Air-broadened Half-width Research Articles

Determination of the water vapor continuum absorption by THz pulse transmission using long-range multipass cell

We measured terahertz (THz) pulses using a 18.6 m propagation distance multipass cell according to water vapor density (WVD) from 1.81 to 16.31 g/m3. The measured power attenuation was smaller than that obtained by the recommendation of the Radiocommunication Sector of the International Telecommunication Union (ITU-R) P. 676–12 (08/2019) database. To compensate for this difference, we proposed a modified pseudo-line (known as water vapor continuum absorption) with a center frequency of 2.740 THz, a line strength of 31,900 kHz/hPa, and an air-broadened half-width of 400 GHz/hPa. The calculated power attenuation by the modified ITU-R agreed well with the measurements of different WVDs. Particularly, we determined the water vapor self-continuum (CW) and foreign-continuum (CA) parameters of the water vapor continuum absorption by comparing the power attenuations, which were calculated from the modified ITU-R and high-resolution transmission (HITRAN) molecular absorption databases.

Line shape parameters of air-broadened water vapor transitions in the ν1 and ν3 spectral region

A Bruker IFS-120HR Fourier transform spectrometer located at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington was used to record a series of spectra of pure H2O and air-broadened H2O in the regions of the ν1 and ν3 bands (3450–4000 cm−1) at different pressures, temperatures and volume mixing ratios of H2O in air. Eighteen high-resolution, high signal-to-noise (S/N) ratio absorption spectra were recorded at T = 268, 296 and 353 K using two temperature-controlled absorption cells with path lengths of 9.906(1) and 19.95(1) cm. The resolution of the spectra recorded with the 9.906 cm and 19.95 cm absorption cells was 0.006 and 0.008 cm−1, respectively. A multispectrum nonlinear least squares fitting technique was employed to fit all the eighteen spectra simultaneously to retrieve 313 accurate line positions, 315 intensities, 229 Lorentz air-broadened half-width and 213 air-shift coefficients and their temperature dependences (136 for air-broadened width and 128 for air-shift coefficients, respectively). Room temperature self-broadened half-width coefficients for 209 transitions and self-shift coefficients for 106 transitions were also measured. Line mixing coefficients were experimentally determined for isolated sets of 10 transition pairs for H2O-air and 8 transition pairs for H2O-H2O using the off-diagonal relaxation matrix element formalism, and 85 quadratic speed dependence parameters were measured. Modified Complex Robert-Bonamy (MCRB) calculations of self-, and air-broadened (from N2- and O2-broadening) half-width and air-shift coefficients, and temperature dependence exponents of air-broadened half-width coefficients are made. The measurements and calculations are compared with each other and with similar parameters reported in the literature.

Air- and self-broadened half widths, pressure-induced shifts, and line mixing in the ν2 band of 12CH4

Lorentz self- and air-broadened half width and pressure-induced shift coefficients and their dependences on temperature have been measured from laboratory absorption spectra for nearly 130 transitions in the ν2 band of 12CH4. In addition line mixing coefficients (using the relaxation matrix element formalism) for both self- and air-broadening were experimentally determined for the first time for a small number of transitions in this band. Accurate line positions and absolute line intensities were also determined. These parameters were obtained by analyzing high-resolution (~0.003 to 0.01cm−1) laboratory spectra of high-purity natural CH4 and air-broadened CH4 recorded at temperatures between 226 and 297K using the McMath–Pierce Fourier transform spectrometer (FTS) located at the National Solar Observatory on Kitt Peak, Arizona. A multispectrum nonlinear least squares technique was used to fit short (5–15cm−1) spectral intervals in 24–29 spectra simultaneously. Parameters were determined for ν2 transitions up to J″=16. The variations of the measured broadening and shift parameters with the rotational quantum number index and tetrahedral symmetry species are examined. The present results are also compared with previous measurements available in the literature.

The effects of different HITRAN versions on calculated long-wave radiation and uncertainty evaluation

Four editions of the High Resolution Transmission (HITRAN) databases (HITRAN96, HITRAN2K, HITRAN04, and HITRAN08) are compared by using a line-by-line (LBL) radiative model in the long-wave calculation for six typical atmospheres. The results show that differences in downward radiative fluxes between HITRAN96 and HITRAN08 at the surface can reach a maximum of 1.70 W m−2 for tropical atmospheres. The largest difference in heating rate between HITRAN96 and HITRAN08 can reach 0.1 K day−1 for midlatitude summer atmosphere. Uncertainties caused by line intensity and air-broadened half-widths are also evaluated in this work using the uncertainty codes given in HITRAN08. The uncertainty is found to be 1.92 W m−2 for upward fluxes at the top of the atmosphere (TOA) and 1.97 W m−2 for downward fluxes at the surface. The largest heating rate caused by the uncertainty of line intensity and air-broadened half-width can reach 0.5 K day−2. The differences in optical depths between 1300 and 1700 cm−1 caused by different HITRAN versions are larger than those caused by the uncertainties in intensity and air-broadened half-width. This paper suggests that there is inaccurate representation of line parameters over some spectral ranges in HITRAN and more attention should be paid to these ranges in fields such as remote sensing.

A multispectrum analysis of the ν 4 band of 13CH 4: Widths, shifts, and line mixing coefficients

Line positions, Lorentz air-broadened half width and air pressure-induced shift coefficients have been measured for nearly 200 transitions in the ν 4 band of 13CH 4 from high-resolution spectra recorded with the McMath-Pierce Fourier transform spectrometer. Three room temperature spectra of 13CH 4 used in the previous study of Malathy Devi et al. (Air-broadened Lorentz halfwidths and pressure-induced line shifts in the ν 4 band of 13CH 4. Appl. Opt. 1988; 27: 2296–2308) were analyzed together with a large number of additional spectra of self- and air-broadened CH 4 recorded at 210–314 K and one room-temperature spectrum of self-broadened 13CH 4. Analyses applying the multispectrum nonlinear least squares fitting technique were performed to retrieve the spectral line parameters. In addition to air-broadened half width and shift coefficients, self-broadened half width and shift coefficients were determined for at least 56 13CH 4 ν 4 transitions. Off-diagonal relaxation matrix element coefficients for air-broadened line mixing were also determined for 28 pairs of P and R transitions in a number of J manifolds, and mixing parameters for self-broadening were also determined for some of these pairs. Temperature-dependences of the pressure-induced shift and mixing parameters for the 13CH 4 lines could not be determined from the spectra used in the present analysis, but temperature dependences of the half width coefficients were determined for the strongest transitions. The results of this study are compared with other studies of air- and self-broadened 13CH 4 and 12CH 4.

Theoretical calculations of N 2-broadened half-widths of ν5 transitions of HNO 3

A number of satellite instruments are measuring nitric acid, HNO 3, in the Earth's atmosphere. In order to do retrievals of temperature and concentration profiles, the spectral parameters for many thousands of HNO 3 transitions must be known. Currently the HITRAN database uses a constant estimated value for the air-broadened half-width of HNO 3. To help improve the line shape parameters, complex Robert–Bonamy calculations were made to determine N 2-broadened half-widths for some 5000 transitions of HNO 3 in the ν 5 band. The intermolecular potential is a sum of electrostatic terms (dipole–quadrupole and quadrupole–quadrupole) and the atom–atom potential expanded to eighth order. The trajectory parameters were adjusted to yield better agreement with measurement. Velocity integrated calculations were made at seven temperatures in order to determine the temperature dependence of the half-widths. The half-width data are compared with available rotation band measurements. The average percent difference between the measured and calculated half-widths is −2.38 for N 2-broadening and −0.65 for air-broadening. The temperature, vibrational, and rotational state dependence of the half-width are investigated.

Air-broadened halfwidth and pressure shift coefficients of 12C 16O 2 bands: 4750–7000 cm −1

Previously we obtained self-broadened halfwidth and self-induced shift coefficients at room temperature for 15 near infrared CO 2 bands between 4750 and 7000 cm −1 [R.A. Toth, L.R. Brown, C.E. Miller, V.M. Devi, D.C. Benner, J. Mol. Spectrosc., 239 (2006) 243–271]. The present study expands our work on the near infrared line parameters of CO 2 to include air broadening coefficients. Here we report nearly 400 air-broadened half width and air-induced pressure shift coefficients spanning 11 different CO 2 vibrational bands in the 4750–7000 cm −1 region. Retrievals have been performed using Voigt line profiles over three distinct spectral intervals: (a) 4750–5200 cm −1, covering the 20011 ← 00001, 20012 ← 00001, and 20013 ← 00001 Fermi Triad and three associated hot bands 21111 ← 01101, 21112 ← 01101, 21113 ← 01101; (b) 6100–7000 cm −1, covering the 30011 ← 00001, 30012 ← 00001, 30013 ← 00001 and 30014 ← 00001 Fermi Tetrad; (c) near 6950 cm −1 for the 00031 ← 00001 overtone band. The air-broadened halfwidth and air-induced pressure shift coefficients have been modeled with empirical expressions and compared to other measurements available in the literature.

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.

Multispectrum analysis of 12CH 4 from 4100 to 4635 cm −1: II. Air-broadening coefficients (widths and shifts)

Room temperature Lorentz air-broadened halfwidth and pressure-induced air-shift coefficients were measured for 1011 transitions in the octad region of methane between 4100 and 4635 cm −1. These measurements were made by analyzing 10 laboratory absorption spectra recorded at 0.011 cm −1 resolution using the McMath–Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak, Arizona. The spectra were obtained using two absorption cells with path lengths of 2.05 and 150 cm. The total sample pressures ranged from 99 to 400 torr with CH 4 volume mixing ratios of 0.01 in dry air. The spectral line parameters were retrieved using a multispectrum nonlinear least squares technique. Transitions belonging to five bands of the octad polyad were observed, namely ν 2 + 2 ν 4, ν 1 + ν 4, ν 3 + ν 4, 2 ν 2 + ν 4, and ν 2 + ν 3. The numbers of measurements by bands are: 33 for ν 2 + 2 ν 4, 180 for ν 1 + ν 4, 635 for ν 3 + ν 4, 33 for 2 ν 2 + ν 4, and 130 for ν 2 + ν 3. Transitions with rotational quantum number J up to 16 are included in the analysis. The measured width and shift coefficients vary according to the tetrahedral symmetry species and the rotational quantum numbers of the transitions. The retrieved parameters from this study are compared with prior results, in part to estimate absolute accuracy and determine the extent of vibrational dependence of widths and shifts.

The effect of the half-width of the 22-GHz water vapor line on retrievals of temperature and water vapor profiles with a 12-channel microwave radiometer

We show that observed biases in retrievals of temperature and water vapor profiles from a 12-channel microwave radiometer arise from systematic differences between the observed and model-calculated brightness temperatures at five measurement frequencies between 22 and 30 GHz. Replacing the value for the air-broadened half-width of the 22-GHz water vapor line used in the Rosenkranz absorption model with the 5% smaller half-width from the HITRAN compilation largely eliminated the systematic differences in brightness temperatures. An a priori statistical retrieval based on the revised model demonstrated significant improvements in the accuracy and vertical resolution of the retrieved temperature and water vapor profiles. Additional improvements were demonstrated by combining the MWRP retrievals with those from the GOES-8 sounder and by incorporating brightness temperature measurements at off-zenith angles in the retrievals.

Air-Broadening and Shift Coefficients of O3Lines in the ν2Band and Their Temperature Dependence

Room temperature measurements of self- and air-broadening coefficients are reported for over 370 transitions covering a range of 0 ≤J″ ≤ 45 and 1 ≤Ka″≤ 12 for the ν2ozone band in the 630 to 800 cm−1spectral region. In addition, the temperature dependence of air-broadened halfwidth and air-induced pressure shift coefficients have been determined for over 350 spectral lines. A total of 29 O3absorption spectra (0.005-cm−1resolution) recorded at various temperatures (29 to −63°C) with a Fourier transform spectrometer were used in the analysis. The spectral line parameters were deduced by analyzing all of the 29 spectra simultaneously using a nonlinear least-squares fitting technique. The results are compared with similar measurements obtained in the ozone ν1band.

The HITRAN database: 1986 edition

A description and summary of the latest edition of the AFGL HITRAN molecular absorption parameters database are presented. This new database combines the information for the seven principal atmospheric absorbers and twenty-one additional molecular species previously contained on the AFGL atmospheric absorption line parameter compilation and on the trace gas compilation. In addition to updating the parameters on earlier editions of the compilation, new parameters have been added to this edition such as the self-broadened halfwidth, the temperature dependence of the air-broadened halfwidth, and the transition probability. The database contains 348043 entries between 0 and 17,900 cm(-1). A FORTRAN program is now furnished to allow rapid access to the molecular transitions and for the creation of customized output. A separate file of molecular cross sections of eleven heavy molecular species, applicable for qualitative simulation of transmission and emission in the atmosphere, has also been provided.