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. Volcanic ash is known to nucleate ice when immersed insupercooled water droplets. This process may impact the properties anddynamics of the eruption plume and cloud as well as those of meteorologicalclouds once the ash is dispersed in the atmosphere. However, knowledge ofwhat controls the ice-nucleating activity (INA) of ash remains limited,although it has been suggested that crystalline components in ash may play animportant role. Here we adopted a novel approach using nine pairs of tephraand their remelted and quenched glass equivalents to investigate theinfluence of chemical composition, crystallinity, and mineralogy on ash INAin the immersion mode. For all nine pairs studied, the crystal-bearing tephranucleated ice at warmer temperatures than the corresponding crystal-freeglass, indicating that crystalline phases are key to ash INA. Similar tofindings for desert dust from arid and semi-arid regions, the presence offeldspar minerals characterizes the four most ice-active tephra samples,although a high INA is observed even in the absence of alkali feldspar insamples bearing plagioclase feldspar and orthopyroxene. There is evidence ofa potential indirect relationship between chemical composition and ash INA,whereby a magma of felsic to intermediate composition may generate ashcontaining ice-active feldspar or pyroxene minerals. This complex interplaybetween chemical composition, crystallinity, and mineralogy could help toexplain the variability in volcanic ash INA reported in the literature.Overall, by demonstrating the importance of crystalline phases in the INA ofash, our study contributes insights essential for better appraising the roleof airborne ash in ice formation. Among these is the inference thatglass-dominated ash emitted by the largest explosive volcanic eruptions mightbe less effective at impacting ice-nucleating particle populations thancrystalline ash generated by smaller, more frequent eruptions.

The importance of crystalline phases in ice nucleation by volcanic ash