Simple modelling of Q-branch absorption-I. Theoretical model and application to CO₂ and N₂O

A simple theoretical approach of Q-branch absorption is developed. It is based on the classical approximation of the rotational distribution and a Strong-Collision-type modelling of line-coupling coefficients. The Q-branch absorption is then represented by a very simple analytical expression which depends on six average parameters; pressure and wave-number dependences are included in the model so that the parameters only depend on the molecular system (active molecule + perturber), band, and temperature. Tests show that, provided effective parameters are used, our model enables very satisfactory predictions of the pressure, temperature, and wavenumber dependences of Q-branch absorption. These effective parameters can be deduced from experimental spectra and no previous knowledge of neither the spectroscopic nor collision parameters is required. This makes the present approach suitable for species (most of those of atmospheric interest) for which no alternative and more accurate approach is yet available.