Black Carbon Particles Do Not Matter for Immersion Mode Ice Nucleation

The role of black carbon (BC) in ice crystal formation via immersion freezing relevant for mixed-phase cloud formation is uncertain. Previous studies report either negligible or significant contributions of BC particles to cloud glaciation via immersion freezing. Despite conflicting evidence, immersion freezing by BC particles is included in several cloud models. Here we show that fossil fuel soot and commercially available hydrocarbon BC is inactive as immersion freezing nuclei for atmospherically relevant particle sizes and surface areas. Instead, temperatures <235 K are necessary for freezing droplets with immersed soot particles, implying homogeneous freezing, rather than immersion freezing by soot. A comparison of the results to previous studies using larger soot aggregates and dust reveals the ineffectiveness of soot as immersion ice nucleating particles. We conclude that soot particles with properties like those investigated here can be neglected for simulating ice nucleation in mixed-phase clouds.