Revisiting\nContrail Ice Formation: Impact of Primary\nSoot Particle Sizes and Contribution of Volatile Particles

Aircraft contrails,\nformed largely on soot particles in current\nflights, are important for aviation’s non-CO<sub>2</sub> climate\nimpact. Here we show that the activation of nonvolatile soot particles\nduring contrail formation is likely determined by the sizes of primary\nsoot particles rather than the effective sizes of soot aggregates\nas assumed in previous studies, which can explain less-than-unity\nfractions of soot particles forming contrail ice particles as recently\nobserved during ECLIF (Emission and CLimate Impact of alternative\nFuels) campaigns. The smaller soot primary sizes compared to aggregate\nsizes delay the onset of contrail ice formation, increase the maximum\nplume supersaturation reached in the contrail plume, and thus increase\nthe probability of small volatile particles contributing to the total\ncontrail ice particle number. This study suggests that the range of\nconditions for volatile plume particles to contribute significantly\nto the contrail ice number budget is wider than previously thought.\nAs the aviation industry is moving toward sustainable aviation fuel\nand/or lean-burning engine technology, which is expected to reduce\nnot only the emission index of nonvolatile soot particles but also\nthe sizes of primary soot particles, this study highlights the need\nto better understand how the combined changes may affect contrail\nformation, contribution of volatile particles, and climate impacts.