Water vapor absorption coefficients in the 8–13-μm spectral region: a critical review

Abstract

Measurements of water vapor absorption coefficients in the thermal IR atmospheric window (8–13 μm) during the past 20 years obtained by a variety of techniques are reviewed for consistency and are compared with computed values based on the AFGL spectral data tapes. The methods of data collection considered were atmospheric long path absorption with a CO₂ laser or a broadband source and filters, a White cell and a CO₂ laser or a broadband source and a spectrometer, and a spectrophone with a CO₂ laser. Advantages and disadvantages of each measurement approach are given as a guide to further research. Continuum absorption has apparently been measured accurately to about the 5–10% level in five of the measurements reported. However, the effect of oxygen broadening has not been fully considered, since most laboratory measurements were made using nitrogen buffering. Oxygen could lead to a small reduction in the adopted value of the water vapor continuum absorption coefficient in air. Also, the temperature dependence does not seem to have been measured well for temperatures ν₂ line absorption coefficients do not appear to have been determined well in this spectral region except at CO₂ laser line frequencies, because the agreement between such measurements and the AFGL spectral data tapes is generally not good.