The role of aerosol particles in contrail formation: from experiments to model simulations
File(s)
Author(s)
Ponsonby, Joel Ross
Type
Thesis or dissertation
Abstract
Contrails account for most of the climate impact of aviation. Their formation relies on aerosol particles, upon which water vapour can condense and subsequently freeze to form ice crystals. Most contrails form via soot particles under “soot-rich” conditions. However, projected soot emissions reductions may enable the involvement of other particles. Under these “soot-poor” conditions, one such candidate is volatile particles, which nucleate from condensable exhaust gases. However, despite the known importance of particles in contrail formation, their role is not yet fully understood. Specifically, there are no systematic measurements of ice crystal formation from: (i) representative surrogate aircraft soot or (ii) lubrication oil droplets, which contribute towards the volatile mode. Therefore, implications for contrail formation under soot-poor and soot-rich conditions remain unresolved.
Here, we report the first laboratory measurements addressing the contrail-forming ability of (i) and (ii). We also develop two parcel models to assimilate these results, evaluating how contrail properties depend on particle properties and ambient conditions. Experimentally, we find that ice formation on surrogate aircraft soot is regulated by the size of the primary particle(s), not the aggregate, implying that current projections may overestimate their ability to form contrails under soot-rich conditions by up to 30%. We also find that lubrication oil droplets can viably form ice crystals under soot-poor conditions. Both particle types are found to nucleate ice by condensation followed by homogeneous freezing. Computationally, we confirm that volatile particles are critical for contrail formation under soot-poor conditions and can enhance ice concentrations by up to 3 orders of magnitude. Therefore, current simulations may underestimate the climate impact associated with transitioning to soot-poor technologies, particularly when EInvPM ≤ 10^12 particles per kg. We use these findings to comment on the adoption of new fuels or combustion technologies and opportunities for temperature-based contrail mitigation.
Here, we report the first laboratory measurements addressing the contrail-forming ability of (i) and (ii). We also develop two parcel models to assimilate these results, evaluating how contrail properties depend on particle properties and ambient conditions. Experimentally, we find that ice formation on surrogate aircraft soot is regulated by the size of the primary particle(s), not the aggregate, implying that current projections may overestimate their ability to form contrails under soot-rich conditions by up to 30%. We also find that lubrication oil droplets can viably form ice crystals under soot-poor conditions. Both particle types are found to nucleate ice by condensation followed by homogeneous freezing. Computationally, we confirm that volatile particles are critical for contrail formation under soot-poor conditions and can enhance ice concentrations by up to 3 orders of magnitude. Therefore, current simulations may underestimate the climate impact associated with transitioning to soot-poor technologies, particularly when EInvPM ≤ 10^12 particles per kg. We use these findings to comment on the adoption of new fuels or combustion technologies and opportunities for temperature-based contrail mitigation.
Version
Open Access
Date Issued
2025-09-22
Date Awarded
2026-01-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Stettler, Marc
Sponsor
Engineering and Physical Sciences Research Council
Grant Number
EP/S023593/1
Publisher Department
Department of Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)
Rights Embargo Date
2026-06-30