Modelling of the non-LTE populations of the nitric acid and methane vibrational states in the middle atmosphere

A modelling of the non-LTE populations of the HNO₃ and CH₄vibrational levels in the middle atmosphere has been carried out and the results are presented.Thework is oriented to assess the potential impact of non-LTE effects on the remote sensing ofthesegases. The models developed for this purpose include a complete set of radiative andcollisionalprocesses. In order to cover typical and extreme remote sensing scenarios, the modelshave beenapplied to different atmospheric and solar illumination conditions. The vibrationallevelsresponsible for the major emissions of HNO₃ are found to be in LTE up to thelowermesosphere, driven by the dominant V-T processes with the air molecules. In thenon-LTEregion, the absorption from the warmer tropospheric layers and solar direct excitationproducesmall enhancements over the equilibrium populations. The mesospheric CH₄vibrational temperatures are mainly determined by two mechanisms: the radiative absorption oftheupcoming radiation emitted by the lower layers of the atmosphere, and thenear- resonantvibrational coupling between the CH₄ levels and the first vibrationallyexcited level ofO₂. By day, non-LTE is significantly enhanced as a consequence of thecollisionalrelaxation of overtone and combinational states excited by the solar radiation at 3.3 μm.The effects derived from the uncertainties in the parameters of the models have beenstudied.