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 leve ls in the middle atmosphere has been carried out and the results are presen ted. 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 purpos e include a complete set of radiative and collisional processes. In order t o cover typical and extreme remote sensing scenarios, the models have been applied to different atmospheric and solar illumination conditions. The vib rational 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 wi th 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 temperat ures are mainly determined by two mechanisms: the radiative absorption of t he upcoming radiation emitted by the lower layers of the atmosphere, and th e near-resonant vibrational coupling between the CH4 levels and the first v ibrationally excited level of O-2. By day, non-LTE is significantly enhance d as a consequence of the collisional relaxation of overtone and combinatio nal states excited by the solar radiation at 3.3 mu m. The effects derived from the uncertainties in the parameters of the models have been studied. ( C) 1999 Elsevier Science Ltd. All rights reserved.