Chemical trends in the upper atmosphere modeled with COMMA/IAP
The intensification of agriculture and industry lead to injection in the atmosphere such gases as methane, carbon dioxide, and dinitrogen oxide. The anthropogenic changes of the troposphere have been well studied but the impact on the mesosphere-lower thermosphere is not enough clarified. The influence of the rising concentrations of methane, dinitrogen oxide and carbon dioxide since the pre-industrial era upon the chemistry of the mesosphere was studied by the model COMMA/IAP.
The Figure exhibits the variation of the water vapor mixing ratio at 67.5° N again at different height levels (50, 60, 70, 80, 83, 86 and 89 km) for summer solstice (July 1st). The 11-years solar cycle variation begins to be noticeable above 80 km. The H2O level at 70 km ranges above that of 60 km due to autocatalytic water vapor production.
The estimation of the growth of the middle atmospheric water vapor mixing ratio due to the anthropogenic increase of methane is only one aspect of influence interpreting the rising NLC occurrence rate. One can speculate about other influences particularly about the temperature owing to the cooling of the middle atmosphere by the enhanced CO2 burden of the atmosphere and a reduced upper mesospheric ozone concentration as a result of higher hydrogen radical formation from increasing H2O. However, there is no direct evidence of a real cooling of the NLC region thus far. A main problem arises from the uncertainty relating to the pre-industrial level of the water vapor mixing ratio at the hygropause. Plausible assumptions do not need to be true assumptions. Global 3D-calculations have the advantage also to consider the typical dynamical pattern such as the important upward flow in middle and high latitudes in the summer season. As water vapor is auto-catalytically formed below about a height of 65 km caused by the solar radiation dissociating water vapor, in contrast to the domain above, the vertical winds lift relatively more humid air into the area of H2O destruction so that at the height of the lower edge of NLC the water vapor is only less influenced by the solar activity and even a slight positive correlation could be realistic. Above that altitude the solar cycle is mirrored by the water vapor distribution. In this model experiment was found generally the expected increase of the middle atmospheric humidity, however, with a certain time delay of some years due to the long transport time of all constituents through the middle atmosphere and the long chemical life time of methane. The increasing H2O concentration results in a higher hydrogen radical level and that entails a decline of ozone particularly in the upper mesosphere/mesopause region. The reduced ozone concentrations form less amounts of O(1D) being an effective H2O destroyer. As a result the H2O concentration increases in the mesopause region stronger than expected from the methane increase only. A drawback of the model of dynamics and chemistry is that it do not operate interactively thus far and the dynamic model is based on the state of the present atmosphere. Thus the model reproduces the present atmosphere very well but there could be some changes of the dynamical parameters (wind vector, temperature, pressure) for the pre-industrial atmosphere.