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Geochemical Controls on the Ice Nucleating Efficiency of Volcanic Ash

Periodic Reporting for period 1 - INoVA (Geochemical Controls on the Ice Nucleating Efficiency of Volcanic Ash)

Reporting period: 2018-01-22 to 2020-01-21

Ice formation has a major impact on the properties and lifetime of clouds yet remains one of the least well understood processes indirectly affecting the Earth’s climate. Ice-nucleating particles (INPs) promote ice formation in supercooled water droplets at temperatures down to ~-38 °C. Although the abundance of INPs in the atmosphere is low - typically comprising only one in a million particles - they exert a profound influence on clouds. Research that seeks to describe how well and explain why different airborne particles nucleate ice is thus central to our understanding of the atmosphere and climate.

While desert dust lofted by wind is considered one of the most important INP types globally, an impact of volcanic ash from explosive eruptions on ice formation is increasingly recognised, with airborne ash sporadically dominating INP populations. Ash is made up of aluminosilicate glass and minerals and iron(-titanium) oxide minerals. Previous field and laboratory studies present conflicting evidence on the ice-nucleating activity (INA) of ash, and it is not clear what drives the large variation observed. Studies on dust suggest that factors such as chemical composition, crystallinity, and mineralogy of the solid particles can influence their INA; the same may be true for ash but this has not been systematically investigated before now.

The overarching objective of the INoVA project was to quantify the INA of volcanic ash and to relate this to geochemical factors including ash properties and history. Specifically, the following hypotheses were tested: the INA of ash is (H1) influenced by its chemical composition, crystallinity, and mineralogy as determined by the source magma conditions, (H2) reduced by interaction with acidic gases at high temperatures in the eruption plume and cloud, and (H3) reduced by exposure to acidic condensates at ambient temperature in the atmosphere. These hypotheses were studied by combining laboratory approaches (experiments and analyses) across geochemistry and atmospheric science disciplines, providing new insights on factors potentially affecting the role of airborne ash in ice nucleation.
Natural ash from different volcanic eruptions was chosen with collaborators at Ludwig-Maximilians University (LMU). Additionally, synthetic ash corresponding to volcanic glass was produced from natural ash in the laboratory. The synthetic ash is identical in chemical composition to the natural ash but contains no crystalline phases; so enabling us to disentangle the influence of chemical composition and mineralogy on ice nucleation. A subset of the ash samples were subjected to high temperature ash-gas interactions (‘eruptive processing’) at LMU to simulate exposure to various thermal and chemical conditions in the eruption plume. Several ash samples were also subjected to ambient temperature ash-condensate interactions (‘atmospheric processing’) at the University of Leeds (ULeeds) to mimic ash contact with acidic cloudwater during atmospheric transport. The INA of all ash samples was measured using a microlitre Nucleation by Immersed Particle Instrument at ULeeds. Measurements were conducted throughout the project to obtain robust replicate data and to acquire insights on ash INA to help plan continuing laboratory work.

A research paper on the influence of chemical composition, crystallinity, and mineralogy on ice nucleation by ash was published in Atmospheric Chemistry and Physics. Two papers on the effects of eruptive and atmospheric processing on ice nucleation by ash are currently in preparation, the latter with collaborators at Carnegie Mellon University. Findings from this work have also been presented at the AGU Fall Meeting 2018 and the VMSG 2019 Annual Meeting, and will be presented at the EGU General Assembly 2020. Briefly, observations that crystal-bearing natural ash is better at nucleating ice than crystal-free synthetic ash suggest that minerals are key to ice nucleation. In particular, the most ice-active samples tested contained feldspars and/or pyroxenes, leading us to speculate that the eruption of magma of felsic to intermediate composition might give rise to ice-active ash. However, results of eruptive and atmospheric processing experiments illustrate that INA can be enhanced or depressed by various ash-gas and ash-condensate interactions. More research at the molecular scale is needed to understand precisely how changes in ash surface chemical and physical properties from such interactions elicit changes in the ash INA.

The project was additionally exploited to inspire learning at the student level. Three BSc students (from ULeeds, from the US funded by NSF, from Germany funded by DAAD) carried out research projects, under my supervision, on ice nucleation by ash. Findings on this topic were also incorporated into two undergraduate environmental science/chemistry lectures delivered at ULeeds and by invitation at the University of Central Lancashire.
This project bridged atmospheric science expertise in quantifying ice nucleation and geochemistry expertise in characterising ash properties which likely dictate its behaviour. Working across various research groups at ULeeds and LMU, including Ice Nucleation, Geochemistry, and Volcanology, contributed to developing new knowledge on ash-atmosphere interactions. Experiments on a wide range of ash samples allowed us to separate the influence of chemical composition and mineralogy on INA, and simulations of eruptive and atmospheric processes enabled us to study enhancement and depression of INA due to thermal and chemical alteration of ash.

Such insights on links between the INA of ash and its magmatic, eruptive, and atmospheric history are essential to constraining the potential influence of ash emissions on ice formation and thus, ultimately, to better understanding the atmospheric and climatic impacts of explosive eruptions. Laboratory data from this project can be parameterised for use in numerical models to explore and quantify the importance of ash INPs to atmospheric composition and climate under different scenarios. Such results could have value for policy stakeholders outside of academia concerned with airborne ash such as the London Volcanic Ash Advisory Center and UK Met Office Atmospheric Dispersion Group. Further, fundamental knowledge gained from this research concerning the effect of various properties and processes on INA might be relevant to understanding ice nucleation by other solid materials too (e.g. desert dust, glacial flour).

Outreach activities during the project, including a public discussion on Volcanic Hazards Communication at Leeds Central Library and engagement with pupils across the UK in ‘I’m A Scientist, Get Me Out of Here,’ gave me valuable opportunities for exchange with the public. A project website and Twitter profile was also used to share information with a wide audience.

Overall, the main goals of the INoVA project have been successfully achieved and, once all work has been published, it will hopefully represent a foundation of new knowledge on ash ice nucleation and pave the way for future research in this area. This fellowship has also contributed greatly to my professional development and prospects to continue a research career.