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 <20°C. The rotational and
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.
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CO2 Laser Measurements of Strong Water Vapor Line Absorption
Coefficients at 300 K, 10-Torr Partial Pressure, in Synthetic Air, and
αC2H4 = 34.76
atm−1 cm−1, for Selected
CO2 Laser Lines
Ammonia interference possible for this CO2 laser line.
Table V
Comparison of Spectrophone Measurements of Strong Water Vapor Line Absorption
Coefficients Using CO2 Lasers with the Values Using
hitran for 10-Torr Partial Pressure, 300 K,
αC2H4 = 35.0
atm−1 cm−1
Bradley et al.42
Includes 6.24% uncertainty due to the uncertainty in determining
the value of the ethylene absorption coefficient for the
10P(14) CO2 laser line.
JPL and AFGL (Clough37)
determinations with the line strengths thought to be accurate to
±20% and the continuum to ±10%.
Table VI
Comparison of Problems and Advantages for Three Approaches for Measuring
Water Vapor Continuum Absorption Coefficients
Problems
Advantages
All
Approaches
Noncontinuous tunability of CO2
lasers
Low-to-moderate spectral resolution with
broadband radiation sources
Knowledge of water vapor
concentration
Knowledge of rotational/vibrational line
parameters
Sample purity (i.e., having only water
vapor in the measurement path)
CO2 Laser Measurements of Strong Water Vapor Line Absorption
Coefficients at 300 K, 10-Torr Partial Pressure, in Synthetic Air, and
αC2H4 = 34.76
atm−1 cm−1, for Selected
CO2 Laser Lines
Ammonia interference possible for this CO2 laser line.
Table V
Comparison of Spectrophone Measurements of Strong Water Vapor Line Absorption
Coefficients Using CO2 Lasers with the Values Using
hitran for 10-Torr Partial Pressure, 300 K,
αC2H4 = 35.0
atm−1 cm−1
Bradley et al.42
Includes 6.24% uncertainty due to the uncertainty in determining
the value of the ethylene absorption coefficient for the
10P(14) CO2 laser line.
JPL and AFGL (Clough37)
determinations with the line strengths thought to be accurate to
±20% and the continuum to ±10%.
Table VI
Comparison of Problems and Advantages for Three Approaches for Measuring
Water Vapor Continuum Absorption Coefficients
Problems
Advantages
All
Approaches
Noncontinuous tunability of CO2
lasers
Low-to-moderate spectral resolution with
broadband radiation sources
Knowledge of water vapor
concentration
Knowledge of rotational/vibrational line
parameters
Sample purity (i.e., having only water
vapor in the measurement path)