Before the onset of the eruptive phase, the molar ratio between SO4 2-and NH4 + in PM0.95 was around 0.5 (Figure ), suggesting complete sulfate neutralization by ammonium, and that ammonium sulfate ((NH4)2SO4) was the dominant sulfate species. This changed during the volcanic episode, for which highly elevated SO4 2-/NH4 + molar ratios of ~ 6 -9 suggest only a minor contribution of ammonium sulfate, and instead an elevated contribution of other sulfate species. This is most likely sulfuric acid, considering that the negative charge balance of the sampled ionic species (Figure ) suggests highly acidic aerosols 61 . It is known that sulfuric acid droplets can make up a large fraction of volcanic particles , and if volcanic ash particles are present in the plume, they are expected to be coated in aqueous sulfuric acid when SO2 and its oxidation products are present 65 . As sulfuric acid is extremely hygroscopic, these particles absorb atmospheric moisture even when relative humidity is very low 66 , as was the case during the GEM depletion events (RH ~ 15 -20%, see Figure ).