Anthropogenic effects on the distribution of minor chemical constituents in the mesosphere/lower thermosphere – A model study

We investigate the influence that rising concentrations of methane, nitrous oxide and carbon dioxide have had upon the chemistry of the mesosphere since 1961. Calculations were performed using our global 3D-model LIMA (Leibniz-Institute Middle Atmosphere), designed for the investigation of the MLT-region (Mesosphere-Lower Thermosphere) and particularly for the extended mesopause region. LIMA utilizes observed tropospheric and lower stratospheric temperature and horizontal wind data up to 35 km altitude by assimilating ECMWF/ERA-40 and ECMWF operational data. Real Lyman-α flux values are employed to determine the variable water vapor dissociation rate. Three different calculations were carried out and analyzed: (1) use of the same annual variation of the model dynamics in the chemical transport model (CTM) for all years according to the dynamics of the solar minimum year 1964 and employment of a realistic growth of the anthropogenic gases; (2) use of constant concentrations of the anthropogenic constituents at the lower border, but employment of the varying model dynamics; (3) the so-called realistic case, which considers both the long-term increase in the anthropogenic minor constituents and the varying dynamics according to LIMA calculations. The analysis of these three cases shows that the dynamics are able to counteract the impact of anthropogenic growth of minor constituents in the upper mesosphere-mesopause-lower thermosphere region in middle to high latitudes in summer. The water vapor mixing ratio increases due to rising methane concentration. The reason for this lies in a positive feedback process of autocatalytic water vapor production. The change in concentration of the minor constituents impacts both the cooling rate and the chemical heating rate. We present the relative and absolute deviations between the solar activity minimum years 1964 and 2008 for the most important minor constituents. We discuss the long-term behavior, particularly of water vapor, with regard to the impact on the NLC-region.