Atmos. Chem. Phys. Atmospheric Chemistry and Physics, vol. 9 (2009) pp. 2779-2792
We investigate the influence the rising concentra-tions of methane, nitrous oxide and carbon dioxide which have occurred since the pre-industrial era, have had on the chemistry of the mesosphere. For this investigation we use our global 3-D-model COMMA-IAP which was designed for the exploration of the MLT-region and in particular the extended mesopause region. Assumptions and approxima-tions for the trends in the Lyman-α flux (needed for the wa-ter vapor dissociation rate), methane and the water vapor mixing ratio at the hygropause are necessary to accomplish this study. To approximate the solar Lyman-α flux back to the pre-industrial time, we derived a quadratic fit using the sunspot number record which extends back to 1749 and is the only solar proxy available for the Lyman-α flux prior to 1947. We assume that methane increases with a constant growth rate from the pre-industrial era to the present. An un-solved problem for the model calculations consists of how the water vapor mixing ratio at the hygropause should be specified during this period. We assume that the hygropause was dryer during pre-industrial times than the present. As a consequence of methane oxidation, the model simulation indicates that the middle atmosphere has become more hu-mid as a result of the rising methane concentration, but with some dependence on height and with a small time delay of few years. The solar influence on the water vapor mixing ratio is insignificant below about 80 km in summer high lat-itudes, but becomes increasingly more important above this altitude. The enhanced water vapor concentration increases Correspondence to: M. Grygalashvyly (firstname.lastname@example.org) the hydrogen radical concentration and reduces the meso-spheric ozone. A second region of stronger ozone decrease is located in the vicinity of the stratopause. Increases in CO 2 concentration enhance slightly the concentration of CO in the mesosphere. However, its influence upon the chemistry is small and its main effect is connected with a cooling of the upper atmosphere. The long-term behavior of water vapor is discussed in particular with respect to its impact on the NLC region.
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