Constraining the role of soot in ice formation for robust estimates of aviation aerosol-cloud interactions

Non-CO₂ effects from aviation, particularly aerosol-cloud interactions, remain one of the largest uncertainties in the climate impact assessment of the transport sector. A key challenge is how aviation-emitted soot contributes to cirrus formation by acting as a potential ice-nucleating particle (INP). Current estimates span a wide range in both sign and magnitude, largely due to limitations in representing atmospheric ice nucleation and the dynamics governing cirrus formation, as well as the poorly constrained ice-nucleating properties of aviation soot in numerical models.

Here, we present a combined modelling and experimental framework to quantify how aviation soot ice nucleation perturbs cirrus properties and the resulting radiative impacts. We use the Met Office Unified Model (UM) coupled with the two-moment Cloud AeroSol Interaction Microphysics scheme (CASIM), which enables prognostic representation of ice crystal number concentration and explicit simulation of INP-driven perturbations. To represent soot-specific ice nucleation, we implement an active site density parameterisation for soot deposition freezing following Ullrich et al. (2017) within CASIM. In parallel, laboratory measurements are conducted using the Portable Ice Nucleation Experiment (PINE), a cloud expansion chamber, to characterise ice nucleation by aviation-relevant soot under cirrus conditions. The resulting constraints on soot INP efficiency across temperature and ice supersaturation are used to evaluate and refine the model parameterisation. Together, the laboratory constraints and regional simulations provide physically based estimates of aviation soot impacts on cirrus and associated radiative forcing.

The model is run in a regional configuration over Europe, focusing on high-traffic flight corridors and selected meteorological case studies relevant for cirrus formation. Model experiments compare baseline and aviation-perturbed simulations and explore sensitivity to assumptions on soot INP activity and emissions. Aviation soot emissions are prescribed using a new emission inventory developed in this work, built upon the GAIA (Global Aviation emissions Inventory based on ADS-B; Teoh et al., 2024). GAIA provides high-resolution, real-world flight activity and emissions, while our inventory explicitly separates contrail-processed and unprocessed soot particles. This separation captures the potential influence of contrail processing on soot ice-nucleating ability and provides a more comprehensive representation of the soot INP population available for cirrus formation.