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We develop an idealized, physically based model describing combined effects of ice nucleation and sublimation on ice crystal number during persistent contrail formation. Our study represents the first effort to predict ice numbers at the point where contrails transition into contrail cirrus-several minutes past formation-by connecting them to aircraft soot particle emissions and atmospheric supersaturation with respect to ice. Results averaged over an observed exponential distribution of ice supersaturation (mean value 15%) indicate that large reductions in soot particle numbers are needed to lower contrail ice crystal numbers significantly for soot emission indices around 1015 (kg fuel)(-1), because reductions in nucleated ice number are partially compensated by sublimation losses. Variations in soot particle (-50%) and water vapor (+10%) emission indices at threefold lower soot emissions resulting from biofuel blending cause ice crystal numbers to change by -35% and <5%, respectively. The efficiency of reduction depends on ice supersaturation and the size distribution of nucleated ice crystals in jet exhaust plumes and on atmospheric ice supersaturation, making the latter another key factor in contrail mitigation. We expect our study to have important repercussions for planning airborne measurements targeting contrail formation, designing parameterization schemes for use in large-scale models, reducing uncertainties in predicting contrail cirrus, and mitigating the climate impact of aviation.

Plain Language Summary The formation and modification of ice crystals in persistent aircraft condensation trails (contrails) is an important component in evaluating the climate impact of aviation. We connect for the first time contrail ice numbers at the end of the formation stage to aircraft soot emissions and atmospheric ice supersaturation. We offer a framework to estimate changes in ice number and to compare effects of mitigation options. Our results show that large reductions in soot emissions are required to lower contrail ice numbers significantly, and those ice numbers are insensitive to small variations of the amount of water vapor emitted by aircraft jet engines. Our study has important implications for planning field measurements, reducing uncertainties in numerical predictions of contrail cirrus effects on climate and for mitigating this impact.