A multi-method assessment of the regional sensitivities between flight altitude and short-term O₃ climate warming from aircraft NO _x emissions

Flight altitude is relevant to the climate effects resulting from aircraft emissions. Other research has shown that flying higher within the troposphere leads to larger warming from O-3 production. Aircraft NOx emissions are of particular interest, as they lead to warming via the short-term production of O-3, but also to reduced warming via processes like CH4 depletion. We focus on short-term O-3 production, as it constitutes one of aviation's largest warming components. Understanding how O-3 formation varies altitudinally throughout the upper troposphere/lower stratosphere is essential for designing climate-compatible aircraft and routing. We quantify this variation by performing simulations with a global atmospheric chemistry model for three representative cruise altitudes, five regions and two seasons using three methods: Eulerian tagging, perturbation and Lagrangian tagging. This multi-method, regional approach overcomes limitations of previous studies that utilize only one of these methods and apply global emission inventories biased towards present-day flight distributions, thus limiting their applicability to future aviation scenarios. Our results highlight that underrepresenting emissions in areas with growing flight activity (e.g. Asia Pacific) may lead to significant, regional underestimations of the altitudinal sensitivity of short-term NOx-related O-3 warming effects in certain cases. We find that emitting in Southern regions, like Australasia, leads to warming larger by a factor of two when compared to global averages. Our findings also suggest that flying lower translates to lower warming from short-term O-3 production and that this effect is strongest during the local summer. We estimate differences ranging from a factor of 1.2-2.6 between tagging and perturbation results that are attributable to non-linearities of NOx-O-3 chemistry, and derived regional correction factors for a widely-used sub-model. Overall, we stress that a combination of all three methods is necessary for a robust assessment of aviation climate effects as they address fundamentally different questions.