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

Abstract

Abstract. The amplitude, the temperature dependence and the physical origin of the water vapor absorption continuum are a long standing issue in molecular spectroscopy with direct impact in atmospheric and planetary sciences. In the recent years, we have determined the self-continuum absorption of water vapor at different spectral points of the atmospheric windows at 4.0, 2.1, 1.6 and 1.25 µm, by highly sensitive cavity enhanced laser techniques. These accurate experimental constraints have been used to adjust the last version (V3.2) of the semi-empirical MT_CKD model (Mlawer-Tobin_Clough-Kneizys-Davies) 15 widely incorporated in atmospheric radiative transfer codes. In the present work, the self-continuum cross-sections, C S , are newly determined at 3.3 µm (3007 cm -1 ) and 2.0 µm (5000 cm -1 ) by optical-feedback-cavity enhanced absorption spectroscopy (OFCEAS) and cavity ring-down spectroscopy (CRDS), respectively. These new data allow completing the spectral coverage of the 4.0 and 2.1 µm windows, respectively, and testing the recently released V3.2 version of the MT_CKD3 continuum. By complementing high temperature literature data to the present data, the temperature dependence 20 of the self continuum is presented.

Temperature Dependence of the Collision‐Induced Absorption Band of O2 Near 1.27 µm

“…Recently, cavity enhanced absorption techniques like cavity‐ring‐down spectroscopy (CRDS) have been applied to the O 2 CIA measurements (Karman et al., 2018; Mondelain et al., 2019) and more generally to the continua measurements (see for example Campargue et al., 2016 and references herein; Lechevallier et al., 2018; Mondelain et al., 2017 and references herein; Richard et al., 2017). These techniques were found particularly suitable as they provide both a high sensitivity and a high baseline stability of the spectra.…”

Accurate Laboratory Measurement of the O2 Collision‐Induced Absorption Band Near 1.27 μm

“…These techniques, in particular, cavity ring down spectroscopy (CRDS), have an inherent higher sensitivity than traditional techniques and allow achieving a very high baseline stability of the spectra, which is the most important criterion for the measurement of weak broadband continua. In particular, the long‐standing debate on the amplitude of the very weak water absorption continuum in the atmospheric transparency window has been recently closed from accurate CRDS and OFCEAS measurements (Campargue et al, , and references herein; Lechevallier et al, ; Richard et al, ), the obtained experimental data set being used to adjust the last version (V3.2) of the MT_CKD continuum (Mlawer et al, ).…”

Sensitivity of near‐infrared transmittance calculations for remote sensing applications to recent changes in spectroscopic information

“…Thus, currently there is no consensus on the strength of the MT_CKD 2.5 continuum in some near‐IR windows. A good number of measurements in the near‐IR have pointed out that the MT_CKD 2.5 continuum may be underestimating the strength of the water vapour continuum in some spectral windows (e.g., Ptashnik et al ., , ; Baranov and Lafferty, ; Mondelain et al ., ; Campargue et al ., ; Reichert and Sussmann, ; Lechevallier et al ., ). Although these measurements do not agree on how big this underestimation is, the differences are more important for remote sensing techniques that use these windows for retrieval.…”

“…It should be noted that the MT_CKD also includes the continua model of several gases such as CO 2 , O 3 and O 2 . There have been unofficial updates of the MT_CKD 2.5 continuum (Lechevallier et al ., ; O'Dell et al ., ), but these revisions have not been carried out using adequate experimental constraints. Thus, currently there is no consensus on the strength of the MT_CKD 2.5 continuum in some near‐IR windows.…”

“…Unlike most of the reported near‐IR measurements, which were mostly dedicated to validating the MT_CKD continuum model (see, for example, Lechevallier et al ., , and references therein), Ptashnik et al . (, ) used high‐resolution laboratory measurements to derive the CAVIAR (Continuum Absorption at Visible and Infrared Wavelengths and its Atmospheric Relevance) water vapour continuum model that can be used at all wavelengths in the near‐IR.…”