On the origin of the water vapor continuum absorption within H2O rotational band
It is commonly accepted that the water vapor continuum absorption originates from collisional interaction of H2O molecules, namely from wings of resonance lines and water dimer (stable and metastable) absorption. However, the relative contribution of each of these mechanisms has to be clarified.
The water vapor continuum absorption has been investigated in the pure rotational band of H2O molecule. Series of measurements using Fourier Transform spectrometer on SOLEIL synchrotron facility were carried out to record water vapor absorption spectra in a broad frequency region (15-700 cm-1) under various thermodynamic conditions (1-15 mbar, 296-326 K) [2-4]. On the one hand, the continuum absorption in this spectral region is of particular importance since it covers the maximum of the Earth thermal emission. On the other hand, it is advantageous for styding of nature of the water vapor continuum because only this spectral range is supported by ab initio calculations of the water dimer spectrum .
The retrieved self-continuum cross-sections are found to be in a good agreement with literature values available in the lower and upper parts of the studied frequency region [5-7]. All experimental data indicate the need for correction of the empirical continuum model MT_CKD  widely used for atmospheric applications.
A comparative analysis of the retrieved data on the continuum absorption with the results of ab initio calculations of the water dimer spectrum  provides (i) a confirmation of the dominant role of water dimers in formation of the water vapor continuum in the mm-submm range and (ii) evidence of a significant contribution of other mechanisms of continuum formation (wings of resonance lines and metastable dimers) in the higher-frequency far-IR region.
The study is partly supported by RFBR-CNRS project 18-55-16006.
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