Accurate laboratory determination of the near‐infrared water vapor self‐continuum: A test of the MT_CKD model

Abstract

The semi‐empirical MT_CKD model of the absorption continuum of water vapor is widely used in atmospheric radiative transfer codes of the atmosphere of Earth and, recently, of exoplanets but lacks of experimental validation in the atmospheric windows. Recent laboratory measurements by Fourier transform Spectroscopy led to self‐continuum cross sections much larger than the MT_CKD values in the near‐infrared transparency windows. We report on accurate measurements by Cavity Ring Down Spectroscopy (CRDS) and Optical Feedback‐Cavity‐Enhanced Absorption Spectroscopy (OF‐CEAS) at selected spectral points of the transparency windows centered around 4.0, 2.1, and 1.25 µm. The temperature dependence of the absorption continuum at 4.38 µm is measured in the 23–39°C range. The self‐continuum water vapor absorption is derived either from the baseline variation of water vapor spectra recorded for a series of pressure values over a small spectral interval or from baseline monitoring at fixed laser frequency during pressure ramps. After subtraction of the local water monomer lines contribution, self‐continuum cross sections, Cs, are determined with an accuracy better than 10% from the pressure squared dependence of the continuum absorption measured up to about 15 Torr. Together with our previous measurements in the 2.1 and 1.6 µm windows, the present results provide a unique set of water vapor cross sections for testing the MT_CKD model in four transparency windows. Although showing some important deviations of the absolute values (up to about a factor 4), our accurate measurements validate the overall frequency dependence of the most recent version (V2.8) of the MT_CKD model.

The water vapour self-continuum absorption in the infrared atmospheric windows: new laser measurements near 3.3 and 2.0 µm

“…More recently, we have implemented highly sensitive cavity enhanced laser techniques at different spectral points of the 4.0, 2.1, 1.6 and 1.25 µm windows to measure in the laboratory the self-continuum absorption of water vapour Campargue et al, 2016;Ventrillard et al, 2015;Richard et al, 2017). Overall, these measurements by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OFCEAS) and Cavity Ring-down Spectroscopy (CRDS) were found closer to the MT_CKD model than to the FTS results.…”

“…The strong disagreement with FTS measurements (Ptashnik et al, 2011a(Ptashnik et al, , 2013(Ptashnik et al, , 2015 has been discussed by Shine et al (2016) and Campargue et al (2016). The measurement of a very small variation of a FTS spectrum baseline induced by the injection of water vapour in an absorption cell requires high baseline stability during the entire measurement period.…”

The water vapor self-continuum absorption in the infrared atmospheric windows: New laser measurements near 3.3 μm and 2.0 μm

“…As illustrated in Fig. 6 The strong disagreement with FTS measurements (Ptashnik et al, 2011a(Ptashnik et al, , 2013(Ptashnik et al, , 2015 has been discussed by Shine et al, 2016 andCampargue et al 2016. The measurement of a very small variation of a FTS spectrum baseline induced by the injection of water vapor in an absorption cell requires very high baseline stability during the entire measurement period.…”

“…More recently, we have implemented highly sensitive cavity enhanced laser techniques at different spectral points of the 4.0, 2.1, 1.6 and 1.25 µm to measure in the laboratory the self-continuum absorption of water vapor (Mondelain et al 2013;2014Campargue et al 2016;Ventrillard et al 2015;Richard et al 2017). Overall, these measurements by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OFCEAS) and Cavity Ring-down Spectroscopy (CRDS) were found closer to the MT_CKD model than to the FTS results.…”

“…al., 2013; 2015), obtained with a pathlength of 612 m at 289.5 and 287 K, respectively, (iii) the FTS dataset obtained at NIST by Baranov and Lafferty (2011) with a 100 m pathlength between 311 and 363 K. Finally, Burch and Alt (1984) reported continuum measurements in the center of the window using a grating spectrograph and an optical configuration aiming to minimize the spectra baseline drifts. Campargue et al, 2016;Richard et al, 2017;this work); by FTS from Baranov and Lafferty (2011), light blue circles, from CAVIAR (black squares; Ptashnik et al, 2011a), from Tomsk2013 (orange circles; Ptashnik et al, 2013), from Tomsk2015 (orange circles; Ptashnik et al, 2015) and with a grating spectrograph by Burch and Alt (1984) The evolution of the MT_CKD model in the 4.0 µm over the last ten years is illustrated by the 2.4, 2.8, 3.0 and 3.2 versions plotted in Fig. 3.…”