-art instrumentation like high-res spectrometer with widest wavenumber coverage$ absorption cell with wide range of temperature and pressure conditions available for measurements and so on. Here is the time for Robert to take the floor, as he knows about this things much better than me.

3. Our debut in continuum studies: 5000-5600 cm-1
/ NERC Grant on water vapour ( ) /
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 within strongest vibrational water vapour bands (not in band wings investigated most thoroughly so far!) 3

4. Continuum measurements at MSF RAL (2003)
I. Ptashnik, K. Smith, K. Shine, D. Newnham, Q. J. R. Meteorol. Soc., v. 130, (2004)
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
And indeed, the spectral features, very similar to the S&K prediction, were clearly observed in the residual spectrum, derived as a difference between RAL measurement and calculated WM line absorption. These features could not be neither explained nor described by any modern continuum model. One of the main problems in deriving the residual was big uncertainty in WM line parameters.

5. Recent CAVIAR measurements at MSF RAL: 1200 - 8000 cm-1
+  2 
Kevin Smith,
Robert McPheat,
David Paynter
IFS 120HR,
IFS 125HR
Short-path cell
(up to 20m)
Long-path cell
(from 32 to 512m),
Pressures:
mbar
Temperatures: K
Spectral resolution:
0.3 – 0.001cm-1
Coming back to the present time, within CAVIAR grant new laborat. measur-s of the water vapour absorption were performed recently at RAL, which covered four strongest vibrational water bands, shown on this slide. Again, according to S&K theory we wxpwctwd to observe quite pronounced WD absorption features in this bands.

6. Retrieval of the water vapour continuum (1600 cm-1 H2O band)
Transmittance
Optical depth
The initial part of the data processing is quite routine. We derive transm. as a ratio of the signals, obtained from radiation passed through the cell with water vapour and through the empty cell. And the new derive optical depth of the water vapour as a logarithm of the transm.

7. Retrieval of the water vapour continuum
< RAL_measurement – Calculated_H2O_lines_contribution >1cm-1
cm-1
Threshold
gradient
Threshold absorption
The next slide shows experimental spectrum and WM spectrum, calculated this time without any continuum model. The difference between these two spectra (green line) is just what we call continuum absorption and how we derive it from experiment. To reduce the impact of the uncertainty in WM spectral line parameters, which causes this fluctuation in the residual near lince centres, we use additional filtering of the residual points, excluding those close to the line centres. Final continuum is derived by averaging in every 1 cm-1 spectral interval
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. Recent CAVIAR measurements in RAL
Retrieval of the water vapour self-continuum
1) Quadratic PH2O-dependence: Cs (PH2O)2
Roberts, Selby, Biberman (Appl.Opt., 1976):
2) Strong negative T-dependence:
MT_CKD continuum model (1989):
Vigasin (JQSRT, 2000), water dimer model:
Under investigation…
Converting to the
"continuum" units
and is shown on this slide. Here you can see actually two recent results, obtained from different RAL measurements. The are in a good agreement with each other and with the former measurements by Burch and by Tobin et al. However, in our case the continuum for the first time is obtained from high-res measurements. And the quality of the meas. and retrieving procedure gives more smooth and spectrally resolved picture of the continuum. Due to that, for example, we can see quite prominent spectral features, which are not described by CKD model, but are in a reasonable agreement with WD prediction by S&K for this fundam. WD band; and are actually in a very good agreement with the recent prediction by Victoria Buch, that unfortunatelly was not published so far, because of the serious illness of the author.
3) Well pronounced spectral features:

9. Water continuum and water dimers (1600-8000 cm-1)
3(PD),
1(PD) 3(PA)
2(PA),
2(PD)
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 9

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

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
 % 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. 12

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

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
and is shown on this slide. Here you can see actually two recent results, obtained from different RAL measurements. The are in a good agreement with each other and with the former measurements by Burch and by Tobin et al. However, in our case the continuum for the first time is obtained from high-res measurements. And the quality of the meas. and retrieving procedure gives more smooth and spectrally resolved picture of the continuum. Due to that, for example, we can see quite prominent spectral features, which are not described by CKD model, but are in a reasonable agreement with WD prediction by S&K for this fundam. WD band; and are actually in a very good agreement with the recent prediction by Victoria Buch, that unfortunatelly was not published so far, because of the serious illness of the author.
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 < 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. Stable or metastable dimers and free-pair CIA
cm-1
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-pair CIA (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. 18

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 states is 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." 19

20. Self-continuum in all near-IR windows exceeds MTCKD
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 large to 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): The role 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.