An experimental study of volcanic ash chemical reactivity : implications for atmospheric trace chemistry and ocean iron biogeochemistry

Volcanic ash from explosive eruptions is increasingly recognised as an environmental agent, yet knowledge is limited on what controls ash reactivity within atmospheric, aquatic or terrestrial systems. Reactivity is likely driven primarily by interactions at the solid-fluid interface, but the chemical properties of the ash surface which mediate these interactions are poorly understood. Relatedly, the capacity of ash emissions to impact the atmospheric trace gas budget by providing reactive surfaces has not been quantified. Further, ash deposition to the ocean is capable of affecting marine primary production by supplying iron, but factors determining ash iron solubility have yet to be elucidated. The doctoral study sheds light on these issues through experimental investigation. Probe gas uptake measurements reveal varying densities of acid-base and redox surface sites on ash. Ash surface chemistry, originally defined by magma composition and mineralogy and subsequently modified by interaction with volcanic gases and condensates during eruption, in turn influences ash reactivity towards sulphur dioxide and ozone gas. Iron mobilisation from ash is initially governed by surface characteristics imparted by plume processing and later reflects dissolution of bulk glass and crystalline components. Interaction with water and acid gases during eruption and exposure to pH fluctuations during transport can significantly affect iron solubility and speciation in airborne ash. Our findings suggest that a combination of magmatic, eruptive and atmospheric factors underlie ash chemical reactivity. By providing new insight on the role of ash as a chemical agent in the atmospheric and oceanic environments, this work contributes to better understanding the potential impacts of explosive eruptions on the Earth System.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 201