Temperature dependence and physical interpretation of the self-continuum absorption of water vapor in the 1.6  <i>μ</i> m window

Abstract

New measurements of the very weak water vapor self-continuum absorption are reported by cavity ring down spectroscopy (CRDS) in the 1.6 μm window. For 30 selected spectral points ranging between 5702 and 6855 cm−1, the temperature dependence of the self-continuum cross section is studied by monitoring the absorption during pressure ramps of pure water vapor at 285, 296, 308, 317, and 332 K. For each measurement point, the self-continuum cross section, CS(T), was retrieved from the purely quadratic pressure dependence of the measured absorption coefficient, after subtraction of the monomer lines contribution. The temperature dependence was studied with a highly stable temperature regulated CRDS cell, allowing for a significant gain compared to a first study [Mondelain et al., J. Geophys. Res. Atmos. 119, 5625–5639 (2014)]. Although some significant deviations are evidenced, an overall good agreement with the MT_CKD model of the self-continuum is noted. In the low energy range of the window, the predictions based on the far-wing line function obtained by Ma et al. [J. Chem. Phys. 128, 124313 (2008)] show an excellent agreement with the measurements. This is not the case in the high-energy range of the window where far-wing predictions underestimate both the cross sections and the temperature dependence. Semi-empirical simulations indicate that the missing absorption in that interval is due to the contribution of the quasi-bound water dimers. The comparison of the simulations with the MT_CKD model in the 0–10000 cm−1 range indicates that the hybrid interpretation of the origin of the continuum proposed in the 1.6 μm window applies to other transparency windows.