The impact of weather pattern and related transport processes on aviation's contribution to ozone and methane concentrations from NO x emissions

Abstract. Aviation attributed climate impact depends on a combination of composition changes in trace gases due to emissions of carbon dioxide (CO2) and non-CO2 species. Nitrogen oxide (NOx = NO + NO2) emissions lead to an increase in ozone (O3) and a depletion of methane (CH4), whereas water vapour (H2O) can additionally lead to the formation of persistent contrails. In comparison to CO2, non-CO2 contributions to the atmospheric composition are short lived and are thus characterised by a high spatial and temporal variability. In this study, we investigate the influence of weather pattern and their related transport processes on composition changes caused by aviation attributed NOx emissions, by using the atmospheric chemistry model EMAC (ECHAM/MESSy). Representative weather situations are simulated in which unit NOx emissions are initialised in specific air parcels at typical flight altitudes over the North Atlantic flight sector. By explicitly calculating composition changes induced by these emissions, interactions between trace gas composition changes and weather conditions along the trajectory of each air parcel are investigated. The resulting climate impact from NOx via changes of O3 mainly depends on the magnitude of the maximum induced composition change. In general, the earlier the maximum O3 change occurs the larger the total O3 change and therefore the resulting climate impact. In this study we show that subsidence in high pressure systems leads to an earlier O3 maximum and that the maximum O3 change is limited by atmospheric NOx and HO2 during summer and winter, respectively. The resulting climate impact due to composition changes of CH4 depends only on the magnitude of the induced depletion of CH4, where a larger depletion of CH4 leads to a larger cooling effect. We show that a large CH4 depletion is only possible if a strong formation of O3 occurs and if large atmospheric H2O concentrations are present. Only air parcels which are transported into tropical areas, due to high pressure systems, experience high concentrations of H2O and thus a large CH4 depletion. Re-routing flight trajectories based on the experimental setup used in this study is currently too computationally expensive. This work demonstrates that transport processes are of most interest when identifying the climate impact from aviation NOx emissions. The insights gained in this study suggest an approach to re-route flights in the future, by performing less computationally expensive purely dynamic simulations.