10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Effect of Mineralogy, Particle Size, and Composition on the Immersion Freezing Properties of Three Central American Volcanic Ashes
LEIF JAHN, Daniel Williams, Michael Polen, William Fahy, Ryan Sullivan, Carnegie Mellon University
Abstract Number: 1422 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Ash generated from explosive volcanic eruptions represents a large source of material periodically injected into the atmosphere. Recent studies have suggested that volcanic ash is a potentially significant source of ice nucleating particles (INP). However, the current body of research regarding how the chemical composition of volcanic ash relates to its ability to nucleate ice is lacking, especially in atmospherically relevant particle size ranges. In this work, we analyzed three volcanic ash samples collected in the vicinity of recent volcanic eruptions (Volcàn de Fuego, Guatemala, 2015; Santiaguito, Guatemala, 2011; Soufrière Hills, Montserat, 2011). Each of the samples were sieved to a <37 μm size fraction, and the Fuego sample was further separated into <2.5 μm, 2.5-10 μm, and 10-37 μm fractions using a custom dust generator and MOUDI sampler. X-ray diffraction (XRD) analysis indicates that the ashes are mostly composed of Na- and Ca-rich feldspars, as well as varying amounts of silica polymorphs and pyroxenes. Additionally, XRD was used to quantify the amount of amorphous non-crystalline material, which likely has weak ice nucleation ability. Freezing properties were examined in the immersion mode using a droplet freezing cold plate. Analysis of the droplet freezing temperature spectra shows that the ashes contain efficient INP in the range of -10 to -25 C that are consistent with the presence of feldspars—some of the most ice-active mineral phases. The Fuego ash sample demonstrated freezing ability comparable to that of some of the most ice-active K-rich feldspar samples reported in the literature, which we attribute to the presence of Na-rich/K-poor anorthoclase feldspar. Additionally, smaller particle sizes were observed to be more ice-active, even when accounting for differences in particle surface area. Ash elemental composition at the population level was examined through ion chromatography, and individual particle elemental distribution and morphology was examined through scanning electron microscopy. Our results indicate that these volcanic ash aerosols can be a significant source of INP and that this freezing ability is driven by the crystalline phase content, mineralogy, and quantity of feldspar minerals in the ash.