Comparison of near-room temperature measurements of the water vapour self-continuum cross-sections between 2000 and 7000 cm-1. [61] D. Mondelain, A. Aradj, S. Kassi, A. Campargue, The water vapour self-continuum by CRDS at room temperature in the 1.6 lm transparency window, J. Quant. Spectrosc. Radiat. Transfer 130 (2013) 381–391, http://dx.doi.org/10.1016/j.jqsrt.2013.07.006.
Spectroscopic catalogues, such as GEISA and HITRAN, do not yet include information on the water vapour continuum that pervades visible, infrared and microwave spectral regions. This is partly because, in some spectral regions, there are rather few laboratory measurements in conditions close to those in the Earth’s atmosphere; hence understanding of the characteristics of the continuum absorption is still emerging. This is particularly so in the near-infrared and visible, where there has been renewed interest and activity in recent years. In this paper we present a critical review focusing on recent laboratory measurements in two near-infrared window regions (centred on 4700 and 6300 cm−1) and include reference to the window centred on 2600 cm−1 where more measurements have been reported. The rather few available measurements, have used Fourier transform spectroscopy (FTS), cavity ring down spectroscopy, optical-feedback – cavity enhanced laser spectroscopy and, in very narrow regions, calorimetric interferometry. These systems have different advantages and disadvantages. Fourier Transform Spectroscopy can measure the continuum across both these and neighbouring windows; by contrast, the cavity laser techniques are limited to fewer wavenumbers, but have a much higher inherent sensitivity. The available results present a diverse view of the characteristics of continuum absorption, with differences in continuum strength exceeding a factor of 10 in the cores of these windows. In individual windows, the temperature dependence of the water vapour self-continuum differs significantly in the few sets of measurements that allow an analysis. The available data also indicate that the temperature dependence differs significantly between different near-infrared windows. These pioneering measurements provide an impetus for further measurements. Improvements and/or extensions in existing techniques would aid progress to a full characterisation of the continuum – as an example, we report pilot measurements of the water vapour self-continuum using a supercontinuum laser source coupled to an FTS. Such improvements, as well as additional measurements and analyses in other laboratories, would enable the inclusion of the water vapour continuum in future spectroscopic databases, and therefore allow for a more reliable forward modelling of the radiative properties of the atmosphere. It would also allow a more confident assessment of different theoretical descriptions of the underlying cause or causes of continuum absorption.
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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.
