Yu. I. BARANOV, and W. J. LAFFERTY Optical Technology Division Optical Technology Division National Institute of Standards and Technology, Gaithersburg, MD , USA THE WATER-VAPOR CONTINUUM ABSORPTION IN THE MID-INFRARED WINDOWS AT TEMPERATURES FROM 311 K TO 363 K

Motivation  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.

Motivation  Water vapor is the major absorber of IR- radiation in the atmosphere and it is the single “green-house” gas with a positive feedback. Global warming should led to increase of water content in the atmosphere and to increase of its role in radiative processes.

Motivation  A detailed knowledge of the radiance attenuation over this windows is required in spectroscopic and radiometric measurements, optical imaging, remote sensing, and many other application.

Experimental set-up view

Experimental conditions For the main set of measurements Spectral resolution 0.1 cm -1 Spectral range 800 to 3500 cm -1 MCT detector, BaF 2 windows Temperature K (±0.3K) Pressure range kPa (torr) Path length m Number of spectra to 6.07 (21.2 to 45.5) to 7.42 (25.5 to 55.7) to 11.5 (33.7 to 86.3) to 12.3 (39.1 to 92.0) to 15.1 (43.2 to 113) to 13.7 (41.1 to 103)

An example of IR water vapor spectrum Res = 0.1 cm -1 Θ=339.2 K P=89.5 torr L=9217 cm

An example of IR water vapor spectrum Yu. I. Baranov, W. J. Lafferty, Q. Ma, R. H. Tipping Water vapor continuum absorption in the cm -1 spectral region at temperatures from 311 to 363 K. JQSRT, 109, , (2008) Res = 0.1 cm -1 Θ=339.2 K P=89.5 torr L=9217 cm

An example of IR water vapor spectrum Yu. I. Baranov, W. J. Lafferty, Q. Ma, R. H. Tipping Water vapor continuum absorption in the cm -1 spectral region at temperatures from 311 to 363 K. JQSRT, 109, , (2008) !!! Res = 0.1 cm -1 Θ=339.2 K P=89.5 torr L=9217 cm

An example of IR water vapor spectrum The continuum CKD model S. A. Clough, F. X. Kneizys, and R. W. Davies, Atmos. Res. 23, (1989). ??? Res = 0.1 cm -1 Θ=339.2 K P=89.5 torr L=9217 cm

Temperature K (±0.3K) Pressure range kPa (torr) Path length m Number of spectra to 6.16 (18.9 to 46.2)84, to 9.11 (35.4 to 68.3)10039 An example of IR water vapor spectrum Additional measurements with InSb detector

Comparison of the data obtained with different detectors (Circles – InSb, dots – MCT)

The water vapor continuum in the 3 to 5 μm spectral region Overtone of O-H-O bending mode at 3215 cm -1 D.P.Schofield et al., CPPC, 2003, 5,

The water vapor continuum in the 3 to 5 μm spectral region Paynter D.J., Ptashnik I.V., Shine K.P., Smith K.M. Geophys. Res. Lett. 2007, 34, L12808.

The continuum CKD model in comparison with experimental data ?

The data validation experiment Measurements at pressures extended up to 200 torr

The data validation experiment Measurements at pressures extended up to 200 torr 1128 cm cm K

Promise ▼ Can we detect nitrogen-broadened continuum at these conditions? Measurements at pressures extended up to 200 torr

Promise Wavenumber, cm -1

Promise Effective atmospheric path: Self-broadening ~ 20 km; Nitrogen-broadening ~ 6 km Wavenumber, cm -1

Promise Effective atmospheric path: Self-broadening ~ 20 km; Nitrogen-broadening ~ 6 km CKD predictions Wavenumber, cm -1

Promise Effective atmospheric path: Self-broadening ~ 20 km; Nitrogen-broadening ~ 6 km CKD predictions NIST self-continuum Wavenumber, cm -1

Promise Effective atmospheric path: Self-broadening ~ 20 km; Nitrogen-broadening ~ 6 km CKD predictions NIST self-continuum Wavenumber, cm -1

Promise Effective atmospheric path: Self-broadening ~ 20 km; Nitrogen-broadening ~ 6 km CKD predictions NIST self-continuum Wavenumber, cm -1

Promise A. Brown, R. H. Tipping “Collision-induced absorption in dipolar molecule – homonuclear diatomic pairs” In: Camy- Peyret C., Vigasin A. A., editors. Weakly interacting molecular pairs: Unconventional Absorbers of radiation in the atmosphere, Kluwer Academic Publishers, Netherlands 2003:93-99.

Promise A. Brown, R. H. Tipping “Collision-induced absorption in dipolar molecule – homonuclear diatomic pairs” In: Camy- Peyret C., Vigasin A. A., editors. Weakly interacting molecular pairs: Unconventional Absorbers of radiation in the atmosphere, Kluwer Academic Publishers, Netherlands 2003: ~3e-25 cm -1 (atm*molec/cm 3 ) -1 at 296 K

Promise A. Brown, R. H. Tipping “Collision-induced absorption in dipolar molecule – homonuclear diatomic pairs” In: Camy- Peyret C., Vigasin A. A., editors. Weakly interacting molecular pairs: Unconventional Absorbers of radiation in the atmosphere, Kluwer Academic Publishers, Netherlands 2003: ~3e-25 cm -1 (atm*molec/cm 3 ) -1 at 296 K 5.8e-25 cm -1 (atm*molec/cm 3 ) -1 at 339 K

Promise MT_CKD nitrogen broadened continuum in comparison with our first estimates 339K

Conclusions The measured over 3 to 5 microns self- broadened water vapor continuum is in good agreement with MT_CKD model around 5 microns. At shorter wavelengths disagreement growths dramatically reaching one order of magnitude at 4 microns. The clear feature at 3200 cm -1 belonging presumably to stable water dimer has been observed.

Conclusions Several measurements at extended water vapor pressures provide the evident proof of reported data. These measurements show very high promise for accurate measurements of the nitrogen broadened continuum in the 3 to 5 microns window. The first preliminary estimate of the continuum absorption coefficients C f exceeds MT_CKD model values for about two orders of magnitude.