(Formerly Poster 750) The Hygroscopic Properties of Volcanic Ash and Implications for the Evolution of Volcanic Plumes in the Atmosphere

Volcanic eruptions are long known to have profound impacts on the Earth System and society, which result from the atmospheric emissions and transport of volcanic ash. The microphysical evolution of volcanic plumes is key to understanding their atmospheric lifetime. Volcanic ash is composed primarily of silicate glass and crystal and is injected into an environment initially rich in volcanically derived gases, including water vapor. Limited observational data exists on the physical interactions between the water vapor and ash particles; yet it is thought that these interactions can strongly impact the coagulation efficiency and microphysical evolution of volcanic ash. In this study, we investigate the hygroscopic properties of fine volcanic ash (less than 125 micro-meter diameter) from a variety of sources, including the eruptions of Mount St. Helens in 1980, Tungurahua in 2006, Chaiten in 2008, Redoubt in 2009, and Eyjafjallajökull in 2010. These recent eruptions were selected to encompass a range of composition, crystallinity, and eruptive style. The hygroscopicity of the ash particles is quantified by their ability to nucleate cloud droplets under controlled levels of water vapor supersaturation. The dependence of critical supersaturation vs. dry particle diameter is used to i) explore the origin of particle hygroscopicity (being from the presence of deliquescent soluble material or adsorption onto insoluble surfaces), and, ii) determine the level of humidity required to coat volcanic ash with water, a requirement for large coagulation efficiencies.

Our results show that fresh volcanic ash tends to follow adsorption activation theory and supports the suggestion that ash particles are sufficiently hygroscopic to be coated with a monolayer of water under subsaturated conditions. The range of interactions varies strongly, and follows what is expected from their composition. These experiments provide new insights on ash-water interactions, which can be used to enhance predictions of the atmospheric evolution and transport of volcanic ash.