Water vapor-nitrogen absorption at CO(2) laser frequencies

The water vapour self- and water-nitrogen continuum absorption in the 1000 and 2500 cm(-1) atmospheric windows

The pure water vapour and water-nitrogen continuum absorption in the 1000 and 2500 cm(-1) atmospheric windows has been studied using a 2 m base-length White-type multi-pass cell coupled with a BOMEM DA3-002 Fourier transform infrared spectrometer. The measurements were carried out at the National Institute of Standards and Technology (NIST, Gaithersburg, MD) over the course of several years (2004, 2006-2007, 2009). New data on the H(2)O:N(2) continuum in the 1000 cm(-1) window are presented and summarized along with the other experimental results and the continuum model. The experimental data reported on the water vapour continuum in these atmospheric windows basically agree with the most reliable laboratory data from the other sources. The MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) continuum model significantly departs from the experimental data in both windows. The deviation observed includes the continuum magnitude, spectral behaviour and temperature dependence. In the 2500 cm(-1) region, the model does not allow for the nitrogen fundamental collision-induced absorption (CIA) band intensity enhancement caused by H(2)O:N(2) collisions and underestimates the actual absorption by over two orders of magnitude. The water vapour continuum interpretation as a typical CIA spectrum is reviewed and discussed.

Infrared water vapor continuum absorption at atmospheric temperatures

We have used a continuous-wave carbon dioxide laser in a single-mode realization of cavity ring-down spectroscopy to measure absorption coefficients of water vapor at 944 cm(-1) for several temperatures in the range 270-315 K. The conventional description of water vapor infrared absorption is applied, in which the absorption is modeled in two parts consisting of local line absorption and the remaining residual absorption, which has become known as the water vapor continuum. This water vapor continuum consists of distinct water-water, water-nitrogen, and water-oxygen continua. The water-water continuum absorption coefficient is found to have a magnitude of C(s)(296 K) = (1.82+/-0.02) x 10(-22) cm(2) molecule(-1) atm(-1), and the water-nitrogen coefficient has a magnitude of C(n)(296 K) = (7.3 +/- 0.4) x 10(-25) cm(2) molecule(-1) atm(-1). The temperature dependences of both the water-water and the water-nitrogen continua are shown to be well represented by a model describing the expected behavior of weakly bound binary complexes. Using this model, our data yield dissociation energies of D(e) = (-15.9 +/- 0.3) kJ/mole for the water dimer and D(e) = (-3.2 +/- 1.7) kJ/mole for the water-nitrogen complex. These values are in excellent agreement with recent theoretical predictions of D(e) = -15.7 kJ/mole (water dimer) and D(e) = -2.9 kJ/mole (water-nitrogen complex), as well as the experimentally determined value of D(e) = (-15.3 +/- 2.1) kJ/mole for the water dimer obtained by investigators employing a thermal conductivity technique. Although there is reasonably good agreement with the magnitude of the continuum absorption coefficients, the agreement on temperature dependence is less satisfactory. While our results are suggestive of the role played by water dimers and water complexes in producing the infrared continuum, the uncertain spectroscopy of the water dimer in this spectral region prevents us from making a firm conclusion. In the meantime, empirical models of water vapor continuum absorption, essential for atmospheric radiative transfer calculations, should be refined to give better agreement with our low-uncertainty continuum absorption data.

Influence of CO2-laser linewidth on the measured absorption coefficients of atmospheric water vapor and ammonia

We describe the results of analysis of the experimental and calculated data on the water vapor and ammonia absorption coefficients of CO2-laser radiation. We believe that the different halfwidths of CO2-laser lines, caused by different pressures of the working mixtures in specific experiments, are probably the reason for discrepancies between data presented in various papers.

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

Measurements of water vapor absorption coefficients in the thermal IR atmospheric window (8-13 microm) 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 CO(2) laser or a broadband source and filters, a White cell and a CO(2) laser or a broadband source and a spectrometer, and a spectrophone with a CO(2) 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 degrees C. The rotational and v(2) line absorption coefficients do not appear to have been determined well in this spectral region except at CO(2) laser line frequencies, because the agreement between such measurements and the AFGL spectral data tapes is generally not good.

Water-vapor continuum CO2 laser absorption spectra between 27 degrees C and -10 degrees C

Water continuum CO2 laser absorption spectra are reported for temperatures between 27 and -10 degrees C. The continuum is found to possess a negative temperature coefficient. The results obtained suggest that the magnitude of this temperature coefficient increases with increasing water pressure and decreasing temperature. The temperature coefficients between 27 and 10 degrees C for air mixtures containing 3.0- and 7.5-Torr water vapor are -2.0 +/- 0.4 and -2.9 +/- 0.5%/ degrees C, respectively. For mixtures with 3.0-Torr water the 10-0 degrees C temperature coefficient is -7.7 +/- 0.2%/degrees C. The temperature and water pressure dependencies observed for the continuum suggest that while both collisional broadening and water dimer mechanisms contribute to the continuum, the dimer mechanism is more important over this temperature range.

Atmospheric water vapor differential absorption measurements on vertical paths with a CO2 lidar

Ground based vertical path differential absorption measurements were obtained up to a height of 1.5 km with a CO2 lidar transmitting alternatively on the R(20) (10.247-microm) and R(18) (10.260-microm) lines during daylight in conditions of both strong and weak temperature inversions. The differential absorption between these lines for typical middle latitude lower atmosphere water vapor concentrations appears to be well suited to this type of measurement as the power loss on the more absorbed backscattered line [R(20)] is not too great as to unduly restrict the operating range, while the power differential is still sufficiently large to be readily measureable. In one set of measurements a strong temperature inversion at a height of 1 km resulted in a rapid vertical lapse in aerosol concentration with a consequent loss of SNR on the returns and severe distortion to the differential absorption profiles at this level. Water vapor profiles were derived from all measurements except in the region of the strong temperature inversion where the atmospheric backscattering cross section decayed rapidly. Reasonable results were obtained through the weak inversion region. The measurement capability of the lidar was found to be restricted by the length of the laser pulse tail and an inadequate signal-to-noise performance in regions of strong temperature inversions due to the associated decreases in aerosol concentration.

Water vapor absorption at isotopic CO2 laser wavelengths

Water vapor absorption at 161 wavelengths, from 9.2 to 11.9 micron, of the 12C1602, 13C1602, and 14CI602 lasers was measured using a resonant optoacoustic spectrometer. Results were obtained at several precisely determined vapor concentrations in a flow of pure air at a total pressure of 1 atm. Since the same apparatus and methodology were used for all measurements, a reliable assessment can be made of the relative merits of the three lasers in applications such as atmospheric propagation and ranging.

Carbon dioxide laser absorption spectra of toxic industrial compounds

CO(2) laser absorption cross-section data are reported for acrolein, styrene, ethyl acrylate, trichloroethylene, vinyl bromide, and vinylidene chloride. These data indicate that sub parts per billion level, interference-free detection limits should be possible for these compounds by the CO(2) laser photoacoustic technique. Photoacoustic detectabilities below 40 ppb should be possible for these compounds in the presence of ambient air concentrations of water vapor and other anticipated interferences. These compounds are also found not to be important interferences in the detection of toxic hydrazine-based rocket fuels by CO(2) laser spectroscopic techniques.

Characteristics of a photoacoustic air pollution detector at CO(2) laser frequencies

The characteristics of a photoacoustic detection system for measurement of ambient trace atmospheric pollutants at CO(2) laser frequencies are analyzed and described. Several photoacoustic variables are optimized in this study. An alternate-traverse acoustic cell to eliminate cell window signals is described. An ac biased 16-cm(2) microphone is used. The observed modulation frequency dependence of the acoustic signal and a theoretical analysis are also presented. Trace gas interferents limit the practical sensitivity of the system, however, even with interferents present, a minimum measurable pressure change of 2.43 x 10(-9) atm, and apparent absorptivity of 8.42 x 10(-9)/cm was obtained. Techniques to minimize effects of interferents are reported along with the methodology of the photoacoustic technique in trace gas measurements.