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 CH₄ vibrational levels in the middle atmosphere has been carried out and the results are presented. The work is oriented to assess the potential impact of non-LTE effects on the remote sensing of these gases. The models developed for this purpose include a complete set of radiative and collisional processes. In order to cover typical and extreme remote sensing scenarios, the models have been applied to different atmospheric and solar illumination conditions. The vibrational levels responsible for the major emissions of HNO₃ are found to be in LTE up to the lower mesosphere, driven by the dominant V-T processes with the air molecules. In the non-LTE region, the absorption from the warmer tropospheric layers and solar direct excitation produce small enhancements over the equilibrium populations. The mesospheric CH₄ vibrational temperatures are mainly determined by two mechanisms: the radiative absorption of the upcoming radiation emitted by the lower layers of the atmosphere, and the near-resonant vibrational coupling between the CH₄ levels and the first vibrationally excited level of O₂. By day, non-LTE is significantly enhanced as a consequence of the collisional relaxation 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 been studied.