On the Potential Role of Lubrication Oil Particles in Contrail Formation for Kerosene and Hydrogen Combustion

The radiative forcing caused by contrail cirrus is nonlinearly influenced by the number of ice crystals formed in the exhaust plume behind an aircraft engine. This ice crystal number strongly depends on the properties (number, size, and solubility) of condensation nuclei in the plume available for droplet activation with subsequent freezing. In this ice crystal formation process, the relative importance of plume particles originating from lubrication oil is unclear. Therefore, we have extended the particle-based Lagrangian Cloud Module in the box model approach to simulate the formation of ice crystals on such oil particles. With this extended box model, we performed simulations for many different scenarios of emitted oil particle numbers and sizes. We examine the activation of these oil particles and investigate their competition with soot for kerosene combustion and with ambient aerosols for hydrogen combustion. Furthermore, we study the relative importance among weakly soluble/insoluble oil particles and well-soluble volatile particles from the combustion process. For present-day soot emissions, the number of ice crystals formed on oil particles may be masked by those formed on soot. However, these oil particles may be abundant enough to dominate ice crystal formation in soot-poor or hydrogen combustion scenarios. This may occur when a small amount of lubrication oil (on the order of a few milliliters per hour) evaporates in the hot exhaust plume and nucleates new particles with a size of a few nanometers during the subsequent plume cooling.