Absorption spectral Shapes of Greenhouse Gases for their Remote Sensing – ASGGRS

On one hand, the effects of the mixing between lines due to the collisions (line-mixing effects) on the line shape are calculated using a model based on a semi-empirical approach (Energy Corrected Sudden approximation). On the other hand, theoretical calculations are done by molecular dynamics simulations in which the classical equations of motion are solved numerically for a large number of molecules (several millions). These calculations provide, for each molecule, the time evolutions of the position and velocity of its center of mass, as well as of the vectors defining its orientation and rotation angular momentum. From these calculations, the absorption (or Raman scattering) spectrum, as well as the parameters involved in some of the analytical line shape models, can be obtained. Note that this method does not involve adjustable parameters, contrary to most of the semi-empirical methods used in the literature.

On one hand, the CO2 database+software previoulsy developped was updated and led to a significant improvement of the modelisation of these effects in CO2 spectra. On the other hand, ab initio calculations of line-mixing effects in CO2 infrared and raman bands have been done for the first time using classical molecular dynamics simulations. The comparisons between calculated and experimental spectra demonstrate the quality of the theoretical model. This opens promising perspectives for first principle predictions of line-mixing effects in spectra of various systems involving linear molecules.

Once validated, the new theoretical approaches will be put at the disposal of the spectroscopic and remote sensing communities. Our hope is that the use of the new models will yield a significant reduction of the errors on the geophysical parameters determined through the inversion procedures. These new approaches should be important for the success of the actual (and future) instruments, and so for reliable prediction of the effect of the greenhouses gases.

J. Lamouroux et al., J. Chem. Phys. 138(2013), 244310. doi:10.1063/1.4811518.
J. Lamouroux,J.-M. Hartmann,H. Tran,M. Snels,S. Stefani,G. Piccioni,Ab initio classical dynamics simulations of CO2 line-mixing effects in Infrared bands,68th International Symposium on Molecular Spectroscopy (USA).

The influence of some greenhouses gases (carbon dioxide and methane) on the Earth climate is a major environmental, societal, and political issue. Numerous instruments (ground-based or satellite-borne) aim at monitoring these gases in the Earth atmosphere with very high accuracy. The analysis of the measured data, mostly done through the so-called "inversion" procedures, requires the knowledge of the intrinsic spectroscopic parameters of absorption lines (positions, intensities,...). However the collisions between the molecules also have to be considered as their effects yield a modification of the line shape for most of the atmospheric physical conditions (pressure, temperature). The collisionnal parameters describing these effects (half-widths, line shifts, and their temperature dependence, line-mixing parameters,...), and the line shape models are now recognized as being the main source of systematic errors for the remote determination of the greenhouse gases abundances through the inversion procedures. Most of these procedures use the Voigt line shape, convolution of the Gaussian and Lorentzian profiles, despite its well-known deficiencies which yield biases on the remotely sensed parameters. In order to take into account the deviations from the Voigt profile, numerous empirical (or semi-empirical) models, based on one (or more) ad-hoc parameters adjusted on available experimental data, were developed. Due to their empirical nature, these models do not have strong physical bases so that they can lead to miss-interpretations and erroneous predictions when used for extrapolations.
This project aims to answer some of the questions raised by the deviations to the Voigt profile, needed for an accurate determination of the concentrations of the greenhouses gases. Hence, the proposed research should provide new physically-based models and associated software, involving no adjustable parameter when possible, that would be used in the remote sensing and radiative transfer codes. For that, the main tasks proposed here are : (1) the in-depth study of the currently available models; (2) the development of new theoretical approaches and subsequent software; (3) the laboratory measurements; and (4) the study of the atmospheric spectra with the new approaches, and their consequences on the determination of molecular abundances. The methodology proposed for the resolution of these tasks is recent but some have been validated on CO2 by some studies. The hopes are that these new approaches, robust and validated using laboratory measurements and atmospheric spectra, will reduce the uncertainty on the abundances of CO2 and CH4, thus contributing to the success of the actual and future satellite missions devoted to the quantitative cartography of the sinks and sources of these greenhouse gases.