Assessment of Exhaust Plume Microphysics for Quantification of Contrail Climate Impacts

Current estimates of the climate impacts of aviation condensation trails (contrails) are highly uncertain, primarily due to limited observational data and inconsistencies among different contrail models. However, contrails are thought to contribute substantially to the overall climate impacts of aviation, even at the lower end of the estimated uncertainty. One potential mitigation strategy is to reduce ice-forming emissions through advancements in fuel and engine technology. However, our current understanding of the sensitivities of contrail formation and radiative forcing to engine design variables and fuel properties is limited. Initial studies show that the early plume microphysics modeling (EPM) in contrail models such as the Aircraft Plume Chemistry, Emissions, and Microphysics Model (APCEMM) do not sufficiently capture the role of various emission species. These limitations include lack of representation of ice formation through homogeneous freezing of volatile aerosols, the effect of chemi-ions on aerosol coagulation, and a simplified treatment of nvPM and their activation.

To address these gaps, we aim to improve the existing EPM by including first principle-based modeling, supported by experimental results. In particular, the physics of condensation at the single-particle level is key to determining the transition towards ice crystals. This study investigates the role of nvPM activation via sulfates and organics, as well as the role of pore condensation and freezing, in contrail formation.  The presence of sulfuric precursors can promote the activation of initially hydrophobic soot particles. Such particles have complex fractal shapes that include regions with high surface energy associated with open nano-and micro-pores, favoring the nucleation of critical water droplets. First results have shown that liquid fills the gaps between the primary particles of soot aggregates to form pendular rings which can develop even in a low saturation environment (S_r<1). The model used in the present study captures the realistic internal mixing of soot aggregates, from the activation up to heavy coating states, e.g., soot cores hosted within spherical droplets. The properties of the growing aerosols are measured throughout, including their size, density, and mass, as well as their optical signature, and will be communicated within APCEMM.

In addition, the effect of chemi-ions on aerosol coagulation will be considered. Implementing these microphysical considerations into the EPM, we can capture the soot-rich to soot-poor continuum of ice formation and test the impact of desulfurized fuel as well as low-soot emission indices on long term contrail evolution and the associated radiative forcing through APCEMM. With this work we hope to quantify the relative importance of various engine design parameters and fuel types to contrail formation, evolution, and, subsequently, climate impacts.