High temperature SO2 chemisorption on model systems. Implications for in-plume processes.

Volcanic volatile species are scavenged by silicate ash during transition through eruption plumes. Scavenging may form S, F and Cl salts and acids on ash surfaces, though the mechanisms and controlling variables remain poorly understood. A limited mechanistic understanding impedes estimation of volcanic volatile budgets and may also prevent assessment of environmental impacts resulting from volatile scavenging. Limited assessment of these impacts may have implications for local communities affected by ashfall onto vegetation, soils and into water bodies, and ashfall from very large eruptions may have global impacts. Through experimental techniques, this study examined SO2 scavenging mechanisms on silicate ash surfaces. SO2 uptake experiments were conducted on model Ca-aluminosilicate xerogels and on glasses with chemical compositions of common ash types. The materials were characterised using bulk and surface-sensitive techniques to gain insight into the mechanisms of scavenging and the reaction products formed. SO2 chemisorption onto glass surfaces may occur on non-bridging oxygens of network modifying cations (Ca), forming sulphate salts (CaSO4) and initiates diffusion mechanisms which resupplies the surface with Ca. The chemisorption-diffusion mechanism may be most efficient at high temperature, and may become significant after a few minutes of SO2 exposure. The proposed scavenging mechanism may occur during the eruption within the high temperature volcanic conduit and in the core of the plume. High temperature SO2 scavenging could deposit the soluble S salts inferred to exist on ash and may dictate its surface chemistry for later reactions during transport through the plume and dispersion in the atmosphere and/or environment. It is not yet possible to quantify this mechanism or compare it to other scavenging mechanisms (aqueous acid condensation, high temperature salt condensation), and so future studies should attempt to constrain all volatile scavenging mechanisms occurring on ash surfaces, with particular focus on high temperatures, even in the subterranean environment.