Aerosol formation in basaltic lava fountaining: Eyjafjallajokull volcano, Iceland

A short-lived episode of basaltic lava fountaining at Eyjafjallajokull volcano (March-April 2010) produced a low- altitude, ash- poor plume. We measured the composition of aerosol particles (sampled using a cascade impactor and filter packs), gases (sampled using filter packs), and volatile species scavenged by scoria and external water in order to investigate the formation and speciation of near- source aerosol (< 2 min from emission). Samples were analyzed for volatile species (S, Cl and F) and metals (Na, K, Ca and Mg). The aerosol mass showed two unusual features: the prevalent size mode was finer than typically found in volcanic plumes (similar to 0.2 mu m, compared to > 0.4 mu m), and its composition was dominated by chloride rather than sulfate. We used two thermodynamic equilibrium models (E- AIM and HSC Chemistry v5.1) to show that the formation of particulate Cl- by condensation of HCl gas is more responsive to changes in ambient temperature than the oxidation of SO2 to SO42-, so that a low SO42-/Cl- ratio in aerosol particles is characteristic of volcanic emissions in cold climates. Field measurements suggested that the efficiency of SO2 to SO42- conversion inside the vent increased with lower explosivity. Volatiles adsorbed on the surface of scoria had significantly higher SO42-/halogen molar ratios than the aerosol samples. Several potential explanations for these differences are discussed.