The water vapour self- and foreign-continua in the 1.6 µm and 2.3 µm windows by CRDS at room temperature

Abstract 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, C S , 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 C s 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 C S 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, C F , 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, C F values were derived from the linear variation of the foreign-continuum absorption with the water vapour partial pressure. C F 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.