Effect of humidity on the absorption continua of CO2 and N2 near 4 µm: Calculations, comparisons with measurements, and consequences for atmospheric spectra.

We present a theoretical study of the effects of collisions with water vapor molecules on the absorption, around 4 µm, in both the high frequency wing of the CO2 ν3 band and the collision-induced fundamental band of N2. Calculations are made for the very first time, showing that predictions based on classical molecular dynamics simulations enable, without adjustment of any parameter, very satisfactory agreement with the few available experimental determinations. This opens the route for a future study in which accurate temperature-dependent (semi-empirical) models will be built and checked through comparisons between computed and measured atmospheric spectra. This is of interest since, as demonstrated by simulations, neglecting the humidity of air can lead to significant modifications of the atmospheric transmission (and thus also emission) between 2000 and 2800 cm-1.