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Ice nucleation of volcanic ash controls both ash aggregation and cloudglaciation, which affect atmospheric transport and global climate.Previously, it has been suggested that there is one characteristic icenucleation efficiency for all volcanic ash, regardless of its composition,when accounting for surface area;  however, this claim is derived from datafrom only two volcanic eruptions. In this work, we have studied thedepositional and immersion freezing efficiency of three distinct samples ofvolcanic ash using Raman microscopy coupled to an environmental cell. Ashfrom the Fuego (basaltic ash, Guatemala), Soufrière Hills (andesiticash, Montserrat), and Taupo (Oruanui eruption, rhyolitic ash, New Zealand)volcanoes were chosen to represent different geographical locations andsilica content. All ash samples were quantitatively analyzed for bothpercent crystallinity and mineralogy using X-ray diffraction. In the presentstudy, we find that all three samples of volcanic ash are excellentdepositional ice nuclei, nucleating ice from 225 to 235 K at ice saturationratios of 1.05 ± 0.01, comparable to the mineral dust proxy kaolinite.Since depositional ice nucleation will be more important at coldertemperatures, fine volcanic ash may represent a global source of cold-cloudice nuclei. For immersion freezing relevant to mixed-phase clouds, however,only the Oruanui ash exhibited appreciable heterogeneous ice nucleation activity.Similar to recent studies on mineral dust, we suggest that the mineralogy ofvolcanic ash may dictate its ice nucleation activity in the immersion mode

Deposition and immersion-mode nucleation of ice by three distinct samples of volcanic ash