Contrail Cirrus Climate Effects for Hydrogen-Propelled Aircraft

Introducing hydrogen as an aviation fuel offers a promising pathway to significantly mitigate the climate impact of the aviation sector. While this transition demands substantial technological advancements and logistical transformations, hydrogen combustion produces zero CO₂ emissions. However, significant uncertainties remain regarding the non-CO₂ effects of hydrogen-powered aviation, particularly the impact of contrails cirrus. For conventional aircraft, contrail cirrus— the later stages of condensation trails—are estimated to exert an effective radiative forcing comparable to that of CO₂ emissions.

This study models contrail cirrus and their radiative forcing associated with hydrogen combustion. Using a theoretical framework for ice particle formation and a modified version of the CoCiP (Contrail Cirrus Prediction) model for contrail development and radiative forcing, we address key uncertainties specific to hydrogen combustion. These uncertainties include fuel burn, which depends on aircraft and engine design, and the characterization of the exhaust. Unlike conventional jet fuel combustion, which relies primarily on soot particles as condensation nuclei, hydrogen exhaust lacks soot, shifting the role of nucleation to entrained ambient aerosols and lubrication oil particles.

First, to address fuel burn variability, we model three tube-and-wing hydrogen-powered aircraft (short-, medium-, and long-range) with 2050 technology assumptions for realistic fuel flow estimates. Second, given the limited availability of empirical data of lubrication oil and the potential to optimize size and quantity in the exhaust through future engine designs, we evaluate the impact of lubrication oil particles on contrail cirrus by varying particle size distributions.

Our results show a consistent reduction in contrail cirrus radiative forcing across all lubrication oil particle size distributions when realistic hydrogen fuel flow is assumed. The largest reductions are observed in cases with larger mean particle radius and smaller variance. These findings provide insights into the potential for hydrogen-powered aviation to reduce the climate impact of contrail cirrus and highlight opportunities to steer engine design for further mitigation.