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

Estimation of the Total Atmospheric Water Vapor Content and Land Surface Temperature Based on AATSR Thermal Data

The total atmospheric water vapor content (TAWV) and land surface temperature (LST) play important roles in meteorology, hydrology, ecology and some other disciplines. In this paper, the ENVISAT/AATSR (The Advanced Along-Track Scanning Radiometer) thermal data are used to estimate the TAWV and LST over the Loess Plateau in China by using a practical split window algorithm. The distribution of the TAWV is accord with that of the MODIS TAWV products, which indicates that the estimation of the total atmospheric water vapor content is reliable. Validations of the LST by comparing with the ground measurements indicate that the maximum absolute derivation, the maximum relative error and the average relative error is 4.0K, 11.8% and 5.0% respectively, which shows that the retrievals are believable; this algorithm can provide a new way to estimate the LST from AATSR data.

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.

Algorithm for the retrieval of columnar water vapor from hyperspectral remotely sensed data

A new algorithm for the retrieval of columnar water vapor content is presented. The proposed procedure computes the area of the H2O absorption centered about 940 nm to allow its integrated columnar abundance as well as its density at ground level to be assessed. The procedure utilizes the HITRAN 2000 database as the source of H2O cross-section spectra. Experimental results were derived from radiometrically calibrated hyperspectral images collected by the Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) sensor over the Cuprite mining district in Nevada. Numerical simulations based on the MODTRAN 4 radiative transfer code were also employed for investigating the algorithm's performance. An additional empirical H2O retrieval procedure was tested by use of data gathered by the VIRS-200 imaging spectrometer.

Remote Mapping of Vegetation and Geological Features by Lidar in the 9-11-mum Region

We report examples of the use of a scanning tunable CO(2) laser lidar system in the 9-11-mum region to construct images of vegetation and rocks at ranges as far as 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Object classification is based on reflectance spectra, which are sufficiently distinct to allow discrimination between several tree species, between trees and scrub vegetation, and between natural and artificial targets. Limitations imposed by laser speckle noise are discussed.

The Continuum of Water Vapor Mixed with Methane: Absolute Absorption at 239 GHz and Linewidth Calculations

In the aim of achieving a better understanding of the molecular mechanisms involved in the absorption continuum of the water-vapor rotational spectrum, various H(2)O-X mixtures are investigated in an atmospheric window in the mmw range. In the present study, the X mixing gas is methane. Absolute absorption rates have been measured at 239 GHz. The dependence on pressure as well as temperature has been obtained. The experimental data are compared with models using conventional line profiles. As these models require the knowledge of the collisional half-widths of H(2)O broadened by CH(4), theoretical calculations using the complex Robert-Bonamy formalism have been carried out. A "continuum effect" is observed when comparing the experimental absorption with the models, as well as for the magnitude of the absorption discrepancy and for the strong temperature dependence of this absorption. The effect is compared for various X mixing gases to the collisional broadening efficiency in the impact approximation. Copyright 2000 Academic Press.

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.

Submillimeter fourier-transform spectrometer measurements of atmospheric opacity above mauna kea

We present accurately calibrated submillimeter atmospheric transmission spectra obtained with a Fourier-transform spectrometer at the Caltech Submillimeter Observatory on Mauna Kea, Hawaii. These measurements cover the 0.9-0.3-mm wavelength range and are the first in a series aimed at defining the terrestrial long-wave atmospheric transmission curve. The 4.1-km altitude of the Mauna Kea site provides access to extremely low zenith water-vapor columns, permitting atmospheric observations at frequencies well above those possible from sea level. We describe the calibration procedures, present our first well-calibrated transmission spectra, and compare our results with those of a single-layer atmospheric transmission model, AT. With an empirical best-fit continuum opacity term included, this simple single-layer model provides a remarkably good fit to the opacity data for H(2)O line profiles described by either van Vleck-Weisskopf or kinetic shapes.

Quantitative chemical identification of four gases in remote infrared (9-11 mum) differential absorption lidar experiments

A combined experimental and computational approach utilizing tunable CO(2) lasers and chemometric analysis was employed to detect chemicals and their concentrations in the field under controlled release conditions. We collected absorption spectra for four organic gases in the laboratory by lasing 40 lines of the laser in the 9.3-10.8-mum range. The ability to predict properly the chemicals and their respective concentrations depends on the nature of the target, the atmospheric conditions, and the round-trip distance. In 39 of the 45 field experiments, the identities of the released chemicals were identified correctly without predictions of false positives or false negatives.

Experimental investigation of the self- and N(2)-broadened continuum within the ν(2) band of water vapor

We present an experimental study of the self- and N(2)-broadened H(2) O continuum in microwindows within the ν(2) fundamental centered at ~1600 cm(-1). The continuum is derived from transmission spectra recorded at room temperature with a BOMEM Fourier transform spectrometer at a resolution of ~0.040 cm(-1). Although we find general agreement with previous studies, our results suggest that there is significant near-wing super-Lorentzian behavior that produces a highly wave-number-dependent structure in the continuum as it is currently defined.

Analysis of the FASCODE model and its H(2)O continuum based on long-path atmospheric transmission measurements in the 4.5-11.5-µm region

An experimental study performed to evaluate the atmospheric transmission model FASCODE and its water vapor continuum [Clough, Kneizys, and Davies (CKD) model, Atmos. Res. 23, 229-241 (1989)] in the 850-2250-cm-(1) spectral region is presented. The analysis is based on a comparison between model calculations and transmission measurements carried out at the Defence Research Establishment Valcartier over a 5.7-km horizontal path for a wide range of ambient temperature (from -8.6 to 29.4°C) and humidity (from 1.16 to 14.2 g/m(3)) conditions. The agreement between measurements and calculations is good on the average. However, there are three specific spectral intervals where the differences cannot be explained by experimental errors. For summer conditions, it is shown that FASCODE overestimates the transmittance by approximately 3-6% (absolute terms) in the 850-950-cm(-1) region. For winter conditions, measurements are higher than calculations by as much as a factor of 2 at the edges of the 6.3µm absorption band of water vapor, namely near 1250-1380 cm(-1) and 1800-2000 cm(-1). The continuous nature of these differences is interpreted as anomalies that are due to the broadening coefficients of the water vapor continuum (CKD model). A set of coefficients is derived from experimental spectra and compared with coefficients from the CKD model. The results suggest that first the self-broadening coefficients at high temperature, C¯(s)(ν, 296), need to be increased by 10-16% near 850-950 cm(-1) and second the foreign broadening coefficients, C¯(ν), need to be decreased by approximately a factor of 2 near 1250-1380 cm-(1) and 1800-2000 cm(-1) to recover a good model-measurement agreement in these three spectral intervals. A modified continuum (based on coefficients derived from transmittances) has been implemented in FASCODE and used to analyze emission spectra from the High-Resolution Interferometer Sounder instrument. The modified continuum reduces the discrepancy by almost a factor of 5 near 1350 cm(-1).

Temperature dependence of water vapor absorption coefficients for CO(2) differential absorption lidars

A temperature correction of water vapor differential absorption coefficients for the CO(2) transition line pairs (10R20, 10R18) and (10R20, 10R22) for temperatures between -0.5 °C and 20 °C is computed, with a reference temperature of 27 °C, from medium-range CO(2) lidar field measurements. The empirical temperature correction, X(T), is fitted with the polynomial X(T) = α(0) + α(1) × T + α(2) × T(2). For the transition line pair (10R20, 10R18) the temperature dependence ranges from 1.62%/°C to 3.47%/°C, and the temperature correction for the transition line pair (10R20, 10R22) ranges from 1.32%/°C to 2.43%/°C.

Wavelength dependence of backscattering and extinction of kaolin dust at CO(2) laser wavelengths: effect of multiple scattering

A multiple wavelength, pulsed CO(2) lidar system is used to measure spectral backscattering and extinction of kaolin dust of different optical thicknesses. The measurements show that the wavelength dependence of spectral backscattering changes with increased multiple scattering, whereas the spectral extinction remains relatively unchanged. A simple analytical two-stream radiative transfer model is used to confirm the measurements qualitatively. Several equations were derived from the model to show that in general the wavelength dependence of backscatter is less dependent on wavelength for a multiplescattering case. Therefore, the aerosol cloud becomes a diffuse target that is more flat in its spectral reflectance as multiple scattering increases. An application to differential absorption detection is discussed and shows that, in general, the effect of multiple scattering on the backscattered signal from aerosols will tend to reduce the error in deducing the average path-length concentration of the absorbing tr ce gas.

High pulse repetition frequency, multiple wavelength, pulsed CO(2) lidar system for atmospheric transmission and target reflectance measurements

A multiple wavelength, pulsed CO(2) lidar system operating at a pulse repetition frequency of 200 Hz and permitting the random selection of CO(2) laser wavelengths for each laser pulse is presented. This system was employed to measure target reflectance and atmospheric transmission by using laser pulse bursts consisting of groups with as many as 16 different wavelengths at a repetition rate of 12 Hz. The wavelength tuning mechanism of the transversely excited atmospheric laser consists of a stationary grating and a flat mirror controlled by a galvanometer. Multiple wavelength, differential absorption lidar (DIAL) measurements reduce the effects of differential target reflectance and molecular absorption interference. Examples of multiwavelength DIAL detection for ammonia and water vapor show the dynamic interaction between these two trace gases. Target reflectance measurements for maple trees in winter and autumn are presented.

Infrared continuum water vapor absorption coefficients derived from satellite data

In a recent paper W. B. Grant ["Water Vapor Absorption Coefficients in the 8-13-microm Spectral Region: a Critical Review," Appl. Opt. 29, 451-462 (1990)] reviewed experimental measurements of the water vapor continuum absorption coefficients in the 8-13-microm spectral region. In comparing three different groundbased techniques he concluded that the absorption coefficients used in the HITRAN and LOWTRAN-7 codes are in reasonable agreement with the measurements. These coefficients are approximately 20% lower than those used in the LOWTRAN-6 code. A fourth method of experimentally determining the water vapor absorption coefficients is described which suggests an opposite result. A comparison between sets of satellite measured infrared radiances and ship measurements of sea surface temperature using a model of infrared transmission through the atmosphere gives coefficients that are 20-40% larger than those used in LOWTRAN-6. Agreement between the four different measurement techniques is only posible if a much stronger negative temperature dependence of the continuum absorption coefficients is adopted. This results in significant increases in the coefficients at temperatures below 270 K.

Measurements of Lorentz air-broadening coefficients and relative intensities in the H(2)(16)O pure rotational and v(2) bands from long horizontal path atmospheric spectra

Lorentz air-broadening coefficients and relative intensities have been measured for forty-three lines in the pure rotational band and twenty lines in the v(2) band of H(2)(16)O between 800 and 1150 cm(-1). The results were derived from analysis of nine 0.017-cm(-1) resolution atmospheric absorption spectra recorded over horizontal paths of 0.5-1.5 km with the McMath Fourier transform spectrometer and main solar telescope operated on Kitt Peak by the National Solar Observatory. A nonlinear least-squares spectral fitting technique was used in the spectral analysis. The results are compared with previous measurements and calculations. In most cases, the measured pressure-broadening coefficients and intensities are significantly different from the values in the 1986 HITRAN line parameters compilation.

Trace detection of hydrazines by optical homodyne interferometry

A photothermal laser interferometric system is described that has sufficient sensitivity to allow the detection of the hydrazines: hydrazine, monomethylhydrazine, and unsymmetrical dimethyihydrazine at part per billion concentrations. A line tunable CO(2) laser excites the trace hydrazine molecules in one arm of a modified Jamin interferometer illuminated with a single frequency He-Ne laser. The CO(2) laser beam intersects one of the He-Ne beams in the interferometer at a small angle, so there is no interaction of the IR and visible laser beams at any optical components in the system. The system operates with computer control of interferometer alignment, CO(2) excitation laser tuning, and data acquisition.