Model, software, and database for line-mixing effects in the ν3 and ν4 bands of CH4 and tests using laboratory and planetary measurements. I. N2 (and air) broadenings and the Earth atmosphere
Résumé
Absorption spectra of the infrared ν3 and ν4 bands of CH4 perturbed by N2 over large ranges of pressure and temperature have been measured in the laboratory. A theoretical approach accounting for line mixing is proposed to (successfully) model these experiments. It is similar to that of Pieroni et al. [J Chem Phys 1999;110:7717–32] and is based on state-to-state rotational cross-sections calculated with a semi-classical approach and a few empirical parameters. The latter, which enable switching from the state space to the line space, are deduced from a fit of a single room temperature spectrum of the ν3 band at 50 atm. The comparisons between numerous measured and calculated spectra under a vast variety of conditions (ν3 and ν4, 0–500 atm, 170–300 K) then demonstrate the quality and consistency of the proposed model. This success is a first validation of a database and associated software built in order to model the shape of CH4 absorption in air, that are available and suitable for the updating of atmospheric radiative transfer codes. The accuracy of these tools is then further demonstrated using transmission measurements of the Earth atmosphere in the ν3 region (3 μm) recorded in solar absorption with ground and balloon based Fourier transform instruments. Similar tests in the ν4 region using satellite based emission spectra and ground-based transmission measurements confirm the model quality although they show very small line-mixing effects and their masking by strong contributions of other species.