Metal speciation of volcanic aerosols from Mt. Etna at varying aerosol water content and pH obtained by different thermodynamic models†

Mt. Etna, an open-vent, persistently degassing volcano, is the tallest and most active volcano in Europe. Aerosols from the summit (Bocca Nuova crater), downwind (about 10 km from the crater) and control sites were collected during the EUROVOLC EPL-REFLECT field campaign in July 2019 and analysed for aerosol mass determination, major inorganic and organic ions, and soluble and insoluble metals. Computational modelling (using the models E-AIM, ISORROPIA, and Visual MINTEQ) was performed to determine the speciation of metal ions in the deliquescent aerosol phase within the volcanic plume and in aerosol collected in the town of Milo (Catania, Italy), a few km downwind of Mt. Etna and influenced by transport of the volcanic plume. The aerosol liquid water concentration at the summit was strongly dependent on the determination method – with ISORROPIA calculating a water concentration a factor of 10² lower than that of E-AIM, which itself was a factor of 10²–10³ lower than the total water content of the plume measured by infrared spectroscopy. The calculated pH was predominantly acidic (except for ISORROPIA calculations in the three samples), with the highest acidity observed where the water concentration was the lowest. Only a few metals were shown to have significant organic–ligand complexation in the aerosol, i.e., Al(III), Cu(II), and Fe(III) with oxalate, in the deliquescent aerosol within the plume. When considering the total amount of water of the plume, lower complexation was observed because of more diluted species concentration and less acidity.