The very weak water vapor self-continuum has been investigated by high sensitivity Cavity Ring Down Spectroscopy in the 1.6 µm window at five temperatures between 302 K and 340 K. The absorption cross-sections, Cs(ν, T), were retrieved for ten selected wavenumbers from a fit of the absorption coefficients measured in real time during pressure ramps, after subtraction of the contributions of the local water monomer lines and of water adsorbed on the CRDS mirrors. The values measured between 5875 and 6665 cm-1 range between 1.5 × 10-25 and 2 × 10-24 cm2 molec-1 atm-1 with a minimum around 6300 cm-1. At 302 K, an agreement within 50% is observed over the whole window with the cross-sections provided by the MT_CKD V2.5 model. Nevertheless, while our measurements show that the Cs(ν, T) decrease from 302 K to 340 K is no more than 50% for all our selected wavenumbers, the MT_CKD V2.5 model predicts a much more pronounced temperature dependence in the centre of the window, the agreement being better on the edges of the window. The obtained results are discussed in relation with theoretical modeling of the water vapor self-continuum as far-wings of monomer lines or water dimer absorption. For potential atmospheric applications, cross-sections are provided at each temperature with a sampling step of 10 cm-1 for the entire 5850 – 6700 cm-1 range.
GR-Atmospheres publishes articles that advance and improve understanding of atmospheric properties and processes, including the interaction of the atmosphere with other components of the Earth system.
The water vapour self-continuum has been investigated by high sensitivity Cavity Ring Down Spectroscopy at room temperature in the 1.6 mm window. The real time pressure dependence of the continuum was investigated during pressure cycles up to 12 Torr for fifteen selected wavenumber values. The continuum absorption coefficient measured between 5875 and 6450 cm-1 shows a minimum value around 6300 cm-1 and ranges between 1*10-9 and 8*10-9 cm-1 for 8 Torr of water vapour. The continuum level is observed to deviate significantly from the expected quadratic dependence versus the pressure. This deviation is interpreted as due to a significant contribution of water adsorbed on the super mirrors to the cavity loss rate. The pressure dependence is well reproduced by a second order polynomial. We interpret the linear and quadratic terms as the adsorbed water and vapour water contribution, respectively. The derived self-continuum cross sections, Cs(T 1⁄4 296 K), ranging between 3*10-25 and 3*10-24 cm2 molecule-1 atm-1 are found in reasonable agreement with the last version of the MT_CKD 2.5 model but in disagreement with recent FTS measurements. The FTS cross section values are between one and two orders of magnitude higher than our values and mostly frequency independent over the investigated spectral region. The achieved baseline stability of the CRDS spectra (better than 1*10-10 cm-1) level totally rules out water continuum absorption at the FTS level (1.2 *10-7 cm-1 at 9 Torr) in the CRDS cell. In order to find the origin of such conflicting results, the differences and possible experimental biases in the two measurement methods are discussed.
