yu. i. baranov and w. j. lafferty optical technology division optical technology division national...

Yu. I. BARANOVYu. I. BARANOV and W. J. LAFFERTY and W. J. LAFFERTY

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 absorptionThe water vapor continuum absorption in the 8 to 12 in the 8 to 12 μm region at temperaturesμm region at temperatures

from 311 to 352Kfrom 311 to 352K..

Introduction The water vapor continuum absorption in

the atmospheric 8 to 12 μm window strongly affects the Earth’s outgoing radiation and therefore is of great importance for radiative balance calculations.

Over the past twenty years many 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.

Introduction

The other high-sensitive techniques,

like photo-acoustic or cavity ring-down spectroscopy were also employed for these measurements.

Nevertheless the existing data is not adequate to match the current requirements.

Introduction

How to measure the continuum with the 2 m base multi-pass cell available at NIST? Is this possible?

This is certainly not possible at room or lower temperatures because of the low vapor pressure of H2O at these temperatures.

Introduction

The solution is to make the measurements at an elevated temperature. This allows one to fill the cell with water vapor at pressures up to several tens of torrs. This is enough to make continuum absorption measurable with the available path length of about 100 m since the continuum absorbance increases as the square of the water vapor pressure (density).

Experimental set-up view

Specific experimental details for H2Ocontinuum measurements

1. How to avoid water vapor condensation?

All of the protrusive parts of the cell, such as valves, window holders, sleeves etc., were equipped with additional electric heaters and coated with thermoinsulation.

This was done in order to provide uniform temperature all of the parts of the internal cell surface and to avoid water vapor condensation.

Six additional thermocouples were set up to measure and control temperatures.

Specific experimental details for H2Ocontinuum measurements

2. How to get water vapor?

The water vapor sampler is a horizontally positioned stainless steel cylinder with distilled water. The sampler is equipped with electric heater and thermoinsulation.

The liquid water temperature was kept close to the cell body temperature.

In order to save a thermal equilibrium and the optic stability we spent over 30 minutes to fill the cell with water vapor.

Specific experimental details for H2Ocontinuum measurements

3. How to measure water vapor pressure?

The gauge (Barocell 600A, 100 torr) for water vapor pressure measurement is also heated up to the cell body temperature.

We calibrated it periodically filling the cell with nitrogen and measuring the pressure with the second gauge (MKS Baratron 270, 1000 torr), operating under regular temperature.

The calibration dependence was found linear and stable. The pressure estimated systematic error is about 0.6%.

Experimental conditionsSpectral resolution is 0.1cm-1

Experimental conditionsSpectral resolution is 0.1cm-1

Why so low ?

Experimental conditionsSpectral resolution is 0.1cm-1

Possible minimalIntensity of the

instrument’s beam

Why so low ?

Experimental conditionsSpectral resolution is 0.1cm-1

Possible minimalIntensity of the

instrument’s beam

Acceptable signal to noise ratio:

200 – 300 scans

Why so low ?

+

Experimental conditionsSpectral resolution is 0.1cm-1

Possible minimalIntensity of the

instrument’s beam

Acceptable signal to noise ratio:

200 – 300 scans

Reasonable time of scanning:few hours

Why so low ?

+ +

Experimental conditionsSpectral resolution is 0.1cm-1

Possible minimalIntensity of the

instrument’s beam

Acceptable signal to noise ratio:

200 – 300 scans

Reasonable time of scanning:few hours

Why so low ?

= 0.1cm-1+ +

Experimental conditions

TemperatureK (±0.3K)

Pressure rangetorr

Path lengthm

Number of spectra

310.8 21.2 to 45.4 80-116 35

318.0 25.5 to 55.7 84-116 40

325.8 33.7 to 86.3 92-116 43

339.3 42.4 to 90.8 92-116 47

351.9 43.2 to 113 84-116 33

Spectral resolution is 0.1 cm-1

An example of IR water vapor spectrum

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=310.7KP=43.3 torr

L=10805 cm

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

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

The quick data treatment methodThe 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 methodThe 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 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

The quick data treatment methodThe 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

L=944.2 cm-1

Measured absorbance versus density square

0

1

2

3

4

0 0.005 0.01

Density, amagat2

(-ln

T/L

)*1

05,

cm

-1

318K326K339K

The temperature dependence of the continuum binary absorption coefficient at 942 cm-1

0

1.5

3

4.5

6

7.5

270 280 290 300 310 320 330 340 350 360

Temperature, K

Ba

a*1

022 ,

cm

-1/(

atm

*mo

lec

*cm-3

) Coffey, 1977Nordstrom,1978Peterson, 1979Loper, 1983Hinderling, 1987Cormier, 2005Present work

The temperature dependence of the continuum binary absorption coefficient at 942 cm-1

0

1.5

3

4.5

6

7.5

270 280 290 300 310 320 330 340 350 360

Temperature, K

Ba

a*1

022 ,

cm

-1/(

atm

*mo

lec

*cm-3

) Coffey, 1977Nordstrom,1978Peterson, 1979Loper, 1983Hinderling, 1987Cormier, 2005Present work

00

1n

00

kTD1kT

Dexp

kTD1kT

Dexp

TT

KK

A. A. Vigasin, JQSRT, 64, 25, 2000

D=-15.7kJ/moleN. Goldman et al., J. Phys. Chem. A

108, 787 (2004)

T0=296K; K0=Kexp(296)

The temperature dependence of the continuum binary absorption coefficient at 942 cm-1

0

1.5

3

4.5

6

7.5

270 280 290 300 310 320 330 340 350 360

Temperature, K

Ba

a*1

022 ,

cm

-1/(

atm

*mo

lec

*cm-3

) Coffey, 1977Nordstrom,1978Peterson, 1979Loper, 1983Hinderling, 1987Cormier, 2005Present work

00

1n

00

kTD1kT

Dexp

kTD1kT

Dexp

TT

KK

A. A. Vigasin, JQSRT, 64, 25, 2000

D=-15.7kJ/moleN. Goldman et al., J. Phys. Chem. A

108, 787 (2004)

T0=296K; K0=Kexp(296)

T0=311K; K0=Kexp(311)

The water vapor continuum absorption coefficients

0

1

2

3

800 850 900 950 1000 1050 1100 1150

Wavenumber, cm-1

Cs*1

02

2,

cm

2m

ole

c-1a

tm-1

310.7K

352.1K

325.7K

The basic method of the data processing

0

0.2

0.4

0.6

0.8

1

800 900 1000 1100 1200 1300

Wavenumber, cm-1

Tra

nsm

itta

nce

T=318KP=52.1 torr

L=116 m

The basic method of the data processing

0

0.2

0.4

0.6

0.8

1

800 900 1000 1100 1200 1300

Wavenumber, cm-1

Tra

nsm

itta

nce

T=318KP=52.1 torr

L=116 m

The basic method of the data processing

0

0.2

0.4

0.6

0.8

1

800 900 1000 1100 1200 1300

Wavenumber, cm-1

Tra

nsm

itta

nce

T=318KP=52.1 torr

L=116 m

The basic method of the data processing

0

0.2

0.4

0.6

0.8

1

800 900 1000 1100 1200 1300

Wavenumber, cm-1

Tra

nsm

itta

nce

T=318KP=52.1 torr

L=116 m

The basic method of the data processing

0

0.2

0.4

0.6

0.8

1

1050 1060 1070 1080 1090 1100

Wavenumber, cm-1

Tra

nsm

itta

nce

T=318KP=52.1 torr

L=116 m

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 region 800 cm-1 to 1150 cm-1. 

Our data at 944 cm-1 have been compared with the data obtained recently by use of CRD spectroscopy technique. The systematic difference exceeds 10%. But relative changes of absorption coefficient with temperature are in good agreement.

Ro-vibrational structure has been calculated using the HITRAN data base and compared with experimental spectra. This comparison shows that the HITRAN water vapor absorption line intensities need to be corrected.