R. H. Tipping

The hitran molecular database : editions of 1991 and 1992

Abstract We describe in this paper the modifications, improvements, and enhancements to the HITRAN molecular absorption database that have occurred in the two editions of 1991 and 1992. The current database includes line parameters for 31 species and their isotopomers that are significant for terrestrial atmospheric studies. This line-by-line portion of HITRAN presently contains about 709,000 transitions between 0 and 23,000 cm-1 and contains three molecules not present in earlier versions: COF2, SF6, and H2S. The HITRAN compilation has substantially more information on chlorofluorocarbons and other molecular species that exhibit dense spectra which are not amenable to line-by-line representation. The user access of the database has been advanced, and new media forms are now available for use on personal computers.

The 1997 spectroscopic GEISA databank

The current version GEISA-97 of the computer-accessible database system GEISA (Gestion et Etude des Informations Spectroscopiques Atmospheriques: Management and Study of Atmospheric Spectroscopic Information) is described. This catalogue contains 1,346,266 entries. These are spectroscopic parameters required to describe adequately the individual spectral lines belonging to 42 molecules (96 isotopic species) and located between 0 and 22,656 cm-1. The featured molecules are of interest in studies of the terrestrial as well as the other planetary atmospheres, especially those of the Giant Planets. GEISA-97 contains also a catalog of absorption cross-sections of molecules such as chlorofluorocarbons which exhibit unresolvable spectra. The modifications and improvements made to the earlier edition (GEISA-92) and the data management software are described. GEISA-97 and the associated management software are accessible from the ARA/LMD (Laboratoire de Meteorologie Dynamique du CNRS, France) web site: http://ara01.polytechnique.fr/registration.

A far wing line shape theory and its application to the water continuum absorption in the infrared region. I

A theory is presented for the calculation of the continuous absorption due to the far wing contributions of allowed lines. The theory is based on the quasistatic approximation for the far wing limit and the binary collision approximation of one absorber molecule and one bath molecule. In line space, the motion of the dipole moment of the absorber molecule, determined by the time displacement Liouville operator related to the total Hamiltonian of the absorber molecule and the bath molecule, is approximately expressed as the ordered product of two time displacement Liouville operators, one related to the intermolecular potential and the other to the unperturbed Hamiltonian. Using a Laplace transformation, the spectral density is expressed in terms of a Cauchy integral whose integrand is a product of two resolvent operators corresponding to the interaction and to the unperturbed Hamiltonian, respectively. After isolating the effects of the bath variables, the spectral density and subsequently the absorption ...

Water vapor continuum in the millimeter spectral region

A theory is presented for the calculation of the continuous absorption of water molecules in the millimeter spectral region. The theory is based on a generalization of Fano’s theory in which the spectral density, the Fourier transform of the dipole‐moment correlation function, is calculated for a system consisting of a pair of molecules. The internal states are written in terms of the line space of the system, and the resolvent operator is obtained using the well‐known Lanczos algorithm. For the interaction between two water molecules, we include only the leading dipole–dipole term of the long‐range anisotropic potential, and model the isotropic interaction, used to calculate the statistical weight within the quasi‐static approximation, by a Lennard–Jones potential. Using reasonable values for the two Lennard–Jones potential parameters, and the known rotational constants and permanent dipole moment of a water molecule, we calculate the absorption coefficient for frequencies up to 450 GHz for temperatures ...

The averaged density matrix in the coordinate representation: Application to the calculation of the far-wing line shapes for H2O

The far-wing line shape theory within the binary collision and quasistatic framework developed previously for linear molecules using the coordinate representation has been generalized to symmetric- and asymmetric-top molecular systems. However, due to more variables needed to specify the orientation of these complicated molecules, one has to evaluate multidimensional integrals with higher dimensionality and this would be intractable for practical calculations. In cases where the anisotropic interaction contains cyclic coordinates, one can carry out the integration of the density matrix over these coordinates analytically and obtain the “averaged” density matrix. This reduces the dimensionality of the multidimensional integrals and thus dramatically reduces the computational time necessary to obtain converged results. In addition, a new interpolation method that enables one to treat more realistic potential models has been formulated. Using these results, calculations for the band-average far-wing line sha...

Temperature dependences of mechanisms responsible for the water-vapor continuum absorption. I. Far wings of allowed lines.

It is well known that the water-vapor continuum plays an important role in the radiative balance in the Earths atmosphere. This was first discovered by Elsasser almost 70 years ago, and since that time there has been a large body of work, both experimental and theoretical, on this topic. It has been experimentally shown that for ambient atmospheric conditions, the continuum absorption scales quadratically with the H(2)O number density and has a strong, negative temperature dependence (T dependence). Over the years, there have been three different theoretical mechanisms postulated: Far wings of allowed transitions, water dimers, and collision-induced absorption. Despite the improvements in experimental data, at present there is no consensus on which mechanism is primarily responsible for the absorption. The first mechanism proposed was the accumulation of the far-wing absorption of the strong allowed transitions. Later, absorption by water dimers was proposed and this mechanism provides a qualitative explanation for the strong, negative T dependence. Recently, some atmospheric modelers have proposed that collision-induced absorption is one of the major contributors. However, based on improvements in the theoretical calculation of accurate far-wing line shapes, ab initio dimer calculations, and theoretical collision-induced absorptions, it is now generally accepted that the dominant mechanism for the absorption in the infrared (IR) windows is that due to the far wings. Whether this is true for other spectral regions is not presently established. Although all these three mechanisms have a negative T dependence, their T dependences will be characterized by individual features. To analyze the characteristics of the latter will enable one to assess their roles with more certainty. In this paper, we present a detailed study of the T dependence of the far-wing absorption mechanism. We will then compare our theoretical calculations with the most recent and accurate experimental data in the IR windows. The results of our calculations are found to agree very well with measurements in the 800-1200 cm(-1) region. We conclude from this work that the T dependence in the IR window region predicted by the far-wing theory is negative and moderately strong. Its pattern is not simple and it could vary significantly as the frequency of interest varies.

The atmospheric water continuum in the infrared: Extension of the statistical theory of Rosenkranz

The statistical theory proposed by Rosenkranz to calculate the continuous absorption by water molecules in the high‐frequency (infrared) wing of the pure rotational band is reviewed and extended. In the review there is a discussion, in particular, of the approximations that are made, including those that are necessary and which limit the applicability of the theory to other spectral regions, and those that are made for calculational convenience. Then, several extensions to the theory are discussed, including increasing the number of rotational states used to calculate the band‐average relaxation parameter, modifying the definition of this parameter to account for near‐wing effects, and eliminating the boxcar approximation. This last modification, effected by using asymmetric‐top functions instead of symmetric‐top functions to calculate matrix elements of the density operator and to diagonalize the dipole–dipole interaction, results in significant enhancement of the relaxation parameter. This improvement, ...

The Frequency Detuning Correction and the Asymmetry of Line Shapes: The Far Wings of H2O-H2O

A far-wing line shape theory that satisfies the detailed balance principle is applied to the H2O–H2O system. Within this formalism, two line shapes are introduced, corresponding to band averages over the positive and negative resonance lines, respectively. Using the coordinate representation, the two line shapes can be obtained by evaluating 11-dimensional integrations whose integrands are a product of two factors. One depends on the interaction between the two molecules and is easy to evaluate. The other contains the density matrix of the system and is expressed as a product of two three-dimensional distributions associated with the density matrices of the absorber and the perturber molecule, respectively. If most of the populated states are included in the averaging process, to obtain these distributions requires extensive computer CPU time, but only have to be computed once for a given temperature. The 11-dimensional integrations are evaluated using the Monte Carlo method, and in order to reduce the va...

The density matrix of H2O–N2 in the coordinate representation: A Monte Carlo calculation of the far-wing line shape

The far-wing line shape theory within the binary collision and quasistatic framework has been developed using the coordinate representation. Within this formalism, the main computational task is the evaluation of multidimensional integrals whose variables are the orientational angles needed to specify the initial and final positions of the system during transition processes. Using standard methods, one is able to evaluate the seven-dimensional integrations required for linear molecular systems, or the seven-dimensional integrations for more complicated asymmetric-top (or symmetric-top) molecular systems whose interaction potential contains cyclic coordinates. In order to obviate this latter restriction on the form of the interaction potential, a Monte Carlo method is used to evaluate the nine-dimensional integrations required for systems consisting of one asymmetric-top (or symmetric-top) and one linear molecule, such as H2O–N2. Combined with techniques developed previously to deal with sophisticated pote...

Theoretical calculation of the translation-rotation collision-induced absorption in N2–N2, O2–O2, and N2–O2 pairs

Abstract The translation-rotation collision-induced spectra of N 2 –N 2 , O 2 –O 2 and N 2 –O 2 mixtures are calculated theoretically. For N 2 –N 2 , using the matrix elements for the quadrupole and hexadecapole moments and the isotropic and anisotropic polarizabilities obtained previously from a global analysis of the fundamental band spectra, we obtain numerical values for the zeroth moment that are smaller than the measured values by 9–14%, depending on the temperature. By increasing the value for the matrix element of the isotropic polarizability slightly, good agreement with experiment is obtained. For O 2 –O 2 , the theoretical spectrum is significantly smaller than the experimental result. By increasing the matrix element of the hexadecapole moment by a factor of 1.7, we can obtain good agreement. This larger value for the hexadecapole moment will not appreciably affect the agreement found previously in the fundamental region because the hexadecapole contribution to the intensity is very small, unlike the translation-rotation band where it is larger than the contribution due to the quadrupole moment. Using these parameters, we then calculate the collision-induced absorption for N 2 –O 2 mixtures for which no experimental data exist. Finally, we calculate the collision-induced absorption for air, and compare our results with previous work; we express the results for the ratio of the absorption coefficient of air to that of N 2 –N 2 as a function of wavenumber and temperature, R ( ω , T ), which can easily be implemented in atmospheric models.