Atmospheric methane and its isotopic composition in a changing climate: A modeling study

Kurzfassung

Methane (CH4) is after carbon dioxide the second most important anthropogenically influenced greenhouse gas and offers, due to its relative short lifetime, an attractive mitigation potential. However, variations in CH4 growth rates are still poorly understood. The main objective of this thesis is to provide an improved understanding of impacts of and feedbacks on CH4 as a greenhouse gas with the help of a global Chemistry-Climate model. The analysis covers four aspects of atmospheric CH4, namely its sinks, sources, global distribution and climate effect in form of its oxidation products. First, the sink processes of CH4 are studied by means of the main tropospheric sink reaction partner, the hydroxyl radical (OH). For this study, results of 16 simulations are analyzed concerning the CH4 lifetime, which is defined by the reaction of CH4 with OH. The derived average lifetime of 8.11±0.13 a ranges at the lower end of similar studies. The results reveal that the tropospheric CH4 lifetime is not constant and strongly relates not only to OH abundance, but also to temperature. Both, however, are influenced by the model configuration and the assumed future scenario of climate warming. It is found that increasing CH4 emissions increase its lifetime. This is partly compensated by coherently rising temperatures in the atmosphere. Secondly, an inverse optimized emission inventory derived by the fixed-lag Kalman Filter method is presented. It is investigated how the applied forward model and assumed OH distribution influence the estimates of the inverse optimization. The results show that the optimized inventory improves the agreement of simulated CH4 to ground-based observations. The inventories are strongly determined by the applied OH abundance, which is in general poorly constrained. A forward simulation with interactive chemistry and the optimized emission inventory reveals that the OH distribution adjusts with respect to the emission inventory and is in general under-constrained. As a consequence, the uncertainty in the sink of CH4 limits the certainty of estimated CH4 emissions. The third part of this thesis focuses on the global distribution of CH4 . In order to investigate mixing and transport of CH4 from specific sources, the model is extended for the simulation of CH4 isotopologues and its isotopic fractionation effects. The simulation results are evaluated concerning the representation of the vertical and latitudinal gradient. It is further coupled to the isotopologues in the hydrological cycle and previous results concerning the isotopic content in stratospheric water vapor are reproduced. And fourth, the present study re-evaluates the common assumption that two water vapor (H2O) molecules are produced per oxidized CH4 molecule. The systematic analysis comprises three different approaches, focusing primarily on the tropical region. The results reveal that the yield of H2O from CH4 oxidation is smaller than two in the lower stratosphere and upper mesosphere. It also attains a value above two in the upper stratosphere and lower mesosphere due to transported long-lived intermediate molecules of the CH4 oxidation chain. It is concluded that assuming a constant chemical yield of H2O from CH4 oxidation neglects vertical variations in the chemical kinetics as well as secondary chemical processes including the loss of H2O. In summary, this study comprises findings of the sinks, sources, global distribution and climate effective oxidation products of CH4. This thesis emphasizes the decisive linkage of sources and sinks of CH4 with respect to their uncertainties and provides a comprehensive framework for a global simulation of CH4 and its isotopologues in order to analyze these uncertainties further.