Transport Patterns of Global Aviation NO<sub>x</sub> and their Short-term O<sub>3</sub> Radiative Forcing – A Machine Learning Approach

Abstract. Aviation produces a net climate warming contribution that comprises multiple forcing terms of mixed sign. Aircraft NOx emissions are associated with both warming and cooling terms, with the short-term increase in O3 induced by NOx emissions being the dominant warming effect. The uncertainty associated with the magnitude of this climate forcer is amongst the highest out of all contributors from aviation and is owed to the non-linearity of the NOx–O3 chemistry and the large dependency of the response on space and time, i.e., on the meteorological condition and background atmospheric composition. This study addresses how transport patterns of emitted NOx and their climate effects vary with respect to regions (North America, South America, Africa, Eurasia and Australasia) and seasons (January–March and July–September in 2014) by employing global-scale simulations. We quantify the climate effects from NOx emissions released at 250 hPa (~ 10 400 m) in terms of radiative forcing resulting from their induced short-term contributions to O3. The emitted NOx is transported with Lagrangian air parcels within the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model. To identify the main global transport patterns and associated climate impacts of the 14 000 simulated air parcel trajectories, the unsupervised QuickBundles clustering approach is adapted and applied. Results reveal a strong seasonal dependence of the contribution of NOx emissions to O3. For most regions, a negative correlation is found between an air parcel’s downward transport and its mean contribution to O3. NOx emitted in the Northern regions (North America and Eurasia) experiences the longest residence times in upper midlatitudes (40–45 % of their lifetime), while those beginning in the South (South America, Africa and Australasia) remain mostly in the Tropics (45–50 % of their lifetime). Due to elevated O3 sensitivities, emissions in Australasia induce the highest overall radiative forcing, attaining values that are larger by factors of 2.7 and 1.2 relative to Eurasia during January and July, respectively. The location of the emissions does not necessarily correspond to the region that will be most affected – for instance, NOx over North America in July will induce the largest radiative forcing in Europe. Overall, this study highlights the spatially and temporally heterogeneous nature of the NOx–O3 chemistry from a global perspective, which needs to be accounted for in efforts to minimize aviation’s climate impact, given the sector’s resilient growth.