Spectral dependence of the self-continuum cross-section: 19 (x). [19] D. Mondelain, S. Manigand, S. Kassi and A. Campargue, J. Geophys. Res.: Atmos., 2014, 119, 5625–5639.
The room temperature self- and foreign-continua of water vapor have been measured near 4250 cm−1 with a newly developed high sensitivity cavity ring down spectrometer (CRDS). The typical sensitivity of the recordings is αmin ≈ 6 × 10−10 cm−1 which is two orders of magnitude better than previous Fourier transform spectroscopy (FTS) measurements in the spectral region. The investigated spectral interval is located in the low energy range of the important 2.1 μm atmospheric transparency window. Self-continuum cross-sections, CS, were retrieved from the quadratic dependence of the spectrum base line level measured for different water vapor pressures between 0 and 15 Torr, after subtraction of the local water monomer lines contribution calculated using HITRAN2012 line parameters. The CS values were determined with 5% accuracy for four spectral points between 4249.2 and 4257.3 cm−1. Their values of about 3.2 × 10−23 cm2 molecule−1 atm−1 are found 20% higher than predicted by the MT_CKD V2.5 model but two times weaker than reported in the literature using FTS. The foreign-continuum was evaluated by injecting various amounts of synthetic air in the CRDS cell while keeping the initial water vapor partial pressure constant. The foreign-continuum cross-section, CF, was retrieved from a linear fit of the spectrum base line level versus the air pressure. The obtained CF values are larger by a factor of 4.5 compared to the MT_CKD values and smaller by a factor of 1.7 compared to previous FTS values. As a result, for an atmosphere at room temperature with 60% relative humidity, the foreign-continuum contribution to the water continuum near 4250 cm−1 is found to be on the same order as the self-continuum contribution.
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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.
