Contrail cirrus account for the major share of aviation’s climate impact. Yet, the links between jet fuel composition, contrail microphysics and climate impact remain unresolved. Here we present unique observations from two DLR-NASA aircraft campaigns that measured exhaust and contrail characteristics of an Airbus A320 burning either standard jet fuels or low aromatic sustainable aviation fuel blends. Our results show that soot particles can regulate the number of contrail cirrus ice crystals for current emission levels. We provide experimental evidence that burning low aromatic sustainable aviation fuel can result in a 50 to 70% reduction in soot and ice number concentrations and an increase in ice crystal size. Reduced contrail ice numbers cause less energy deposition in the atmosphere and less warming. Meaningful reductions in aviation’s climate impact could therefore be obtained from the widespread adoptation of low aromatic fuels, and from regulations to lower the maximum aromatic fuel content.

We describe results of in situ observations of a 1‐ to 2‐min‐old contrail in the vortex phase generated from soot‐rich exhaust (>1015 emitted soot particles per kilogram of fuel burned). Simultaneous measurements of soot (EIsoot) and apparent ice (AEIice) particle number emission indices show a pronounced anticorrelation in the vertical contrail profile. AEIice decrease by about 75% with increasing distance below the contrail‐producing aircraft, while EIsoot increase by an equivalent relative fraction, therefore strongly suggesting ice particle formation to be soot‐controlled and losses to be caused by sublimation. Quantifying these losses in measurements helps to validate and improve contrail parameterizations used to estimate the climate impact of contrails and contrail cirrus. Our study further demonstrates the challenges in the performance and interpretation of particle measurements in young contrails and lends itself to suggestions for improving contrail data evaluation.

We study losses of ice crystals in a persistent, soot‐rich contrail in the wake behind a medium‐sized aircraft at cruise. Constraining a model covering ice nucleation, growth, and sublimation phases with an aircraft data set, we track the sublimation history over 2 min of contrail age and relate ice crystal numbers to the number of soot particles emitted by the aircraft engines. We analyze the observed vertical distribution of ice numbers, estimating an exponential scale height in the range 50–100 m and wake‐averaged ice numbers (1.3–1.7) × 1015 (kg‐fuel)−1 after sublimation, removing 60% of the ice crystals that originally nucleated on emitted soot particles. We define soot emission‐ and ice supersaturation‐dependent contrail depths, affecting estimates of horizontal spreading rates of contrails. Our findings have ramifications for the representation of long‐lived contrails in global models.