Water vapor absorption at isotopic CO2 laser wavelengths

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.

Ammonia monitoring at trace level using photoacoustic spectroscopy in industrial and environmental applications

An ammonia traces analyser based on photoacoustic spectroscopy is described. The system uses a CO(2) laser and a properly designed resonant photoacoustic cell to achieve ammonia detection at sub-parts-per-billion (ppb) level. The instrument features unattended automatic on-line monitoring of ammonia with a detection limit of 0.1 ppb. Interferences from atmospheric CO(2) and H(2)O are efficiently suppressed by a careful selection of the laser wavelength and a compensation of the water vapour signal made with a high-precision hygrometer. The cell design enables continuous measurement at high flow rates (up to 5 l/min), which guarantees a fast response time of the system for the monitoring of ammonia, a sticky polar molecule that adheres to most surfaces. Various examples of applications of the instrument in the semiconductor industry and for atmospheric pollution monitoring are presented. They demonstrate the excellent performances of the system and its suitability for these applications.

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.

Backscattering measurements of atmospheric aerosols at CO2 laser wavelengths: implications of aerosol spectral structure on differential-absorption lidar retrievals of molecular species

The volume backscattering coefficients of atmospheric aerosol were measured with a tunable CO2 lidar system at various wavelengths in Utah (a desert environment) along a horizontal path a few meters above the ground. In deducing the aerosol backscattering, a deconvolution (to remove the smearing effect of the long CO2 lidar pulse and the lidar limited bandwidth) and a constrained-slope method were employed. The spectral shape beta(lambda) was similar for all the 13 measurements during a 3-day period. A mean aerosol backscattering-wavelength dependence beta(lambda) was computed from the measurements and used to estimate the error Delta(CL) (concentration-path-length product) in differential-absorption lidar measurements for various gases caused by the systematic aerosol differential backscattering and the error that is due to fluctuations in the aerosol backscattering. The water-vapor concentration-path-length product CL and the average concentration C = <CL>/L for a path length L computed from the range-resolved lidar measurements is consistently in good agreement with the water-vapor concentration measured by a meteorological station. However, I was unable to deduce, reliably, the range-resolved water-vapor concentration C(r), which is the derivative of the range-dependent product CL, because of the effect of residual noise caused mainly by errors in the deconvolved lidar measurements.

Measurement of the NH(3), CCl(2)F(2), CHClF(2), CFCl(3), and CClF(3) absorption coefficients at isotopic (13)C(16)O(2) laser wavelengths by photoacoustic spectroscopy

Owing to the linearity of the photoacoustic signal, the absorption coefficients of Freons were measured on isotopic (13)C(16)O(2) laser lines.

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.

CO(2) DIAL measurements of water vapor

CO(2) lidars have heretofore been used to measure water vapor concentrations primarily using the 10R(20) line at 10.247 microm, which has a strong overlap with a water vapor absorption line. This paper discusses the use of that line as well as other CO(2) laser lines for which the absorption coefficients are weaker. The literature on measurement of water vapor absorption coefficients using CO(2) lasers is reviewed, and the results from four laboratories are shown to be generally consistent with each other after they are normalized to the same partial pressure, temperature, and ethylene absorption coefficient for the 1P(14) CO(2) laser line; however, the agreement with the Air Force Geophysics Laboratory's HITRAN and FASCOD 2 spectral data tapes is not good either for the water vapor absorption lines or for the water vapor continuum. Demonstration measurements of atmospheric water vapor have been conducted using the Mobile Atmospheric Pollutant Mapping System, a dual CO(2) lidar system using heterodyne detection. Results are discussed for measurements using three sets of laser line pairs covering a wide range of water vapor partial pressures.

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.