New measurements of the water vapour continuum in the 1.6 µm and 2.3 µm windows are obtained at room temperature using highly stable and sensitive cavity ring down (CRD) spectrometers.
In the 1.6 µm window, self-continuum cross-sections, CS, are derived for 30 selected spectral points between 5700 and 6850 cm−1 using pressure ramps of pure water vapour (up to 15 Torr). Purely quadratic pressure dependence is obtained for each measurement point. Compared to our previous dataset (Mondelain et al., 2014), the retrieved Cs values are more accurate and show a general agreement. The spectral coverage is extended both on the high and low frequency edges of the 1.6 µm window. In addition, new CS values are derived in the 5120–5137 cm−1 interval extending to higher energy the coverage of the 2.3 µm window. The measurements near 5700 cm−1 and 5130 cm−1 are found lower than the MT_CKD_3.2 values by a factor of about 2 and 1.5, respectively.
Foreign-continuum cross-sections, CF, are newly obtained in the 2.3 µm window from CRDS spectra of moist air in flow regime. Spectra were recorded for different water vapour partial pressures while maintaining the total pressure constant in the high finesse CRDS cavity. After subtraction of the monomer and self-continuum contributions, CF values were derived from the linear variation of the foreign-continuum absorption with the water vapour partial pressure. CF values determined at four spectral points between 4430 and 5000 cm−1 are larger by up a factor of five than the corresponding MT_CKD values. Considering these CRDS data at room temperature and literature values at 400 K the temperature dependence of the foreign-continuum is confirmed to be weak.
Papers with the following subject areas are suitable for publication in the Journal of Quantitative Spectroscopy and Radiative Transfer:
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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.
