New Insights into the Ice Nucleation By Volcanic Ash Particles in Mixed-Phase Clouds

Volcanic ash (VA) is one of the major emissions from volcanic eruptions which markedly contribute to aerosol loadings in the atmosphere during volcanic events. Aside from the negative impact of VA on aviation and air quality, these particles can alter the cloud microphysics by triggering ice formation in clouds, therefore impacting on precipitation and climate. However, the behaviour of VA as ice-nucleating particles is not well-understood. Previous ice nucleation studies [1] showed that the ability of different VA in the immersion mode at some temperatures could vary up to 5 orders of magnitude in their ice nucleation active site (INAS) density. In this new study, we present a unique dataset on the ice nucleation behaviour of a wide range of different VA (10 samples) obtained from different volcanoes worldwide. The ice nucleation activities of these VAs were studied in the world-acclaimed Aerosol Interactions and Dynamics in the Atmosphere (AIDA) cloud/aerosol simulation chamber as well as with the Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology (INSEKT) at KIT, Germany. All the VA samples investigated nucleated ice in the immersion freezing mode from 264 K to 237 K with INAS densities ranging from ~ 10⁵ m^-2 to ~ 10¹³ m^-2, respectively. The variabilities observed among the VA samples at any given temperature were within 2 orders of magnitude. A steep gradient in the INAS densities was observed for some samples, which indicates that the active sites are similar and evenly distributed among the particles. At 245 K, VA from Sakurajima, Japan showed the highest activity among all the VA studied to date. With this large dataset, we have developed a more confident parameterization for the immersion freezing ice nucleation by VA within the temperature range studied. This can become very useful for modelling the impact of VA particles on clouds especially, in the regions where these group of natural aerosols are emitted and dispersed. These results will further help to constrain the current uncertainties associated with aerosol-cloud microphysics interactions in the atmosphere.

[1] Mangan TP, Atkinson JD, Neuberg JW, O’Sullivan D, Wilson TW, Whale TF, et al. (2017) Heterogeneous Ice Nucleation by Soufriere Hills Volcanic Ash Immersed in Water Droplets. PLoS ONE 12(1): e0169720. doi:10.1371/journal. pone.0169720.