Weather influence on aviation NOx climate impacts via ozone and methane

Aviation activities contribute substantially to the anthropogenic climate impact. Due to an increasing demand on aviation transport, multiple mitigation strategies have been established to reduce the contribution to climate change by aviation. One promising strategy is to re-route aircraft, such that climate sensitive atmospheric areas are avoided. This mitigation strategy, depends on the scientific understanding of all processes involved. The European project REACT4C assessed the feasibility of such a mitigation technique by simulating the climate impact of NOx, as well as other emissions and contrail formation for eight distinct weather pattern. For each weather pattern, unit emissions of NOx are emitted in the North Atlantic flight sector. Each air parcel, containing the emitted NOx, is tracked within the atmosphere. This unique model set-up allows to analyse concentration changes of O3 and CH4 along each trajectory. In general, due to the emitted NOx, O3 is produced and CH4 is lost. Most recent results showed that by just increasing the operation cost by 1%, the climate impact can be reduced by about 10%. By comparing climate cost functions (CCF), a metric of the climate impact per unit emission, to weather charts, a link between high pressure ridges and the total climate impact of NOx is observed. Therefore, this research focuses on identifying weather influences on the temporal development of O3 and CH4 due to aviation attributed NOx emission. In this thesis, the NOx chemistry, atmospheric transport processes and the model set-up of the REACT4C project is reviewed. The temporal development analysis of O3 is split-up into two parts, the O3 build-up and the O3 depletion. First, all data from the climate model are re-gridded and chemical production and loss rates are isolated from all other loss terms (i.e. diffusion). Certain characteristics of the temporal concentration changes of O3 are identified. A systematic analysis of the background chemical compounds and all important chemical reactions involved, provide insides to identify seasonal and emission altitude differences. With the help from literature and multiple statistical means, weather influences on those production and loss terms and thus the temporal development of O3 and CH4 , are identified. In a final step, inter-seasonal variations are analysed. In general, the chemical processes during the O3 build-up are dominated by the emitted NOx, whereas the chemical processes during the depletion of O3 are dominated by the high O3 concentration. Seasonal differences of the maximum O3 concentration and the total CH4 loss are caused by lower background concentrations of all chemicals involved during winter, which lead to lower production and loss rates of O3 and CH4 . At the same time altitude differences in the production and loss of O3 and CH4 are caused by altitude variations in all chemicals involved. The vertical transport within the atmosphere defines the time when the O3 maximum is reached. If an air parcel containing the emitted NOx, is transported fast to a lower altitude, the O3 maximum occurs sooner. If however the same air parcel would stay for a longer time at a high altitude, a late O3 maximum occurs. It could be identified that this downward motion is caused by the subsidence within a high pressure system. Airparcel with an earlier O3 maxima, experience high subsidence, which leads to a higher chemical activity based on higher temperatures. During summer a high O3 maximum can only be reached, if the background concentration of NOx is low during the O3 build-up. If the background NOx concentration is high, only very low O3 maxima occur. During winter the maximum O3 concentration is limited by the background concentration of HO2 . Only high HO2 background concentrations lead to high O3 maxima. The temporal development of CH4 is mainly influenced by the maximum O3 concentration as well as specific humidity. High O3 and H2O concentrations lead to high OH productions, which lead to a high CH4 losses. A high CH4 loss only occurs, if the maximum O3 concentrations and the specific humidity are high. This study shows that the weather situation each air parcel, containing NOx emissions, experiences has a direct influence on the resulting concentration changes of O3 and CH4 . Therefore, weather has a direct impact on the climate impact of NOx , since the concentration change of O3 and CH4 directly influences the resulting climate impact. The understanding of processes related to the climate impact of aviation attributed NOx emission is increased. This improved understanding shows great potential to improve possibilities to forecast local climate impact resulting from aviation NOx emissions, which is necessary for future re-routing mitigation strategies.