Reduced contrail radiative effect for fleets with low soot and water vapour emissions

Besides the goal of net-zero carbon dioxide (CO${}_2$) emissions, reducing aviation’s climate impact also involves addressing other forcing effects, particularly radiative forcing from contrails. Current understanding suggests that decreasing both soot and water vapour emissions from aircraft engines reduces the occurrence and persistence of contrails, thereby lowering their radiative forcing. This may be achieved by engine concepts that combine water recovery from the exhaust with particle washout. This study presents an idealized sensitivity analysis using the Contrail Cirrus Prediction (CoCiP) model to assess how reductions in soot number and water vapour emissions could affect the radiative impact of contrail cirrus over Europe. The emission scenarios are not tied to any specific engine design but are chosen to explore the physical sensitivity of contrail formation and radiative effects. The number of emitted soot particles is reduced by up to two orders of magnitude, and water vapour emissions by one. We find that the mitigation effect becomes more pronounced with higher emission reductions. While both components contribute to the overall reduction, their combination leads to a stronger effect. The magnitude of the effect depends on their interactions during contrail formation and on subsequent atmospheric processes. The radiative forcing response exhibits marked geographic variability, with the strongest effects over regions with frequent ice-supersaturated air masses. Daily and seasonal variations in contrail occurrence and radiative impact further modulate the overall mitigation potential. The simulations indicate a substantial decrease in contrail radiative effect if technologies that lower soot and water vapour emissions are applied across a broad fleet. A limitation of this study is that other contrail nucleation pathways, such as condensation on volatile particles, are not considered. Thus, our assessment may provide an upper bound on the reduction of contrail cirrus radiative impact from soot and/or water vapour emissions.