Compositional and Mineralogical Effects on Ice Nucleation Activity of Volcanic Ash Atmosphere

Volcanic ash produced during explosive eruptions may serve as ice nuclei in the atmosphere, contributing to the occurrence of volcanic lightning due to tribocharging from ice–ice or ice–ash collisions. Here, different ash samples were tested using deposition-mode and immersion-mode ice nucleation experiments. Results show that bulk composition and mineral abundance have no measurable effect on depositional freezing at the temperatures tested, as all samples have similar ice saturation ratios. In the immersion mode, there is a strong positive correlation between K2O content and ice nucleation site density at −25 °C and a strong negative correlation between MnO and TiO2 content at temperatures from −35 to −30 °C. The most efficient sample in the immersion mode has the highest surface area, smallest average grain size, highest K2O content, and lowest MnO content. These results indicate that although ash abundance—which creates more available surface area for nucleation—has a significant effect on immersion-mode freezing, composition may also contribute. Consequently, highly explosive eruptions of compositionally evolved magmas create the necessary parameters to promote ice nucleation on grain surfaces, which permits tribocharging due to ice–ice or ice–ash collisions, and contribute to the frequent occurrence of volcanic lightning within the eruptive column and plume during these events.