A short-lived episode of basaltic lava fountaining at Eyjafjallajökull 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 (∼0.2μm, compared to >0.4 μm), 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.