EGU2020-5820
https://doi.org/10.5194/egusphere-egu2020-5820
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Solid-gas interactions in the eruption plume can both depress and enhance volcanic ash ice-nucleating activity

Elena Maters1, Ana Casas2, Corrado Cimarelli2, Donald Dingwell2, and Benjamin Murray1
Elena Maters et al.
  • 1School of Earth and Environment, University of Leeds, United Kingdom
  • 2Department of Earth and Environmental Sciences, Ludwig-Maximilians University Munich, Germany

Volcanic ash generated by explosive eruptions can act as ice-nucleating particles, promoting freezing of supercooled water droplets in the eruption plume and the ambient atmosphere, and so impacting processes such as plume electrification, ash aggregation, and cloud glaciation. Our initial study of a compositional range of milled ash and glass materials demonstrated that mineralogy is likely a key property influencing ice nucleation by ash.1 However, the surface properties of ash are modified by interaction with magmatic gases in the hot core of the eruption plume, and it is not known how such in-plume interactions might affect the ice-nucleating activity (INA) of ash.

Here we investigated the influence of high temperature solid-gas interactions on the INA of three milled ash (Tungurahua, Astroni, Etna) and two milled mineral (K-feldspar, quartz) materials. Sub-samples of these materials were exposed to pure water vapour (H2O) or mixtures of water vapour with HCl(g) (H2O-HCl) or SO2(g) (H2O-SO2) under an 800 °C/400 °C heating sequence in the Advanced Gas-Ash Reactor.2 The INA of the non-treated and treated samples was then assessed using a microlitre Nucleation by Immersed Particle Instrument.3 The H2O treatment decreased the INA relative to that of the non-treated sample for all materials, and the H2O-HCl treatment decreased the INA to the same extent or more. Conversely, the H2O-SO2 treatment increased the INA (Tungurahua ash, Etna ash), or decreased the INA 1) to a lesser extent than the other treatments (Astroni ash), 2) to the same extent as the other treatments (quartz), or 3) to a greater extent than the other treatments (K-feldspar).

The depression in INA induced in all cases by the H2O treatment may relate to dehydroxylation of the silicate materials’ surfaces at high temperatures. On the other hand, differing effects on INA of the H2O-HCl and H2O-SO2 treatments is inferred to relate to contrasting reactivities of these materials towards HCl(g) and SO2(g). Water leachates of the samples suggest that chloride and sulphate salts (e.g., NaCl, CaSO4) formed on the H2O-HCl- and H2O-SO2-treated ash surfaces, respectively, but not on the H2O-HCl- and H2O-SO2-treated mineral surfaces. Additional tests suggest that the changes in INA observed for these treated ash samples do not reflect a ‘solute effect’4 imparted by the chloride or sulphate salts in water, implying that the ice-nucleating properties of the ash surfaces themselves are somehow changed by reaction with HCl(g) and SO2(g).

Surface-sensitive analyses could be useful to elucidate how sample surfaces have been modified by the different solid-gas interactions at the scale relevant for ice nucleation, and so potentially shed light on the cause of the depression and enhancement in INA observed here. The possibility that in-plume reaction with SO2(g) can increase the INA of volcanic ash in particular merits further investigation, as a previous line of thought has been that exposure of silicate particles to this acidic gas decreases INA.

 

1Maters et al. (2019) Atmos. Chem. Phys., 19, 5451–5465. doi:10.5194/acp-19-5451-2019

2Ayris et al. (2015) Bull. Volcanol., 77, 104. doi:10.1007/s00445-015-0990-3

3Whale et al. (2015) Atmos. Meas. Tech., 8, 2437-2447. doi:10.5194/amt-8-2437-2015

4Whale et al. (2018) Chem. Sci., 9, 4142-4151. doi:10.1039/C7SC05421A

How to cite: Maters, E., Casas, A., Cimarelli, C., Dingwell, D., and Murray, B.: Solid-gas interactions in the eruption plume can both depress and enhance volcanic ash ice-nucleating activity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5820, https://doi.org/10.5194/egusphere-egu2020-5820, 2020

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