Ash, Sulfate and Ice: Examining Interactions One Particle at a Time

Tuesday, 20 March 2018: 10:00
Salon Vilaflor (Hotel Botanico)
Margaret A Tolbert, Univ Colorado, Boulder, CO, United States, Kimberly D Genareau, University of Alabama, Tuscaloosa, AL, United States, Katherine Primm, University of Colorado at Boulder, Chemistry and Biochemistry-CIRES, Boulder, CO, United States, Shuichi Ushijima, University of Colorado at Boulder, Boulder, CO, United States and S M Cloer, University of Alabama, Geological Sciences, Tuscaloosa, AL, United States
Abstract:
Explosive volcanic eruptions inject large amounts of sulfur into the stratosphere, which can perturb the climate for several years following the eruption. In addition to sulfur and extensive water vapor emissions, such eruptions also generate large amounts of fine ash, which is dispersed into the atmosphere via plumes above volcanic vents and pyroclastic flows. Volcanic lightning is also commonly observed during explosive eruptions due to tribocharging from particle collisions involving ash or ice nucleated on the ash.

A schematic illustrating some of the possible particle interactions of volcanic ash is shown in the figure. Emitted ash can serve directly as a depositional ice nucleus at low temperature. Alternatively, after contact with a pre-existing aqueous aerosol such as ammonium sulfate, several possibilities exist. The ash could serve as a nucleation site for efflorescence of ammonium sulfate, forming a mixed solid phase particle (which could then itself go on to be an ice nucleus ). Alternatively, the ash may become immersed in the droplet, possibly leading to immersion ice nucleation.

Here, we examine these fundamental steps in the life cycle of an ash particle in the laboratory. Sulfate particles are levitated in an optical trap and exposed to collisions with volcanic ash particles. The collision either results in sulfate efflorescence to solid ammonium sulfate or immersion of the ash in the droplet. Similar experiments using mineral dust surrogates are used for comparison. We find that ash is not effective in raising the efflorescence relative humidity of sulfate. Thus, ash-sulfate interactions will likely lead to immersed ash. We next use Raman microscopy to study ice nucleation on ash in both deposition and immersion mode. Again, comparisons are made with ice nucleation of mineral dust particles. Here we find ash particles from several volcanic eruptions are excellent depositional ice nuclei, while immersion nucleation efficacy varies with the chemical and physical properties of the ash.

The experiments performed will be illustrated with movies of contact events between particles as well as ice nucleation videos. Implications for volcanic lightning will be discussed.

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