1 Annual CAVIAR meeting, 16.12.2008, Imperial College London
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 Slide 2 2
Annual CAVIAR meeting, 16.12.2008, Imperial College London Recent
CAVIAR measurements at MSF RAL: 1600 - 7200 cm -1 Simulated spectra
for 15 mbar pure water vapour and water dimers + 2 According to the
S&K ab initio calculations and pressurised steam measurements
most pronounced water dimer features should be expected within
strongest vibrational water vapour bands Slide 3 3 Annual CAVIAR
meeting, 16.12.2008, Imperial College London Recent CAVIAR
measurements at MSF RAL: 1600 cm -1 H 2 O band Transmittance
Optical depth Slide 4 4 Annual CAVIAR meeting, 16.12.2008, Imperial
College London (RAL) Recent CAVIAR measurements at MSF RAL: 1600 cm
-1 H 2 O band Slide 5 5 Annual CAVIAR meeting, 16.12.2008, Imperial
College London Recent CAVIAR measurements at MSF RAL: 1600 cm -1 H
2 O band 17.3 mb H 2 O (150.33 mb total) MSF RAL Line-by-line
(Hitran+CKD) MT_CKD continuum model Slide 6 6 Annual CAVIAR
meeting, 16.12.2008, Imperial College London Recent CAVIAR
measurements at MSF RAL Water vapour continuum retrieval from
experiment: 1cm -1 Threshold gradient Threshold absorption Slide 7
7 Annual CAVIAR meeting, 16.12.2008, Imperial College London Recent
CAVIAR measurements at MSF RAL (1600 cm -1 band) Water vapour
continuum retrieval: 1cm -1 First high-resolution measurements of
the continuum in this spectral region Slide 8 8 Annual CAVIAR
meeting, 16.12.2008, Imperial College London Recent CAVIAR
measurements at MSF RAL (3700 cm -1 band) Water vapour continuum
retrieval: 1cm -1 Slide 9 9 Annual CAVIAR meeting, 16.12.2008,
Imperial College London Recent CAVIAR measurements at MSF RAL (3700
cm -1 band) Water vapour continuum retrieval: 1cm -1 Y. Bouteiller
and J.P. Perchard (Chem. Phys., 2004) S. Kuma, M. Slipchenko, T.
Momose, A. Vilesov (Chem. Phys. Lett., 2007) Slide 10 10 Annual
CAVIAR meeting, 16.12.2008, Imperial College London Recent CAVIAR
measurements at MSF RAL (5300 cm -1 band) Water vapour continuum
retrieval: 1cm -1 1 (PD)+ 2 (PD) 2 (PA)+ 3 (PA) 1 (PA)+ 2 (PA) 2
(PD)+ 3 (PD) Slide 11 11 Annual CAVIAR meeting, 16.12.2008,
Imperial College London Recent CAVIAR measurements at MSF RAL (7200
cm -1 band) Water vapour continuum retrieval: 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) Slide
12 12 Annual CAVIAR meeting, 16.12.2008, Imperial College London
First estimation of the continuum absorption was made in the far
band wings. However, uncertainty is still too big and further work
is required. Recent CAVIAR measurements at MSF RAL: Water continuum
in band wings Slide 13 13 Annual CAVIAR meeting, 16.12.2008,
Imperial College London 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." Slide 14 14 Annual CAVIAR
meeting, 16.12.2008, Imperial College London 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. Far wings of
water monomer lines OR water dimers: Water monomer lines
contribution Slide 15 15 Annual CAVIAR meeting, 16.12.2008,
Imperial College London Water dimer: Equilibrium constant K eq (T)
P WD = P H2O. K eq (T) Slide 16 16 Annual CAVIAR meeting,
16.12.2008, Imperial College London Temperature dependence of WD
absorption within bands The temperature dependence of the
equilibrium constant K eq (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. Slide 17 17 Annual CAVIAR meeting,
16.12.2008, Imperial College London Band wings: Ma & Tipping ab
initio continuum model Both MT_CKD and M&T continuum model
deviates up to 30 - 40% from experimental continuum in middle-IR at
temperatures 296 - 363 K. 326 K 363 K 311 K 296 K 311 K 326 K 363 K
(Ma&Tipping) Slide 18 18 Annual CAVIAR meeting, 16.12.2008,
Imperial College London 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 Band wings: Ma & Tipping ab initio continuum
model Slide 19 19 Annual CAVIAR meeting, 16.12.2008, Imperial
College London (Ma & Tipping) Band wings: Ma & Tipping and
CKD continuum models k(,T) = k o ( ) e T o (1/T o - 1/296) Both
MT_CKD and M&T continuum model can not explain spectral
variation of T-dependence observed in experiment. Slide 20 20
Annual CAVIAR meeting, 16.12.2008, Imperial College London 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 smaller than the observed
water vapor continuum..." "...Hence, it is highly likely that
collisional broadening is the predominant mechanism responsible for
the water vapor continuum..." Slide 21 21 Annual CAVIAR meeting,
16.12.2008, Imperial College London 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.
