Asymptotic line wing theory allows one to construct the line shape describing the frequency and temperature dependence of the self-broadened H2O continuum in the 3–5 μm spectral region obtained experimentally by CAVIAR and NIST. The H2O transmission functions are adequately described as well, using this line shape up to temperatures of ∼675 K and pressures of ∼10 atm.
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The pure water-vapor continuum absorption in the 2.88 to 5.18 μm spectral region has been measured using a Fourier-transform infrared spectrometer at a resolution of 0.1 cm−1. The sample temperatures and pressures varied from 311 to 363 K and from 2.8 kPa (21 Torr) to 34.5 kPa (259 Torr), respectively. The path lengths used in the study ranged from 68 to 116 m. Under these conditions, the continuum absorption in the middle of the 4 μm window is quite detectable reaching as high as 4%. The spectral processing included calculations to fit and remove the H2O ro-vibrational structure. In the region around 5 μm, the absorption coefficients obtained are in good agreement with those of the commonly used MT_CKD continuum model. However at shorter wavelengths, the observed values significantly deviate from the model. Inspection of the present data as well as that of previous measurements leads to the conclusion that the MT_CKD model despite the latest updates significantly underestimates the rate of the continuum temperature dependence over the 4 μm atmospheric window. Line strengths for 189 H2O transitions were obtained from the spectral processing. The deviation of these measured intensities from those listed in the HITRAN database is randomly scattered around zero to within several percents and no systematic trends were detected.
