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 O₃ production. Aircraft NO _x emissions are of particular interest, as they lead to warming via the short-term production of O₃, but also to reduced warming via processes like CH₄ depletion. We focus on short-term O₃ production, as it constitutes one of aviation’s largest warming components. Understanding how O₃ 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 NO _x -related O₃ 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 O₃ 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 NO _x -O₃ 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.

Aviation produces a net climate warming contribution that comprises multiple forcing terms of mixed sign. Aircraft NO_x emissions are associated with both warming and cooling terms, with the short-term increase in O₃ induced by NO_x 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 nonlinearity of the NO_x–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 NO_x 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 NO_x emissions released at a representative aircraft cruise altitude of 250 hPa (∼10 400 m) in terms of radiative forcing resulting from their induced short-term contributions to O₃. The emitted NO_x 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 algorithm is adapted and applied. Results reveal a strong seasonal dependence of the contribution of NO_x emissions to O₃. For most regions, an inverse relationship is found between an air parcel's downward transport and its mean contribution to O₃. NO_x emitted in the northern regions (North America and Eurasia) experience the longest residence times in the 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, NO_x 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 NO_x–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.

A hard landing incident in Pico Aerodrome (LPPI) involving an Airbus A320-200 aircraft is investigated using airborne observations and forecasts of the AROME (Applications of Research to Operations at Mesoscale) model. A second flight is also analyzed. The severity of the wind shear during both flights is quantified using the intensity factor “I” that is based on aerial data and recommended by ICAO (International Civil Aviation Organization). During Flight 1, 36% of the landing phase (below 2100 ft) occurred under “severe” wind shear conditions and 16% occurred under “strong” conditions. Upstream characteristics included southwest winds, stable stratification and a Froude number close to 1. According to the AROME model, these circumstances triggered the development of vertically propagating mountain waves, with maximum vertical velocities above 400 ft/min and exceeding 200 ft/min in the flight path. These conditions, together with the severe wind shear, may have caused the incident. During the second flight, a wake with lee vortices and reversed flow developed in the region of the flight path, which is consistent with a low upstream Froude number and/or with the flow regime diagram of previous studies. During the approach phase of this flight, “severe” wind shear conditions were absent, with “strong” ones occurring 4% of the time. It predominantly displayed “light” conditions during 68% of this phase. As a result of the comparison between “I” and the AROME turbulence indicators, preliminary thresholds are proposed for these indexes. Lastly, this study provides an objective verification of AROME wind forecasts, showing a good agreement with airborne observations for wind speeds above 10 kt, but a poor skill for weaker winds.

Reliable prediction of aviation’s environmental impact, including the effect of nitrogen oxides on ozone, is vital for effective mitigation against its contribution to global warming. Estimating this climate impact however, in terms of the short-term ozone instantaneous radiative forcing, requires computationally-expensive chemistry-climate model simulations that limit practical applications such as climate-optimised planning. Existing surrogates neglect the large uncertainties in their predictions due to unknown environmental conditions and missing features. Relative to these surrogates, we propose a high-accuracy probabilistic surrogate that not only provides mean predictions but also quantifies heteroscedastic uncertainties in climate impact estimates. Our model is trained on one of the most comprehensive chemistry-climate model datasets for aviation-induced nitrogen oxide impacts on ozone. Leveraging feature selection techniques, we identify essential predictors that are readily available from weather forecasts to facilitate the implementation therein. We show that our surrogate model is more accurate than homoscedastic models and easily outperforms existing linear surrogates. We then predict the climate impact of a frequently-flown flight in the European Union, and discuss limitations of our approach.

Aviation-induced aerosols, particularly composed of sulfate (SO4), can interact with liquid clouds by enhancing their reflectivity and lifetime, thereby exerting a cooling effect. The magnitude of these interactions, however, remains highly uncertain and may even offset the combined warming from aviation's other climate forcers depending on spatiotemporal factors such as emission altitude and season. Here, we introduce AIRTRAC v2.0, the latest advancement of the Lagrangian tagging submodel within the Modular Earth Submodel System (MESSy), and the first submodel to provide aviation-specific sulfate tagging in this framework. AIRTRAC contributes to lowering uncertainty by tracking global contributions of aviation-emitted sulfur dioxide (SO2) and sulfuric acid (H2SO4) to SO4 formation. Using a sulfur-species tagging approach for SO2, H2SO4 and SO4, it enables the characterization of transport patterns and highlights atmospheric regions with enhanced potential for aerosol–cloud interactions. In contrast to some of the existing sulfate tagging models, AIRTRAC considers a full range of microphysical processes along trajectories. To investigate sulfate transport from aviation, two global simulations were performed for January–March and July–September 2015, using pulse emissions of SO2 and H2SO4 distributed across a cruise altitude of 240 hPa (∼ 10.6 km) based on the aviation SO2 inventory of the Coupled Model Intercomparison Project Phase 6 (CMIP6). Comparisons of AIRTRAC-derived SO4 distributions with perturbation-based simulations under analogous conditions show reasonable agreement. Using AIRTRAC v2.0, we estimate median SO2 and SO4 lifetimes of 22 d and 2.1 months, respectively, in northern winter, and 14 d and 2.2 months in summer, consistent with volcanic eruption modeling studies and observational benchmarks involving high-altitude SO2 injection. The median SO4 production efficiency during summer was found to be statistically significantly larger by 144 % compared to winter, likely due to a more efficient oxidation of SO2. Large-scale circulation patterns and emission latitude may enhance SO4 lifetimes: tropical emissions can be upwelled into the stratosphere past 100 hPa (∼ 16 km) over time, while high-latitude emissions can persist longer because they may be directly injected above the climatological tropopause. AIRTRAC v2.0 currently excludes SO2 oxidation from aviation nitrogen oxides (NOx) and does not tag other species such as black carbon. Owing to its flexible design, however, the approach can be readily extended to additional aerosols. Overall, AIRTRAC v2.0 offers the novel capability to track the atmospheric transport of aviation-emitted SO2, H2SO4 and SO4, providing critical insights into one of aviation's most uncertain climate impacts.

Aviation produces a net climate warming contribution that comprises multiple forcing terms of mixed sign. Aircraft NO_x emissions are associated with both warming and cooling terms, with the short-term increase in O₃ induced by NO_x 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 NO_x–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 NO_x 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 NO_x emissions released at 250 hPa (~ 10 400 m) in terms of radiative forcing resulting from their induced short-term contributions to O₃. The emitted NO_x 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 NO_x emissions to O₃. For most regions, a negative correlation is found between an air parcel’s downward transport and its mean contribution to O₃. NO_x 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, NO_x 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 NO_x–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.