Water vapour foreign continuum cross-section. Experimental data of FTS measurements (H2O + N2) in 2500 cm-1 window [75]. [75] Y.I. Baranov, The continuum absorption in H2O + N2 mixtures in the 2000-3250 cm-1 spectral region at temperatures from 326° to 363°K, Journal of Quantitative Spectroscopy & Radiative Transfer, 112 (2011) 2281-2286. 10.1016/j.jqsrt.2011.06.005
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 absorption spectra of H2O+N2 mixtures, as well, as the spectra of pure gases, have been measured using a Fourier-transform infrared spectrometer at a resolution of 0.1 cm−1. The sample temperatures were 326, 339, 352, and 363 K. Water vapor pressures varied from 8 (60 torr) to 34.5 kPa (259 torr). The nitrogen pressure was kept constant at about 414 kPa (4.1 atm). The path length was 100 m. The continuum absorption coefficients obtained in the spectral range 2000–3250 cm−1 (3.1–5 μm) do not depend significantly on temperature, as is predicted by the well known MT_CKD model. But there are significant deviations in the continuum spectral behavior and magnitude. Around 2050 cm−1 the measured absorption coefficients Cf are about two times larger than those of the model. This deviation grows rapidly at shorter wave lengths, reaching a maximum of two orders of magnitude in the middle of the window at 2500 cm−1. At this point, the deviation starts to decrease significantly and around 3100 cm−1 our results are in agreement with the MT_CKD model. This behavior of the deviation is due to the broad and structureless feature in the region of the nitrogen fundamental band. Most likely, this feature is the N2 fundamental band component, induced by collisions between H2O and N2 molecules. The data obtained and a comparison with the results from the other available sources are presented.
