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.
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All published work on the water-vapour continuum in the region 8 μ to 50 μ is reviewed in the light of some new experimental results in the 11 to 21 -μ region. This new work employed a 15.5 m multiple-reflection cell, giving total path lengths up to 500 m, and a high-resolution (1-2 cm-1) grating spectrograph. Control of temperature (20-40°C), and water-vapour partial pressure (3-35 mb) allowed simulation of tropospheric conditions and their extension to rather higher humidities, but the total pressure could not be varied from atmospheric.
The results suggest that two mechanisms of absorption are involved. One of them is almost certainly the foreign-broadening of water-vapour lines and it is responsible for most of the continuum absorption noted by previous workers. The origin of the other is not clear; it gives rise to absorption which increases with partial water-vapour pressure, e, equalling the foreign-broadened component at e 15 mb. This behaviour was first reported by Bignell, Saiedy and Sheppard (1963) who then attributed it to self-broadening of H2O lines. However, the present more sensitive work casts doubt on this interpretation because the absorption is found to have a negative temperature dependence of 2 per cent per °C over the range 21° to 45°C, which is greater than that of any water-vapour line. Absorption of the same type, though weaker, is present near 4 μ.
Reinterpretation of previous open-air measurements in atmospheric conditions ranging from warm and damp to cold and dry shows that the systematic differences they exhibit might be a manifestation of the new absorption.
No firm explanation can be given, but the possibility of continuum absorption by the recently reported water dimer molecule (H2O)2 should be investigated.
