Contrail formation: generalised theory and a mitigation proposition for fuel-cell-propelled aircraft

Abstract This study presents a generalised theory that describes the thermodynamical processes during mixing of a moist aircraft exhaust with the ambient air and allows one to decide whether or not a contrail forms. Usage of alternative fuels like hydrogen or ammonia increases the moisture content in aircraft plumes compared to current kerosene combustion. Our analysis compares the thermodynamic plume evolution for the classical mixing line and a novel generalised formulation. Additionally, both formulations are used to evaluate the limiting ambient temperature, above which an aircraft does not produce a contrail. We find that the inaccuracies introduced by the classical mixing line cancel each other out, leading to negligible differences between both formulations. Furthermore, the impact of potential heat and water vapour recuperation systems on contrail formation behind fuel-cell-propelled aircraft is investigated. Reducing the exhaust’s thermal energy by technical means increases the contrail formation propensity. Especially, if fuels with high hydrogen content are used, plumes with reduced heat content could reach supersaturation values above 500 percent sign 500 % $ 500{\%}$ . This can trigger liquid water droplet formation directly from the gas phase, a process absent in conventional contrail scenarios, and may increase the number of formed ice crystals drastically. A concurrent increase in ice crystal numbers and contrail formation propensity would increase the contrail-cirrus climate impact. To mitigate this scenario, our analysis identifies requirements on the reduction of exhaust water vapour to suppress contrail formation by technical means and reduce the potential contrail climate impact by fuel-cell-propelled aircraft.