A double-box model for aircraft exhaust plumes based on the MADE3 aerosol microphysics

Aviation emissions at typical cruise altitude (~9-13 km) consists of a blend of chemical components including aerosols and their precursor gases, affecting 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. A double-box aircraft exhaust plume model is developed to explicitly simulate the aerosol microphysics inside the dispersing aircraft exhaust plume, together with a simplified representation of the vortex regime (which begins ∼ 10 s after emission and captures the dynamics of aerosol particle interactions with contrail ice particles). This study focuses specifically on sulfate (SO₄) and soot aerosols, as well as the total number concentration of aviation-induced aerosol particles. The plume model is used to quantify aviation-induced aerosol number concentrations at the end of the dispersion regime where the exhaust has dispersed on scales resolved by global models (~46 h), and the results are compared with those from the instantaneous dispersion approach commonly used in global models. The difference between the two approaches is defined as the plume correction. For typical North Atlantic cruise conditions, the plume correction ranges from −15% (with contrail ice in the vortex regime) to −4.2% (without contrail ice). A tendency-based process analysis shows that the negative value of the plume correction is due to the higher efficiency of coagulation process in the plume approach, leading to lower total particle number concentrations compared to the instantaneous dispersion approach. Sensitivity studies performed for different world regions highlight the role of background conditions for the plume-scale processes, with the plume correction varying between −12 % for Europe and −42 % for China. Parametric studies performed on various aviation emission parameters used to initialise the plume model demonstrate the strong influence of contrail ice in the vortex regime, which substantially reduces aerosol number concentrations in the plume approach. They also show a large sensitivity towards aviation fuel sulfur content, as SO₂ emissions and subsequent H₂SO₄ formation are key drivers of nucleation. The plume model can be directly implemented in coarse-resolution global aerosol–climate models or used as offline parametrisation to constrain quantifications of the climate effects of aviation-induced aerosol particles.