Yu. I. BARANOV, W. J. LAFFERTY, and G. T. Fraser Optical Technology Division Optical Technology Division National Institute of Standards and Technology, Gaithersburg, MD , USA The water-vapor continuum and selective absorption in the 8 to 12 μm and 3 to 5 μm windows at temperatures from 311 to 363K.

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.

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

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 H 2 O continuum.

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, (1989).

Experimental set-up view

Experimental conditions Spectral resolution is 0.1 cm -1 Spectral range 800 to 3500 cm -1 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

The quick data treatment method

The basic data treatment method Two spectra at: Θ=318K L=116m P=51.2 torr Θ=352K L=116m P=111.9 torr

The basic data treatment method

Two spectra at: Θ=318K L=116m P=51.2 torr Θ=352K L=116m P=111.9 torr

The basic data treatment method Two spectra at: Θ=318K L=116m P=51.2 torr Θ=352K L=116m P=111.9 torr

The basic data treatment method Every data array for a given temperature Θ was fitted by function: using standard least square method.

Water vapor continuum binary absorption coefficients C s in cm -1 (mol/cm 3 ) -1 atm -1 compared with CKD model values Wavenumber, cm -1 The CKD values are shown as solid lines 310.8K 325.8K 351.9K

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

The temperature dependence of the continuum binary absorption coefficient at 1203 cm -1 Montgomery, 1978 Burch et al., 1982 NIST, 2006 Clough CKD model

The temperature dependence of the continuum binary absorption coefficient Temperature, 1000/Θ, K -1

On a possible continuum origin

Pure nitrogen roto-translational band and CIA spectrum

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?

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?

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, (2003) Kluwer Academic Publisher

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.