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Aviation accounts for approximately 5% of the current anthropogenic climate impact. Up to two thirds of the warming generated by airplanes is attributed to non-CO2 effects with contrail cirrus as the largest contributor. Hydrogen as aviation fuel promises zero carbon emission but the non-CO2 effects of this new fuel are poorly known. In this study we investigate the generation of contrail cirrus from hydrogen combustion using a modified version of the Contrail Cirrus Prediction model (CoCip). In the absence of soot-emissions ice particles in hydrogen contrail are assumed to form on entrained aerosols, ultrafine volatile particles and lubrication oil. The calculation of the number of ice particles formed on entrained aerosols is approximated by previously published simulation results and theory. Ultrafine volatile particles and lubrication oil both activate into water droplets at lower temperatures than soot and aerosols due to the Kelvin effect (small radius) and hydrophobicity respectively and are implemented using theory and published experimental results. Using hydrogen fuel contrails can, according to the Schmidt-Appleman criteria, form at lower altitudes than with jet fuel due to the increase in water vapor in the exhaust. Despite this our preliminary results show an overall decrease in both warming and cooling contrails for hydrogen compared to standard jet fuel. We do find that hydrogen contrails can generate more radiative forcing than jet contrails at very low temperatures mainly due to the activation of lubrication oil in combination with the larger amount of water vapor. For the bulk of flights however, hydrogen fuel leads to either equal or less contrail radiative forcing than jet fuel even with reduced soot-emissions in line with lean-burn engines.
Pettersson et al. (Mon,) studied this question.