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

Natural water dimers absorption: known and unknown experimentWater vapour continuum absorption
in near- and middle-IR:
MSF, Rutherford Appleton Laboratory:
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Recent CAVIAR measurements at MSF RAL: 1600 - 7200 cm-1
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Recent CAVIAR measurements at MSF RAL:
1600 cm-1 H2O band
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(RAL)
1600 cm-1 H2O band
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1600 cm-1 H2O band
Annual CAVIAR meeting, 16.12.2008, Imperial College London
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Recent CAVIAR measurements at MSF RAL
Water vapour continuum retrieval from experiment:
< RAL_measurement – Calculated_H2O_lines_contribution >1cm-1
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Recent CAVIAR measurements at MSF RAL (1600 cm-1 band)
Water vapour continuum retrieval:
First high-resolution measurements of the continuum in this spectral region
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Y. Bouteiller and J.P. Perchard (Chem. Phys., 2004)
S. Kuma, M. Slipchenko, T. Momose, A. Vilesov (Chem. Phys. Lett., 2007)
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21(PD) 1+3(PA) 23(PD)
T. Salmi, V. Hanninen, A. Garden, H. Kjaergaard, J. Tennyson, L. Halonen, J. Phys. Chem. (2008)
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First estimation of the continuum absorption was made in the far band wings. However, uncertainty is still too big and further work is required.
Water continuum in band wings
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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."
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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
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Water dimer: Equilibrium constant Keq(T)
PWD = PH2O . Keq(T)
The temperature dependence
Keq(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
water dimer spectral lines.
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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
(Ma&Tipping)
296 K
311 K
326 K
363 K
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Ma, Tipping, Leforestier
with the experiment at 944.2 cm-1.
No any comments are given…
Band wings: Ma & Tipping ab initio continuum model
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(Ma & Tipping)
k(,T) = ko() eTo (1/To - 1/296)
Both MT_CKD and M&T continuum model can not explain
spectral variation of T-dependence observed in experiment.
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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..."
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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.
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7100
7200
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7500
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Keq=0.04 atm
MSF RAL (2007), 293K
2000
3000
4000
5000
6000
7000
10
-25
10
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10
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10
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All continuums (except Burch data) are with the 'Base term' subtracted !!!
Wavenumber, cm
Burch (1981) 350K
2000
3000
4000
5000
6000
7000
10
-25
10
-24
10
-23
10
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All continuums (except Burch data) are with the 'Base term' subtracted !!!
Wavenumber, cm
Burch (1981) 350K
cimulate homodginious path
Z[km] P[mbar] T[K] H2O[ppmv] DIMER[ppmv] Keq=0.041 (M.Caro-Curtiss for 293K)
0.000 .200E+02 293.0 .100E+07 .809E+03
0 ! M_RUN- Multi run flag: 0- one LBL run; 1- two LBL runs (second time uses lbl.in2)
0 0 ! ITR- Path's kind. (0-hor.; 1-vert.); IREDZ (1-RED.,0-NO)
0. 104. ! Z1, Z2 (Z2-IF ITR=1) - heights [KM]
1.00e-5 ! LEN - Length of horizontal path [KM]
0. ! TETA - Zenith angle [GRAD]
10. 10000. 0 ! FREQ1, FREQ2; Freq. units: 0-[CM-1], 1-[MKM], 2-[NM]
25. 0.001 0 ! [CM-1] WINGs; H-MIN.SETKA (IF IAVTH=0, AUTOMAT. IF>0)
0.001 0 ! [CM-1] RES.STEP; ISRES=0 -USR, >0 H*ISRES; -1 -'FILTER'
2.0 0 ! [CM-1] OUT STEP; ISOUT: 0-USER, >0- OUT=H*ISOUT;
1 25. ! NG- No.of used gases; H2O_DIM >0 - H2O dimer HWHM instead of HCl (hit.ind.15)
15 2 7 15 4 5 6 7 22 4 6 15 3 7 4 5 6 7 15 9 11 12 10 ! NGAS(NG) - gases indexes
C:
HITRAN_2004
0 0 ! CONTINUUM CKD24 1-ON, 0-OFF (H2O,CO2,O3,O2,N2); IONLY_CONT. 1-ON, 0-OFF
1 0 ! OUT: 1-OPT.DEPTH, 2-ABS., 3-TRANS.; IOPT_ML: 1-MULTILAYER OPT.DEPTH OUT
1 ! INTERPOLATION: 1-LINEAR; 2-KVADRAT.
MSF RAL (2007)
H2O lines (Voigt)
Dimers / Continuum
C:HITRAN_200401_hit06_02_07_22_hit04+dimer2008-2003.par !
0 ! ISEL - SELECTION: 1 - ON, 0 - OFF
1.e-9 1.e-11 ! TAU0 TAU01 (IF ISEL=1)
3 ! READ LINES: 3-HITRAN-2004; 2-HITRAN96; 1-HITRAN91; 0-FROM LBL_ATL.DAT
0 0 ! output: if 0- uniform out step, seted above (OUT STEP);
Wavenumber (cm
0.0
0.2
0.4
0.6
0.8
1.0
10020030040050060070080090010001100
10
-23
10
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10
-21
10
-20
Dimers / Continuum
1.e-9 1.e-11 ! TAU0 TAU01 (IF ISEL=1)
Wavenumber (cm
Dimers / Continuum
1.e-9 1.e-11 ! TAU0 TAU01 (IF ISEL=1)
Wavenumber (cm
0.0
0.2
0.4
0.6
0.8
1.0
10020030040050060070080090010001100
10
-23
10
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10
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10
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Dimers / Continuum
1.e-9 1.e-11 ! TAU0 TAU01 (IF ISEL=1)
Wavenumber (cm
conditions to simulate homoginious path
Z[km] P[mbar] T[K] H2O[ppmv] DIMER[ppmv]
(Keq=0.0375, Curtiss)
Dimers HWHM = 25, 20, 20 cm
-1
respectively
/Path length = 10 m; Pure H2O (15 mbar), T=293K/
Optical depth
CKD-2.4 model
Transmittance
[cm
-1
Transmittance
120013001400150016001700180019002000
0
1
2
3
4
5
Optical depth
MT_CKD continuum
Optical depth
MT_CKD continuum
Optical depth
LBL (Hit04 lines without 25 cm base)
RAL continuum (RAL - LBL)
McPheat et al. (RAL, 2007) /295K, 128m/
1300140015001600170018001900
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0.4
0.6
0.8
1.0
Burch, 1981 /308K/
Keq=0.04 atm

1300140015001600170018001900
0.0
0.2
0.4
0.6
0.8
1.0
Burch, 1981 /308K/
Keq=0.04 atm

Keq=0.04 atm
0.000 .200E+02 293.0 .100E+07 .809E+03
C:
HITRAN_2004
MSF RAL (2007)
Ikawa
0.000 .200E+02 293.0 .100E+07 .809E+03
C:
HITRAN_2004
MSF RAL (2007)
Ikawa
Ikawa
Keq=0.04 atm
Everything (excluding Burch data) is with the 'Base term' subtracted !!!
Wavenumber, cm
C
s
, 10
-21
cm
2
*molec
-1
atm
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MSF RAL (2007), 293K
Fitting to Burch (1985)
Transmittance
[cm
-1
Transmittance
120013001400150016001700180019002000
0
1
2
3
4
5
Optical depth
MT_CKD continuum
Optical depth
MT_CKD continuum

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

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