The potential role of lubrication oil in contrail formation

Contrail cirrus clouds contribute significantly to the climate impact of aviation. This impact depends non-linearly on the number of ice crystals formed in nascent contrails. For conventional kerosene combustion, ice crystals primarily form on soot particles emitted from the engine. However, in scenarios with reduced or no soot emissions (such as hydrogen combustion) other particles in the exhaust plume become relevant. Lubrication oil is one potential source of such particles.
Modern aircraft engines rely on lubrication systems to cool and lubricate rotating components such as bearings. When lubrication oil escapes into the environment—whether controlled or uncontrolled—it can become a source of volatile ultrafine particles. In the hot exhaust plume, the oil may evaporate and upon cooling of the plume nucleate new particles. Even if only a small amount of oil (on the order of a few milliliters per hour) forms new particles, the oil particle numbers can exceed the soot particle numbers of conventional engines if the resulting particles are only a few nanometers in size.
Although it is unclear to this point in how far this process occurs in the exhaust, this numerical study explores the potential role of oil particles in contrail ice crystal formation. By investigating many different scenarios, we study their activation behavior and importance compared to other ice-forming particles. The findings indicate that oil particles could substantially contribute to ice crystal formation in soot-poor or hydrogen combustion scenarios. While in-situ flight measurements are necessary to evaluate their actual formation, number, and size distribution, the study highlights the need to minimize oil particle emissions in aircraft exhaust as part of the transition to more sustainable aviation technologies.