Water Vapor Absorption Line Research Articles

Water vapor total column measurements using the Elodie Archive at Observatoire de Haute Provence from 1994 to 2004

Abstract. Water vapor total column measurements at Observatoire de Haute Provence (5°42´ E, +43°55´ N), south of France, were obtained using observations of astronomical objects made between July 1994 and December 2004 on the 193-cm telescope with the high-resolution spectrometer Elodie. Spectra of stars, nebulae, and other astronomical objects were taken regularly during 10 years. More than 18 000 spectra from 400 nm to 680 nm are available on-line in the Elodie Archive. This archive, usually explored by astronomers, contains information to study the atmosphere of the Earth. Water vapor absorption lines appear in the visible in delimited bands that astronomers often avoid for their spectral analysis. We used the Elodie Archive with two objectives: firstly, to retrieve seasonal variability and long-term trend of atmospheric water vapor, and secondly, to remove signatures in spectra for further astronomical or geophysical use. The tools presented here (the workflow, the interoperable Elodie Archive and the web service Tellodie) are developed following, when possible, formats and standards recommended by the International Virtual Observatory Alliance.

Calculation of solar radiation atmospheric absorption with different H 2O spectral line data banks

A comparison of the atmospheric absorption calculated with different data banks of water vapour absorption lines is made. The HITRAN database, Barber-Tennyson line list (BT2), calculation of Partridge and Schwenke (PS) are considered. The contribution of H 2O lines, absent in HITRAN, to the atmospheric transmission, calculated with 10 cm −1 spectral resolution in the 10 000–20 000 cm −1 spectral region is up to 1.5% for a vertical path and 4% for a solar zenith angle of 70 deg. The highest difference is observed in the 940 nm band. The incoming fluxes of solar radiation, measured by a rotating solar spectroradiometer, were modeled with BT2 and HITRAN database. The difference between measured and calculated fluxes does not exceed the instrumental uncertainties.

A comparison of water vapor line parameters for modeling the Venus deep atmosphere

The discovery of the near infrared windows into the Venus deep atmosphere has enabled the use of remote sensing techniques to study the composition of the Venus atmosphere below the clouds. In particular, water vapor absorption lines can be observed in a number of the near-infrared windows allowing measurement of the H 2O abundance at several different levels in the lower atmosphere. Accurate determination of the abundance requires a good database of spectral line parameters for the H 2O absorption lines at the high temperatures (up to ∼700 K) encountered in the Venus deep atmosphere. This paper presents a comparison of a number of H 2O line lists that have been, or that could potentially be used, to analyze Venus deep atmosphere water abundances and shows that there are substantial discrepancies between them. For example, the early high-temperature list used by Meadows and Crisp [Meadows, V.S., Crisp, D., 1996. J. Geophys. Res. 101 (E2), 4595–4622] had large systematic errors in line intensities. When these are corrected for using the more recent high-temperature BT2 list of Barber et al. [Barber, R.J., Tennyson, J., Harris, G.J., Tolchenov, R.N., 2006. Mon. Not. R. Astron. Soc. 368, 1087–1094] their value of 45 ± 10 ppm for the water vapor mixing ratio reduces to 27 ± 6 ppm . The HITRAN and GEISA lists used for most other studies of Venus are deficient in “hot” lines that become important in the Venus deep atmosphere and also show evidence of systematic errors in line intensities, particularly for the 8000 to 9500 cm −1 region that includes the 1.18 μm window. Water vapor mixing ratios derived from these lists may also be somewhat overestimated. The BT2 line list is recommended as being the most complete and accurate current representation of the H 2O spectrum at Venus temperatures.

High-efficiency uni-travelling-carrier photomixer at 1.55 µm and spectroscopy application up to 1.4 THz

The fabrication of InGaAs/InP uni-travelling-carrier photodiodes integrated with broadband horn antennas and demonstration of photomixing up to 1.8 THz are reported. A radiated power of 1.1 µW at 940 GHz was measured for a photocurrent of only 2.75 mA (50 mW optical power), demonstrating the high efficiency of the device. The conversion efficiency obtained has been increased by more than a decade compared to the best reported values in the literature. The photomixer is used to identify the 1411 GHz water vapour absorption line at atmospheric pressure.

First airborne water vapor lidar measurements in the tropical upper troposphere and mid-latitudes lower stratosphere: accuracy evaluation and intercomparisons with other instruments

Abstract. In the tropics, deep convection is the major source of uncertainty in water vapor transport to the upper troposphere and into the stratosphere. Although accurate measurements in this region would be of first order importance to better understand the processes that govern stratospheric water vapor concentrations and trends in the context of a changing climate, they are sparse because of instrumental shortcomings and observational challenges. Therefore, the Falcon research aircraft of the Deutsches Zentrum für Luft- und Raumfahrt (DLR) flew a zenith-viewing water vapor differential absorption lidar (DIAL) during the Tropical Convection, Cirrus and Nitrogen Oxides Experiment (TROCCINOX) in 2004 and 2005 in Brazil. The measurements were performed alternatively on three water vapor absorption lines of different strength around 940 nm. These are the first aircraft DIAL measurements in the tropical upper troposphere and in the mid-latitudes lower stratosphere. Sensitivity analyses reveal an accuracy of 5% between altitudes of 8 and 16 km. This is confirmed by intercomparisons with the Fast In-situ Stratospheric Hygrometer (FISH) and the Fluorescent Advanced Stratospheric Hygrometer (FLASH) onboard the Russian M-55 Geophysica research aircraft during five coordinated flights. The average relative differences between FISH and DIAL amount to −3%±8% and between FLASH and DIAL to −8%±14%, negative meaning DIAL is more humid. The average distance between the probed air masses was 129 km. The DIAL is found to have no altitude- or latitude-dependent bias. A comparison with the balloon ascent of a laser absorption spectrometer gives an average difference of 0%±19% at a distance of 75 km. Six tropical DIAL under-flights of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board ENVISAT reveal a mean difference of −8%±49% at an average distance of 315 km. While the comparison with MIPAS is somewhat less significant due to poorer comparison conditions, the agreement with the in-situ hygrometers provides evidence of the excellent quality of FISH, FLASH and DIAL. Most DIAL profiles exhibit a smooth exponential decrease of water vapor mixing ratio in the tropical upper troposphere to lower stratosphere transition. The hygropause with a minimum mixing ratio of 2.5 µmol/mol is found between 15 and 17 km. A high-resolution (2 km horizontal, 0.2 km vertical) DIAL cross section through the anvil outflow of tropical convection shows that the ambient humidity is increased by a factor of three across 100 km.

Water vapor absorption line intensities in the 1900–6600 cm −1 region

Water vapor infrared spectra have been measured using the Bruker IFS 120 HR Fourier transform spectrometer at the Physikalisch-Chemisches Institut of the Justus-Liebig-Universität Giessen. Spectra were recorded at pressure-broadening-limited resolution and at room temperature in the range of 1900–6600 cm −1. The use of fully evacuated transfer optics and a White-type multireflection cell made it possible to obtain pressure×pathlength products up to 31.27 mbar×288.5 m. These spectra have previously been used to determine experimental values of rovibrational line positions and upper energy levels of the 2 ν 2, ν 1, and ν 3 bands [Mikhailenko SN, Tyuterev VlG, Keppler KA, Winnewisser BP, Winnewisser M, Mellau G, et. al. The 2 ν 2 band of water: analysis of new FTS measurements and high- K a transitions and energy levels. J Mol Spectrosc 1997;184: 330–49] and of the 3 ν 2, ν 1+ ν 2, and ν 2+ ν 3 bands [Mikhailenko SN, Tyuterev VlG, Starikov VI, Albert KK, Winnewisser BP, Winnewisser M, et al. Water spectra in the region 4200–6250 cm −1, extended analysis of ν 1+ ν 2, ν 2+ ν 3, and 3 ν 2 bands and confirmation of highly excited states from flame spectra and from atmospheric long-path observations. J. Mol. Spectrosc. 2002; 213: 91–121]. This work presents the intensities of 3769 lines for the weak and medium transitions in the spectral range indicated. These data provide an independent source of experimental information which is complementary to intensity data available in the literature and can thus help to evaluate experimental errors and the reliability of these spectral line parameters.

Suppression of spectral interferences due to water-vapor rotational transitions in terahertz time-domain spectroscopy

Absorption lines of atmospheric water vapor are commonly present in terahertz spectra measured in ambient air. These spectral interferences are caused by the rotational transitions of water molecules. In this study, we develop an effective method for suppression of the water-vapor effects by modeling the absorption lines using Lorentzian line shape and spectroscopic parameters extracted from the HITRAN database and then subtracting them iteratively in the frequency domain. The free-space terahertz pulse and the absorbance spectrum of an explosive material are restored successfully in the experimental verification.

Generation and spectroscopic application of tunable continuous-wave terahertz radiation using a dual-mode semiconductor laser

We present a tunable continuous-wave terahertz radiation source based on photomixing of a tunable dual-mode semiconductor laser on a log-periodic antenna. The discontinuously tunable dual-mode emission of the laser is achieved by spectrally filtered feedback in a double-external-cavity configuration. Two different two-frequency lasing regimes have been identified: pure dual-mode and dual-mode-comb emission. We spectrally investigate the laser emission in these two regimes and study the consequences for the generation of terahertz radiation. Under pure dual-mode emission, we achieve higher power, higher coherence and good tunability of the THz radiation. We demonstrate by detecting absorption lines of water vapor and hydrochloric acid gas that our concept for a tunable continuous-wave terahertz radiation source offers attractive properties for spectroscopic applications.

Calculation of the self-broadening coefficients of water vapor absorption lines using an exact trajectory model

The broadening coefficients γ and δ of vibrational-rotational absorption lines of water vapor by self-pressure are calculated. The exact trajectory model is used, in terms of which the real and imaginary parts of the resonance wave functions are calculated. The reconstruction accuracy of the coefficients γ is considerably improved compared to the model of rectilinear trajectories.

Mode referencing of an external cavity diode laser for continuous frequency stabilization

The output of an external cavity diode laser (ECDL) is frequency stabilized to a water (H2O) vapor absorption line at 935 nm using wavelength modulation spectroscopy (WMS) by means of a dual-feedback locking loop. A novel mode-referencing technique is presented that, in conjunction with the feedback locking mechanism, helps improve the overall frequency-locking system. The mode-referencing enables the device output to maintain its locked frequency in the presence of environmental and other effects, which may cause the operational characteristics of the laser to drift.

Current updates of the water-vapor line list in HITRAN: A new “Diet” for air-broadened half-widths

The current edition of the HITRAN compilation employed a sophisticated algorithm for combining measurements available for the air-broadened half-widths of water-vapor absorption lines with theoretical values. Nevertheless, some of the values in the HITRAN database were found to be far from ideal, due to large dispersions that still exist in the experimental or theoretical methods. Therefore, new criteria were developed for introducing the best available air-broadened half-widths into HITRAN, based on physical principles and statistics. This update concerns the three most abundant isotopologues of water, with the values for H 2 17O and H 2 18O being the ones from analogous transitions of the principal isotopologues. The new parameters have been tested in different remote-sensing applications and improved constituent profiles were obtained. In total, air-broadened half-width values were updated for 11,787 transitions of water vapor in the HITRAN database (6789 for H 2 16O, 2906 for H 2 17O, and 2092 for H 2 18O). Some additional updates to the water-vapor line list are also presented. The resultant file (01_hit06.par) was uploaded to the HITRAN website ( http://www.cfa.harvard.edu/hitran/) in September 2006.

Absolute Frequency Stabilization of Diode Lasers Around 0.94 $\mu\hbox{m}$

We report on the frequency stabilization of extended-cavity diode lasers against rotovibrational absorption lines of water vapor at around 0.94 mum, by means of the frequency modulation spectroscopy technique. A minimum frequency instability of ~ 30 kHz has been obtained for integration times in the range from 1 to 100 s

Measurement of acetaldehyde in exhaled breath using a laser absorption spectrometer

A high-resolution liquid-nitrogen-free mid-infrared tunable diode laser absorption spectroscopy (TDLAS) system was used to perform real-time measurement of acetaldehyde concentrations in human exhaled breath following ingestion of an alcoholic beverage. Acetaldehyde absorption features were measured near 5.79 mum (1727 cm(-1)) using a IV-VI semiconductor laser, a 100 m long path optical gas cell, and second- harmonic detection coupled with wavelength modulation. Acetaldehyde levels were measured with a minimum detection limit of 80 ppb for 5 s integration time. The variations in exhaled acetaldehyde levels over time were analyzed prior to and following ingestion of two different amounts of white wine. A method to calibrate acetaldehyde measurements internally using water vapor absorption lines was investigated to eliminate the need for system calibration with gas standards. The potential of a TDLAS system to be used as a noninvasive clinical tool for measurements of large volatile compounds with possible applications in cancer detection is demonstrated.

High-temperature multipass cell for infrared spectroscopy of heated gases and vapors

In absorption spectroscopy, infrared spectra of heated gases or condensed samples in the vapor phase are usually recorded with a single pass heated gas cell. This device exhibits two orders of magnitude lower sensitivity than the high-temperature multipass cell presented in this article. Our device is a novel type of compact long path absorption cell that can withstand aggressive chemicals in addition to temperatures up to 723 K. The construction of the cell and its technical features are described in detail, paying special attention to the mechanisms that compensate for thermal expansion and that allow the user to vary the optical path length under any thermal or vacuum condition. The cell may be used with a laser source or implemented within a Fourier transform infrared spectrometer. Its design is compatible with optical arrangements using astigmatic mirrors or spherical mirrors in a Herriott configuration. Here we implement a homebuilt Herriott-type cell with a total optical path length of up to 35 m. In order to demonstrate the feasibility of the cell, methane and water vapor absorption lines showing dissimilar temperature effects on line intensity were recorded with the help of a mid-infrared laser source tunable between 3 and 4 microm. Emphasis is put on lines that are too weak to be recorded with a single pass cell.

SWAS observations of water vapor in the Venus mesosphere

We present the first detections of the ground-state H 2 16O ( 1 10 – 1 01 ) rotational transition (at 556.9 GHz) and the 13CO (5–4) rotational transition from the atmosphere of Venus, measured with the Submillimeter Wave Astronomy Satellite (SWAS). The observed spectral features of these submillimeter transitions originate primarily from the 70–100 km altitude range, within the Venus mesosphere. Observations were obtained in December 2002, and January, March, and July 2004, coarsely sampling one Venus diurnal period as seen from Earth. The measured water vapor absorption line depth shows large variability among the four observing periods, with strong detections of the line in December 2002 and July 2004, and no detections in January and March 2004. Retrieval of atmospheric parameters was performed using a multi-transition inversion algorithm, combining simultaneous retrievals of temperature, carbon monoxide, and water profiles under imposed constraints. Analysis of the SWAS spectra resulted in measurements or upper limits for the globally averaged mesospheric water vapor abundance for each of the four observation periods, finding variability over at least two orders of magnitude. The results are consistent with both temporal and diurnal variability, but with short-term fluctuations clearly dominating. These results are fully consistent with the long-term study of mesospheric water vapor from millimeter and submillimeter observations of HDO [Sandor, B.J., Clancy, R.T., 2005. Icarus 177, 129–143]. The December 2002 observations detected very rapid change in the mesospheric water abundance. Over five days, a deep water absorption feature consistent with a water vapor abundance of 4.5 ± 1.5 parts per million suddenly gave way to a significantly shallower absorption, implying a decrease in the water vapor abundance by a factor of nearly 50 in less that 48 h. In 2004, similar changes in the water vapor abundance were measured between the March and July SWAS observing periods, but variability on time scales of less than a week was not detected. The mesospheric water vapor is expected to be in equilibrium with aerosol particles, primarily composed of concentrated sulfuric acid, in the upper haze layers of the Venus atmosphere. If true, moderate amplitude (10–15 K) variability in mesospheric temperature, previously noted in millimeter spectroscopy observations of Venus, can explain the rapid water vapor variability detected by SWAS.

ICLAS of weak transitions of water between 11 300 and 12 850 cm −1: Comparison with FTS databases

Very weak water vapor absorption lines have been investigated by intracavity laser absorption spectroscopy (ICLAS) in the 11 335–11 947 and 12 336–12 843 cm −1 spectral regions dominated by the ν 1 + 3 ν 2 + ν 3 and ν 2 + 3 ν 3 bands, respectively. A detectivity on the order of α min ∼ 10 −9 cm −1 was achieved with an ICLAS spectrometer based on a Ti: Sapphire laser. It allowed detecting transitions with an intensity down to 5 × 10 −28 cm/molecule which is about 10 times lower than the weakest line intensities previously detected in the considered region. A line list corresponding to 1281 transitions with intensity lower than 5 × 10 −26 cm/molecule has been generated. A detailed comparison with the line lists provided by the HITRAN database and by recent investigations by Fourier transform spectroscopy associated with very long multi pass cell is presented. The rovibrational assignment performed on the basis of the ab initio calculations of Schwenke and Partridge, has allowed for determining 176 new energy levels belonging to a total of 16 vibrational states.

Narrowband fiber-optic phase-shifted Fabry–Perot Bragg grating filters for atmospheric water vapor lidar measurements

A unique ultranarrowband fiber-optic phase-shifted Fabry-Perot Bragg grating filter for atmospheric water vapor lidar measurements was designed, fabricated, and successfully tested. Customized optical fiber Bragg gratings were fabricated so that two transmission filter peaks occurred: one (89% transmission, 8 pm FWHM) near the 946-nm water vapor absorption line and the other peak (80% transmission, 4 pm FWHM) at a region of no absorption. Both transmission peaks were within a 2.66-nm stop band. Demonstration of tension tuning to the 946.0003-nm water vapor line was achieved, and the performance characterization of custom-made optical fiber Bragg grating filters are presented. These measurements are successfully compared to theoretical calculations using a piecewise-matrix form of the coupled-mode equations.

Processing X/K band radio occultation data in the presence of turbulence

Canonical transform (CT) and full‐spectrum inversion (FSI) methods can be used in processing radio occultation data to retrieve transmission profiles, which are necessary in addition to bending angle profiles for the retrieval of atmospheric humidity independent of temperature. Low Earth orbiter–low Earth orbiter cross‐link occultations with X/K band frequencies can provide transmission data along the wing of the 22 GHz water vapor absorption line. The computation of transmission assumes spherical symmetry of the atmospheric real and imaginary refractivity. This condition is broken in the presence of turbulence, which can result in significant errors in transmission. We suggest the computation of the differential transmission from the differential CT/FSI amplitude to correct for the effect of turbulence. The efficiency of the method is tested by quasi‐realistic numerical end‐to‐end simulations. We explicitly modeled turbulence as random refractivity fluctuation field based on power form of the fluctuation spectrum. The simulations demonstrate that the error in the retrieved transmission can be significant in a single frequency channel, while the differential transmission can be retrieved with high accuracy. The new method does not impose any significant restriction on the frequency difference between the channels and provides very good differential transmission sensitivity if spacings of a few gigahertz are chosen (e.g., 10 and 17 GHz or 17 and 20 GHz).

Semi-empirical calculation of air-broadened half-widths and air pressure-induced frequency shifts of water-vapor absorption lines

This paper describes a semi-empirical calculation of the air-broadened half-widths and the air pressure-induced frequency shifts for the H 2 16O isotopologue. This semi-empirical calculation is based on fits of several recent high-quality measurements and theoretical calculations to the first-order terms in the expansion of the complex Robert–Bonamy (CRB) equations, which yields a second- and first-order polynomial function of the differences in the upper- and lower-state vibrational quantum numbers for the half-width and line shift, respectively. The aim of this work was to obtain a complete set of air-broadened half-widths and air pressure-induced frequency shifts for transitions of H 2 16O present in the HITRAN database from microwave to the visible in order to supplement the observed and calculated values. For around 700 sets of rotational quantum numbers ( J ′ K a ′ K c ′ ← J ″ K a ″ K c ″ ), semi-empirical coefficients describing the vibrational dependence of the air-broadened half-widths and the air pressure-induced frequency shifts have been obtained directly from the fit of experimental and/or theoretical data. The accuracy of the parameters deduced from this calculation is estimated to be between 5% and 10% for the air-broadened half-widths and between 0.001 and 0.01 cm - 1 atm - 1 for the air pressure-induced frequency shifts. For sets of rotational quantum numbers for which either none or insufficient experimental/theoretical data were available to deduce a vibrational dependence, further approximations have been used to obtain a complete set of semi-empirical coefficients.

Measurement of Low Amounts of Precipitable Water Vapor Using Ground-Based Millimeterwave Radiometry

Abstract Extremely dry conditions characterized by amounts of precipitable water vapor (PWV) as low as 1–2 mm commonly occur in high-latitude regions during the winter months. While such dry atmospheres carry only a few percent of the latent heat energy compared to tropical atmospheres, the effects of low vapor amounts on the polar radiation budget—both directly through modulation of longwave radiation and indirectly through the formation of clouds—are considerable. Accurate measurements of PWV during such dry conditions are needed to improve polar radiation models for use in understanding and predicting change in the climatically sensitive polar regions. To this end, the strong water-vapor absorption line at 183.310 GHz provides a unique means of measuring low amounts of PWV. Weighting function analysis, forward model calculations based upon a 7-yr radiosonde dataset, and retrieval simulations consistently predict that radiometric measurements made using several millimeter-wavelength (MMW) channels near the 183-GHz line, together with established microwave (MW) measurements near the 22.235-GHz water-vapor line and ∼31-GHz atmospheric absorption window can be used to determine within 5% uncertainty the full range of PWV expected in the Arctic. This combined capability stands in spite of accuracy limitations stemming from uncertainties due to the sensitivity of the vertical distribution of temperature and water vapor at MMW channels. In this study the potential of MMW radiometry using the 183-GHz line for measuring low amounts of PWV is demonstrated both theoretically and experimentally. The study uses data obtained during March 1999 as part of an experiment conducted at the Department of Energy’s Cloud and Radiation Testbed (CART) site near Barrow, Alaska. Several radiometers from both NOAA and NASA were deployed during the experiment to provide the first combined MMW and MW ground-based dataset during dry Arctic conditions. Single-channel retrievals of PWV were performed using the MW and MMW data. Discrepancies in the retrieved values were found to be consistent with differences observed between measured brightness temperatures (TBs) and forward-modeled TBs based on concurrent radiosonde profiles. These discrepancies are greater than can be explained by radiometer measurement error alone; errors in the absorption models and uncertainty in the radiosonde measurements contribute to the discrepancies observed. The measurements, retrieval technique, and line model discrepancies are discussed, along with difficulties and potential of MMW/MW PWV measurement.