Spectral composition of the water vapour self-continuum absorption within 2.7 and 6.25 µm bands

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Fourier Transform Spectroscopy laboratory measurements of the pure water vapour absorption spectra in a wide temperature range (from –5 to 78 °C) were performed in earlier studies in the near-infrared spectral region, and self-continuum absorption was retrieved within 1600 cm–1 (6.25 µm) and 3600 cm–1 (2.7 µm) absorption bands. In this paper, we derive the proportion of true bound and quasibound water dimers in the equilibrium water vapour by fitting their simulated spectra to the spectral features in the experimentally retrieved continuum. The results are in reasonable agreement with statistical calculations and generally support the idea of a complementary contribution from bound and quasibound water dimers (WDs) to the spectral structure of water vapour continuum within absorption bands. However, the total retrieved equilibrium constants (for true bound and quasibound WDs) exceed the values derived from the second virial coefficient and from ab initio calculations by 30% at 268 K and by 100% at 350 K. Thus, supplementary absorption mechanisms need to be examined to clarify the origin of the still remaining discrepancy. Possible reasons for this deviation are discussed, such as unaccounted absorption due to intermolecular oscillations in WDs and/or super-Lorentzian line shape of the middle line wings of water monomers.