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• New water self-continuum measurements. • Temperature dependence of water self-continuum. • Water self-continuum from dimer contribution. • Impact of water self-continuum on Earth’s radiation budget. Pure water vapor measurements were conducted over a temperature range of 278. 7–353. 0 K and a wavenumber range of 700–2000 cm −1 using a commercial Bruker IFS 125HR Fourier-transform spectrometer with a multi-reflection cell. Line parameters and continuum contributions were derived from the same spectra. In the out-of-band region 700–1350 cm −1, the H 2 O self-continuum was determined at ambient temperature. The measured continuum shows good agreement with a narrow-band measurement at 1185 cm −1. In line with measurements that led to the development of MTCKD 4. 2, it was found that the self-continuum is substantially weaker than had been assumed for decades. Compared to MTCKD 4. 2, even lower values by as much as 17% were found below 1000 cm −1. Since the out-of-band continuum overlaps with the infrared atmospheric window, which is of high importance for the Earth’s radiation budget, “clear-sky” radiative transfer calculations were performed using both MTCKD 4. 2 and the data of this study. The largest difference was found for tropical atmosphere, where this study yielded a 0. 31 W m −2 higher outgoing longwave radiation. The self-continua in the in-band region around ν 2 were fitted using a model that combines bound and quasi-bound dimer spectra, enabling the determination of temperature-dependent equilibrium constants and corresponding formation enthalpies: ΔH 296K, Db = −1202 (32) cm −1 and ΔH 296K, Dq = −748 (168) cm −1. The sum of the equilibrium constants, K Db (T) + K Dq (T), exceeds the value predicted by second virial coefficients, well beyond the uncertainty margin.
Röske et al. (Sat,) studied this question.