[1] Existing studies of the composition of volcanic plumes generally interpret the presence of sulfate aerosol as the result of comparatively slow oxidation of gaseous SO 2 . We report here new observations from Masaya Volcano, Nicaragua, which demonstrate that sulfate aerosol may also be emitted directly from volcanic vents. Simultaneous aerosol and gaseous S, Cl, and F compounds were collected at the rim of the passively degassing crater in May 2001. Mean concentrations of SO 4 2À , Cl À , and F À within the plume were 83, 1.2, and 0.37 mg m À3 , respectively (fine aerosol fraction <2.5 mm) and 16, 2.5, and 0.56 mg m À3 , respectively (coarse aerosol fraction >2.5 mm). The aerosols were highly acidic, with estimated pH of <1.0 in the fine aerosols. Sulfate was present mainly in smaller particles, with the fine fraction accounting for %80% of the mass. The bulk of the sulfate was emitted directly from the magmatic vent. Acidity in the aerosols derived from the presence of sulfuric acid and, to a lesser extent, hydrofluoric acid,] equivalent values of 0.5-0.8 and 0.3-3 for fine and coarse aerosols, respectively. Gas phase/aerosol phase mass ratios were, on average, 458 (S), 330 (F), and 186 (Cl), with ranges of 95-1178, 37-659, and 43-259, respectively. These observations of highly acidic aerosol emitted directly from crater vents have implications for plume chemistry and environmental and health impacts of volcanic degassing.
