Aviation is widely recognised to have global-scale climate impacts through the formation of ozone (O₃) in the upper troposphere and lower stratosphere (UTLS), driven by emissions of nitrogen oxides (NO_X). Ozone is known to be one of the most potent greenhouse gases formed from the interaction of aircraft emission plumes with atmospheric species. This paper follows up on previous research, where a Photochemical Trajectory Model was shown to be a robust measure of ozone formation along flight trajectories post-flight. We use a combination of a global Lagrangian chemistry-transport model and a box model to quantify the impacts of aircraft NO_X on UTLS ozone over a five-day timescale. This work expands on the spatial and temporal range, as well as the chemical accuracy reported previously, with a greater range of NO_X chemistry relevant chemical species. Based on these models, route optimisation has been investigated, through the use of network theory and algorithms. This is to show the potential inclusion of an understanding of climate-sensitive regions of the atmosphere on route planning can have on aviation’s impact on Earth’s Thermal Radiation balance with existing resources and technology. Optimised flight trajectories indicated reductions in O₃ formation per unit NO_X are in the range 1–40% depending on the spatial aspect of the flight. Temporally, local winter times and equatorial regions are generally found to have the most significant O₃ formation per unit NO_X; moreover, hotspots were found over the Pacific and Indian Ocean.

Aircraft emissions play a role in human induced climate change, in particular condensation trails (contrails) formation, NOx and CO2 emissions. Recent years have seen increased research output in contrail studies and mitigation. Geo-engineering has gained attention in recent decades, with the objective to increase the global albedo resulting in reduced radiative forcing. Scenarios for geo-engineering include stratospheric aerosol injection (SAI), Marine Cloud Brightening (MCB), silver iodide injection as well as more niche scenarios. The interaction between aviation and geo-engineering remains underexplored. We explore the role of geo-engineering on contrail formation, persistence and aircraft plume chemistry employing the Aircraft Plume Chemistry, Emissions, and Microphysics Model (APCEMM). We apply this to jet fuel A-1 as well as potential alternative fuels with respective emission indices’ parameters including Synthetic aviation fuels (SAFs), hydrogen internal combustion jet engines and ammonia. We evaluate the chemistry background composition using theoretical adjustments and the G6-Sulfur experiment in CMIP6 model outputs. We explore how the result of increased particulate matter from increased aerosol number density, resulting from geo-engineering, facilitate the seeding of contrail formation. We find changes in ice crystal number density, heterogenous reaction rates of nitric acid, ozone perturbations optical depth of contrails, contrail lifetimes and total extinction, providing insights into the radiative forcings from SAI influenced contrails. [An increase in ice crystal number density is observed due to an abundance of sulfate aerosols acting as condensation nuclei. Reductions in the efficacy of alternative fuels such as hydrogen at minimising contrails, both in lifetime and forcing, when background aerosols replace soot as nucleating particles. Enhanced heterogonous reaction rate were found to increase HNO3 with SAI by 5 % for a 50 % increase in SO2 ]