Laboratory measurements of water vapour continuum absorption in spectral region 5000-5600 cm-1:: Evidence for water dimers

In spite of decades of extensive studies, the role of water dimers (WD) in the atmospheric radiation budget is still controversial. In order to search for evidence of the dimer in the solar near infrared, high spectral resolution pure water vapour absorption spectra were obtained in laboratory conditions for two different pressures and temperatures in the spectral region 5000-5600 cm(-1) (1.785 to 2 mum). The residual was derived as a difference between the measured optical depth and the calculated one for water monomer, using the modified HITRAN database and two different representations of the water vapour continuum: CKD-2.4 (Clough-Kneizys-Davies) and the Ma and Tipping continuum. in both cases the residuals obtained are very similar to those expected from a recent theoretical calculation of the WD absorption. However, the WD band half-width at half maximum (HWHM) and dimerization equilibrium constant, K-eq, required to provide a best fit to the residual, differ for each case. To be in best agreement with the residual calculated by using the Ma and Tipping continuum, the WD bands HWHM should be similar to28 cm(-1), and K-eq = 0.02 +/- 0.0035 atm(-1) and 0.043 +/- 0.0055 atm(-1) for temperatures 342 and 299 K respectively. For the residual calculated using the CKD-2.4 continuum the fitted value of the HWHM is similar to18 cm(-1), and K-eq = 0.011 +/- 0.0025 atm(-1) (342 K) and 0.018 +/- 0.003 atm(-1) (299 K). It is concluded that a substantial part of the WD absorption is already implicitly included within the CKD-2.4 continuum model. The increase in estimated clear-sky global mean absorption of solar radiation due to WD varies from 0.5% to 2.0%, depending on the set of WD parameters used. On the basis of a comparison of the derived K-eq values with others in the literature, the higher estimate is favoured.