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Yu. I. BARANOV, Yu. I. BARANOV, W. J. LAFFERTY, and G. T. Fraser W. J. LAFFERTY, and G. T. Fraser
Optical Technology DivisionOptical Technology DivisionNational Institute of Standards and Technology,National Institute of Standards and Technology,
Gaithersburg, MD 20899-8441, USAGaithersburg, MD 20899-8441, USA
The water-vapor continuum and selective The water-vapor continuum and selective absorption in the 8 to 12 absorption in the 8 to 12 μm and 3 to 5 μm μm and 3 to 5 μm windows at temperatures from 311 to 363Kwindows at temperatures from 311 to 363K..
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Introduction
The water vapor continuum absorption in the atmospheric 8 to 12 and 3 to 5 μm windows strongly affects the Earth’s outgoing and the Sun’s incoming radiation and therefore is of great importance for radiative balance calculations.
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Introduction
Increasing use of lasers, spectrometers, and other IR techniques in atmospheric research, remote sensing, and environment protection also requires more precise data on water vapor continuum absorption coefficients
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Introduction
Over the past twenty years many scientific groups in the world have used long-base (up to 100 m) long-path (up to several thousands m) cells to measure the H2O continuum.
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Introduction
The other high-sensitive techniques, like photo-acoustic or cavity ring-down
spectroscopy (CRDS) have also been employed for these measurements.
Revised and selected data were put on the basis of the CKD(a) continuum model, widely used for atmospheric spectroscopy applications.
a S. A. Clough, F. X. Kneizys, and R. W. Davies, Atmos. Res. 23, 229-241 (1989).
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Experimental set-up view
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Experimental conditionsSpectral resolution is 0.1 cm-1
Spectral range 800 to 3500 cm-1
TemperatureK (±0.3K)
Pressure rangekPa (torr)
Path lengthm
Number of spectra
310.8 2.83 to 6.07 (21.2 to 45.5)
68-116 41
318.0 3.40 to 7.42 (25.5 to 55.7)
84-116 46
325.8 4.49 to 11.5 (33.7 to 86.3)
76-116 48
339.3 5.21 to 12.3 (39.1 to 92.0)
84-108 51
351.9 5.76 to 15.1 (43.2 to 113) 84-116 51
363.6 5.48 to 13.7 (41.1 to 103) 84-116 36
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An example of IR water vapor spectrum
0
0.2
0.4
0.6
0.8
1
800 900 1000 1100 1200 1300 1400
Tra
ns
mit
tan
ce
Continuum absorption
The v2 watervapor fundamental
band
0
0.2
0.4
0.6
0.8
1
1750 1950 2150 2350 2550 2750 2950
Wavenumber, cm-1
Tra
ns
mit
tan
ce
Res=0.1 cm-1
T=339.2KP=89.5 torrL=9205 cm
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The small part of four spectra at different pressures
0.5
0.6
0.7
0.8
0.9
1
940 942 944 946 948 950
Wavenumber, cm-1
Tra
ns
mit
tan
ce
T=318KL=108mP, torr28.735.443.353.9
The quick data treatment methodMeasured absorbance versus
density square
0
1
2
3
4
0 0.005 0.01
Density, amagat2
(-ln
T/L
)*1
05,
cm
-1
318K326K339K
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The basic data treatment method
0.2
0.4
0.6
0.8
1
800 900 1000 1100 1200 1300
Wavenumber, cm-1
Tra
ns
mit
tan
ce
0.4
0.6
0.8
1
1920 2020 2120 2220
Wavenumber, cm-1
Tra
ns
mit
tan
ce
Two spectra at:
Θ=318KL=116mP=51.2 torr
Θ=352KL=116mP=111.9 torr
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The basic data treatment method
,,, structcontobs TTT ,
','',
dTfT linstruct
,25
,
,
,exp,
2222 P
P
P
PSPLT
ii
i
ii
i
i
ilin
Here f(ν-ν’) – instrumental function; L – a path length;
P, Θ – water vapor pressure, and temperature; Si, γi and νi – line parameters from the HITRAN data base; the sum is taken over all lines in a spectral range ν ± 25 cm-1
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The basic data treatment method
0.2
0.4
0.6
0.8
1
800 900 1000 1100 1200 1300
Wavenumber, cm-1
Tra
ns
mit
tan
ce
0.4
0.6
0.8
1
1920 2020 2120 2220
Wavenumber, cm-1
Tra
ns
mit
tan
ce
Two spectra at:
Θ=318KL=116mP=51.2 torr
Θ=352KL=116mP=111.9 torr
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The basic data treatment method
Two spectra at:
Θ=318KL=116mP=51.2 torr
Θ=352KL=116mP=111.9 torr
0.2
0.4
0.6
0.8
1
800 900 1000 1100 1200 1300
Wavenumber, cm-1
Tra
ns
mit
tan
ce
0.4
0.6
0.8
1
1920 2020 2120 2220
Wavenumber, cm-1
Tra
ns
mit
tan
ce
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The basic data treatment method
0.2
0.4
0.6
0.8
1
800 900 1000 1100 1200 1300
Wavenumber, cm-1
Tra
ns
mit
tan
ce
0.4
0.6
0.8
1
1920 2020 2120 2220
Wavenumber, cm-1
Tra
ns
mit
tan
ce
2
1ln
ji
ij
ba
LT
jijT ,
Every data arrayfor a giventemperature Θ
was fittedby function:
using standardleast squaremethod.
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Water vapor continuum binary absorptioncoefficients Cs in cm-1(mol/cm3)-1atm-1
compared with CKD model values
Wavenumber, cm-1
0.0E+00
5.0E-23
1.0E-22
1.5E-22
2.0E-22
2.5E-22
3.0E-22
3.5E-22
800 900 1000 1100 1200 1300 1920 2020 2120 2220 2320
The CKD values are shown as solid lines
310.8K325.8K351.9K
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The temperature dependence of the continuum binary absorption coefficient at 942 cm-1
0
1
2
3
4
270 290 310 330 350 370 390
Temperature, K
Cs*
10
22, c
m-1
/(m
ole
c*c
m-3
)
Nordstrom et al.,1978, (CO2-laser, White cell)Peterson et al., 1979, (CO2-laser, White cell)Eng and Mantz, 1980, (diode laser, White cell)Burch et al., 1982, (spectrometer, White cell)Loper et al., 1983, (CO2-laser, spectrophone)Hinderling, 1987, (CO2-laser, spectrophone)Cormier et al., 2005, (CO2-laser, CRDS)NIST 2006, (spectrometer, White cell)Clough, CKD model
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The temperature dependence of the continuum binary absorption coefficient at 1203 cm-1
0
0.3
0.6
0.9
1.2
1.5
290 340 390 440
Temperature, K
Cs*1
022 ,
cm
-1/(
mo
lec*c
m-3
)
Montgomery, 1978Burch et al., 1982NIST, 2006Clough CKD model
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The temperature dependence of the continuum binary absorption coefficient
1.00
1.50
2.00
2.50
3.00
3.50
2.40 2.60 2.80 3.00 3.20 3.40
ln(C
s*1
023)
850 cm-1
944 cm-1
1100 cm-1
1200 cm-1
The first and the last big figures arereproduced from AFGL-TR-81 (Burch D. E.)
0.30
0.80
1.30
1.80
2.30
2.80
3.30
2.60 2.80 3.00 3.20 3.40
ln(C
s*1
023)
1930 cm-1
1973 cm-1
2050 cm-1
2143 cm-1
Temperature, 1000/Θ, K-1 Temperature, 1000/Θ, K-1
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On a possible continuum origin
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Wavenumber, cm-1
Cs,
cm
-1a
ma
ga
t-2
273K296K330K
The water vapor selfbroadened continuum (as it comes from the CKD programm)
The bottom line represents the distribution of the allowed transition intensities.
δν1/2=246 cm-1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Wavenumber, cm-1
Cs*
10
4 cm
-1 a
ma
ga
t-2
233K273K333K
230K270K346K
Ho, Birnbaum, Rosenberg,J. Chem. Phys., 55, 1028, 1971 NIST, 2003
CO2 roto-translational band and collision induced/dimer spectrum
δν1/2=72 cm-1
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On a possible continuum origin
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Wavenumber, cm-1
Cs,
cm
-1a
ma
ga
t-2
273K296K330K
The water vapor selfbroadened continuum (as it comes from the CKD programm)
The bottom line represents the distribution of the allowed transition intensities.
δν1/2=246 cm-1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Wavenumber, cm-1
Cs*1
05, c
m-1
am
agat
-2
163K296K
Methane roto-translational band
Dore, Moraldi, Poll and BirnbaumMolec. Phys., 66, 363, 1989
δν1/2=311 cm-1
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On a possible continuum origin
0
1
2
3
4
5
0 200 400 600 800 1000
Wavenumber, cm-1
Cs*1
06, c
m-1
am
agat
-2
Bosomworth and Gush,Can. J. Phys. 43, 751, 1965
300K
δν1/2=111 cm-1
2000 2200 2400 2600 2800 3000
NIST, 2004300K
Pure nitrogen roto-translational band and CIA spectrum
0
0.04
0.08
0.12
0.16
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Wavenumber, cm-1
Cs,
cm
-1a
ma
ga
t-2
296K
The N2-broadened continuum(as it comes from the CKD programm)
δν1/2=158 cm-1
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On a possible continuum origin
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Wavenumber, cm-1
Cs,
cm
-1a
ma
ga
t-2
273K296K330K
The water vapor selfbroadened continuum (as it comes from the CKD programm)
The bottom line represents the distribution of the allowed transition intensities.
δν1/2=246 cm-1
0
50
100
150
200
250
300
350
30 40 50 60 70 80 90
Sum of mass of colliding molecules, amu
Ro
to-t
ran
slat
ion
al b
and
hal
f-w
idth
, cm
-1
Methane
Water vapor continuum, CKD
N2 broadened continuum,
Pure nitrogen CO2
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On a possible continuum origin
Is the continuum a cumulative contribution of line far wings?
Yes: Theoretical justification.
No: It is hard to understand continuum’s not uniform temperature dependence. It is hard to explain why the continuum is shaped like typical CIA spectrum?
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On a possible continuum origin
Is the continuum absorption by water dimers?
Yes: Water dimers exist.
No: The main reason for the continuum “dimer” conception is its exponential temperature dependence with the exponent value close to the energy of dimer dissociation. But really it is not exponential and not uniform. There is no reasonable explanation for the nitrogen broadened continuum?It is hard to explain why the continuum is shaped like typical CIA spectrum?
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On a possible continuum origin
Is the continuum a water vapor Collision Induced Spectrum?
Yes: It is shaped like collision induced spectrum.
There is a very easy and clear explanation of the nitrogen broadened continuum.
Water vapor CIA spectrum exists and it is expected to be very strong because of the first order magnitude dipole-dipole induction.
No: A. Brown, R. H. Tipping, “Collision-induced absorption in dipolar molecule-homonuclear diatomic pairs”, C. Camy-Peyret and A. A. Vigasin (eds.), Weakly Interacting Molecular Pairs: Unconventional Absorbers of Radiation in the Atmosphere, 93-99 (2003) Kluwer Academic Publisher
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Summary
Pure water vapor spectra have been recorded over a wide range of temperatures and pressures. Continuum binary absorption coefficients have been determined in the regions 800 to 1300 and 1930 to 2300 cm-1. In the 800 to 1300 cm-1 region our data for lower temperature reasonably agree with data provided with CKD model. But the disagreement increases up to 50% at high temperatures. In the high frequency segment our data satisfactory agree with CKD values around 2000 cm-1. But at higher wavenumbers the measured values greatly exceed the model. The data presented show that over both regions the absorption coefficient temperature dependence is not purely exponential and not uniform.