Ammonia versus kerosene contrails: A review

Hydrogen-rich fuels, such as liquid ammonia (LNH₃), are being considered for new commercial aircraft propulsion systems to reduce aviation’s CO₂ climate impact. It is crucial to ensure that integrating these fuels does not increase non-CO₂ climate impacts, defeating the purpose of decarbonizing aviation. Specifically, there are concerns about increased atmospheric radiative forcing (RF) via more frequent and persistent condensation trails (contrails). Some recent analyses show that ammonia contrails could form at lower altitudes (i.e., warmer air) and more frequently than kerosene contrails. On an equal energy basis, NH₃-powered engines can exhaust six times more mass of water in every kilogram of air per unit Kelvin temperature increase compared to their kerosene-powered counterparts. The vastly different thermodynamic and microphysical conditions in the exhaust plume of NH₃-powered engines query the existing understanding of contrail prediction. Current literature suggests that reducing soot particles as efficient ice nuclei (IN) in plumes of conventional kerosene-fueled engines could eliminate contrails by decreasing ice crystal number density. Such a proposal fails to consider the dissimilar plume properties and a range of microphysical phenomena that affect contrail formation—and thus may not be easily conjectured to NH₃-contrails. Examples include an increase in the supersaturation temperature threshold, ambient particle effects, preexisting soot emitted from airplanes burning carbon-based fuels, the feasibility of a homogeneous freezing mechanism, and any non-soot system-exhausted particles serving as efficient IN. Hence, this review seeks to consolidate knowledge of kerosene and ammonia contrails and offer thermodynamic and microphysical perspectives on contrail formation.