Water vapor self-continuum measurements in the infrared atmospheric window and ν2 band region (700–2000 cm−1)

Abstract

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 H₂O 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 MT_CKD 4.2, it was found that the self-continuum is substantially weaker than had been assumed for decades. Compared to MT_CKD 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 MT_CKD 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 ν₂ 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.