Abstract
Abstract. We investigate the influence the rising concentrations 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 approximations for the trends in the Lyman-α flux (needed for the water 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 unsolved 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 humid 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 latitudes, but becomes increasingly more important above this altitude. The enhanced water vapor concentration increases the hydrogen radical concentration and reduces the mesospheric ozone. A second region of stronger ozone decrease is located in the vicinity of the stratopause. Increases in CO2 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.
Highlights
A trend refers to the tendency of a parameter to change during a sufficiently long time interval excluding the effects of all accidental and quasi cyclic variations
We investigate the influence the rising concentrations of methane, nitrous oxide and carbon dioxide which have occurred since the pre-industrial era, have had on the chemistry of the mesosphere
The main goal of this paper is the reproduction of the observed water vapor increase in the middle atmosphere using the inferred trends of long-lived constituents such as methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2) and taking into account the derived variation of the solar Lymanα radiation which has a strong influence on the distribution of water vapor above the middle mesosphere
Summary
A trend refers to the tendency of a parameter to change during a sufficiently long time interval excluding the effects of all accidental (natural) and quasi cyclic variations. In this connection, trends in measured data are determined through statistical regression analysis and the statistical significance of the derived trend calculated. We use the available derived trends of a few minor constituents as input data for our model study These trends are idealized since we assume that there has been an exponential increase in their value since the beginning of industrialization.
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