Aircraft emissions at typical cruise altitude (approximately 9-13 km) comprise of a diverse array of chemical compounds, including aerosols and their precursor gases. Recent global modelling studies have suggested that these aviation-induced aerosol particles can be transported downward to the lower atmospheric layers, where they may influence and alter the microphysical properties of low clouds such as droplet size and distribution and hence modify their radiative characteristics. However, before these particles are transported downward, they undergo a series of chemical and microphysical transformations within the aircraft exhaust plume, collectively referred to as aging processes. Due to their coarser spatial resolution (∼100 km), the global aerosol-climate models are limited in their ability to accurately represent the microphysical processes at the subgrid-scale level, consequently resulting in large uncertainties in estimating the aviation impact on aerosol particles generated by aircraft emissions. This especially concerns the aerosol number concentration and size, which are key quantities for estimating the aerosol indirect effect for low-level, liquid-phase clouds.

In this thesis, a double-box aircraft exhaust plume model is developed by extending the framework of the well-established MADE3 single-box model, incorporating additional parametrisation to capture the spatial and temporal evolution of aerosol dynamics within the aircraft exhaust plume. The plume model is designed to explicitly simulate the aerosol microphysics inside a gradually dispersing aircraft exhaust plume, together with a simplified representation of the vortex regime (starting ∼10 s behind the aircraft) which simulates the interaction of aerosols with short-lived contrail ice particles. This thesis primarily focuses on sulfate (SO4) and soot aerosols, together with the total number concentration of aerosols emitted in the exhaust plume of an aircraft. The model is specifically designed to provide a more accurate representation of the microphysical processes occurring within an aircraft exhaust plume which alters the aerosol dynamics at the plume scale. The plume model is initialised at the end of the jet phase, approximately 10 seconds after the emission, using measured initial size distribution parameters for standard aircraft operating conditions together with other aircraft operational and emission parameters such as fuel consumption, speed, and emission factors of emitted species. In order to ensure the validity of the double-box plume model, I performed different numerical and parametric tests. The numerical tests confirmed the correct implementation of the extension from single- to double-box plume model. The parametric study in combination with the tendency diagnostics showed that the model reliably captures the expected sensitivity of aerosol number and size to several physical parameters, in line with theory and with previous global applications of MADE3.

The plume model is used to quantify the aviation-induced particle number concentration at the end of the dispersion regime (∼46 h) by comparing the results from the plume approach with the results obtained by the instantaneous dispersion approach commonly applied by the global models. The difference between the plume approach and the instantaneous dispersion approach allows to define a plume correction: for typical cruise conditions over the North Atlantic and typical aviation emission parameters, the plume correction for aviation-induced particle number concentration ranges between –15% and –4% as quantified for the first time in this study, depending on the presence or absence of the short-lived contrail ice in the vortex regime, respectively. These negative corrections indicate that the plume approach simulates a lower aviation induced particle number concentration than the instantaneous dispersion.

In order to understand the influence of the microphysical processes and diffusion dynamics on the aerosol evolution inside an aircraft plume, tendency diagnostics are implemented in the plume model to track the impact of the individual processes on the aerosol properties. This analysis shows that the negative value of the plume correction is due to the higher efficiency of the coagulation in the plume model, partly counteracted by nucleation, leading to a lower number concentration of aviation-induced particles in the plume approach. Sensitivity studies performed over different regions highlight a large variability in the plume correction between –12% for Europe and –43% for China, thus signifying the importance of background conditions for the plume microphysics. Parametric studies performed on various aviation emission parameters used to initialise the plume model further demonstrate the high relevance of short-lived contrail ice in the vortex regime, which accounts for the aerosol-ice interaction. These interactions lead to a considerable reduction in aviation-induced aerosol number concentrations, particularly in the early stages of plume evolution. Moreover, the parametric studies show a large sensitivity towards aviation fuel sulfur content (FSC), driving sulfur dioxide (SO2) emissions and gas-phase sulfuric acid (H2SO4) formation, which in turn is a primary driver for the nucleation process.

The double-box aircraft exhaust plumemodelMADE3 (v4.0) presented in this thesis is ready for application in global model studies. The model configuration is highly flexible with low computational costs which means that it can be effectively implemented for both online and offline parametrisation. The results from the plume model can be used to better initialise the aviation emissions in global model simulations and can contribute to a refined quantification of the climate impact of aviation-induced aerosol particles on clouds.

Aviation emissions of aerosol particles and aerosol precursor gases alter the Earth's radiation budget via both direct and indirect aerosol effects, resulting in a significant climate effect. Current estimates of aviation-induced climate effects are based on coarse-resolution global aerosol-climate models, which are not able to resolve the microphysical processes at the aircraft plume scale. This results in large uncertainties in the aviation-induced impact on aerosol number and size, which are key quantities for estimating the aerosol indirect effect, especially for low-level liquid-phase clouds. In this work, a double-box aircraft exhaust plume model is developed to explicitly simulate the aerosol microphysics inside a dispersing aircraft exhaust plume, together with a simplified representation of the vortex regime (which begins ĝ1/4 10 s after the aircraft emissions and captures the dynamics of aerosol particle interactions with contrail ice particles). The aircraft exhaust plume model is used to quantify the aviation-induced aerosol number concentration at the end of the dispersion regime (ĝ1/446 h) and the results are compared with the result obtained by the instantaneous dispersion approach commonly applied by the global models. The difference between the plume approach (simulated using two boxes) and the instantaneous dispersion approach (simulated by a single box) is defined as the plume correction: for typical cruise conditions over the North Atlantic and typical aviation emission parameters, the plume correction for aviation-induced particle number concentration ranges between -15 % and -4.2 %, depending on the presence or absence of the contrail ice in the vortex regime, respectively. A tendency-based process analysis shows that the negative value of the plume correction is due to the higher efficiency of coagulation and nucleation processes in the plume approach, leading to lower total particle number concentrations compared to the instantaneous dispersion approach. Sensitivity studies over different regions highlight the role of background conditions for the plume microphysics, with the plume correction varying between -12 % for Europe and -42 % for China in a scenario with contrail ice in the vortex regime. Parametric studies performed on various aviation emission parameters used to initialise the plume model demonstrate the high relevance of contrail ice in the vortex regime to considerably reduce the aviation-induced aerosol number concentration in the plume approach. Moreover, the parametric studies show a large sensitivity towards aviation fuel sulfur content, driving sulfur dioxide (SO2) emissions and the sulfuric acid (H2SO4) formation, which in turn is a primary driver for the nucleation process. Thanks to its flexible configuration and minor additional computational costs, the plume model presented here can readily be applied in coarse-resolution global aerosol-climate models or used as offline parametrisation to quantify the climate effects of aviation-induced aerosol particles.

The characteristics of aviation-induced aerosol, its processing, and effects on cirrus clouds and climate are still associated with large uncertainties. Properties of aviation-induced aerosol, however, are crucially needed for the assessment of aviation’s climate impacts today and in the future. We identified more than 1100 aircraft plume encounters during passenger aircraft flights of the IAGOS-CARIBIC Flying Laboratory from July 2018 to March 2020. The aerosol properties inside aircraft plumes were similar, independent of the altitude (i.e., upper troposphere, tropopause region, and lowermost stratosphere). The exhaust aerosol was found to be mostly externally mixed compared to the internally mixed background aerosol, even at a plume age of 1 to 3 h. No enhancement of accumulation mode particles (diameter >250 nm) could be detected inside the aircraft plumes. Particle number emission indices (EIs) deduced from the observations in aged plumes are in the same range as values reported from engine certifications. This finding, together with the observed external mixing state inside the plumes, indicates that the aviation exhaust aerosol almost remains in its emission state during plume expansion. It also reveals that the particle number EIs used in global models are within the range of the EIs measured in aged plumes.