Temperature dependence of the water vapor self‐continuum by cavity ring‐down spectroscopy in the 1.6 µm transparency window

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, C s ( ν , T), were retrieved for 10 selected wave numbers 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 cell mirrors. The values measured between 5875 and 6665 cm −1 range between 1.5 × 10 −25 and 2 × 10 −24  cm 2 mol −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 C s (ν , T) decrease from 302 K to 340 K is no more than 50% for all our selected wave numbers, the MT_CKD V2.5 model predicts a much more pronounced temperature dependence in the center 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.