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1 Water vapour self-continuum within bands: Arguments for dimers Igor Ptashnik 1,2, David Paynter 1, Keith Shine 1 Robert McPheat 3, Kevin Smith 3, Gary Williams 3 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 ) Slide 2 2 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 Laboratory FTS measurements at RAL (2003-2009) Slide 3 3 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!) Our debut in continuum studies: 5000-5600 cm -1 / NERC Grant on water vapour (2001-2003) / Slide 4 4 Continuum measurements at MSF RAL (2003) I. Ptashnik, K. Smith, K. Shine, D. Newnham, Q. J. R. Meteorol. Soc., v. 130, 2391-2408 (2004) Measurements: 10 m, 98 mbar H 2 O, 342 K (heated cell) 128 m, 20 mbar H 2 O, 299 K (long path) Data processing: Measurement HITRAN _ & _ M&T -continuum P Dimer = K eq (T) P H2O Slide 5 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: 20 - 280 mbar Temperatures: 293 - 350K Spectral resolution: 0.3 0.001cm -1 Slide 6 6 Retrieval of the water vapour continuum (1600 cm -1 H 2 O band) Transmittance Optical depth Slide 7 7 Retrieval of the water vapour continuum 1cm -1 cm -1 Threshold gradient Threshold absorption 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. Slide 8 8 Recent CAVIAR measurements in RAL Retrieval of the water vapour self-continuum 1) Quadratic P H2O -dependence: C s (P H2O ) 2 3) Well pronounced spectral features: Converting to the "continuum" units Roberts, Selby, Biberman (Appl.Opt., 1976): 2) Strong negative T-dependence: MT_CKD continuum model (1989): Vigasin (JQSRT, 2000), water dimer model: Under investigation Slide 9 9 Water continuum and water dimers (1600-8000 cm -1 ) 2 (PA), 2 (PD) 3 (PD), 1 (PD) 3 (PA) 1 (PD)+ 2 (PD) 2 (PA)+ 3 (PA) 1 (PA)+ 2 (PA) 2 (PD)+ 3 (PD ) 2 1 (PA) 1 + 3 (PD) 2 1 (PD) 1 + 3 (PA) 2 3 (PD) First high spectral resolution measurement Slide 10 10 Water continuum and water dimers (1600-8000 cm -1 ) 1cm -1 Bouteiller & Perchard (2004) intensities are scaled (factor 0.52) using Slipchenko, Kuyanov, Sartakov, Vilesov (2006) absolute intensity measurement of WD trapped in He droplets. 1 (PD) 3 (PD), 3 (PA) 2 2 (PD) inter + intra Vigasin, Jin & Ikawa (Molec. Phys., 2008) Slide 11 11 Water continuum and water dimers (1600-8000 cm -1 ) 50-100% error is required to explain this deviation from MTCKD What is the possible impact of the error in line parameters? From 100 to 300% error is required to explain deviation from MTCKD Slide 12 12 From 20 to >100% error in the intensities and self-widths of the strongest H 2 O lines in HITRAN-2008 is required to explain deviation from MT_CKD in the most spectral intervals within bands. Possible impact of the error in line parameters 1cm -1 Slide 13 13 Line parameters fitting from 5mm MSF RAL measurements 1300 - 2000 cm -1 3400 - 4000 cm -1 Possible impact of the error in line parameters Slide 14 14 Temperature dependence of the self-continuum Slide 15 15 Temperature dependence of the self-continuum Slide 16 16 Recent CAVIAR measurements in MSF RAL Temperature dependence of the continuum Paynter, Ptashnik, Shine, Smith, McPheat, Williams, JGR-2009 Slide 17 17 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 H 2 O unit are very close to the respected WM bands (shift < 10 - 15 cm -1 ), while oscillations in the donor H 2 O 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). Water dimer and monomer's spectral features Slide 18 18 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 CO 2 " (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). cm -1 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 CO 2 pairs. Slide 19 19 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." Stable or metastable dimers and free-pair CIA Slide 20 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! Slide 21 21 1.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.). 2.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. 3.Observed spectral features are often shifted towards smoothed water monomer spectra within bands. 4.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. 5.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. 6.Water vapour self-continuum in near-IR windows is about an order of magnitude stronger than is predicted by MT_CKD model. Conclusion