Crystalline silica (mostly cristobalite) was produced by vapor-phase crystallization and devitrification in the andesite lava dome of the Soufriere Hills volcano, Montserrat. The sub-10-micrometer fraction of ash generated by pyroclastic flows formed by lava dome collapse contains 10 to 24 weight percent crystalline silica, an enrichment of 2 to 5 relative to the magma caused by selective crushing of the groundmass. The sub-10-micrometer fraction of ash generated by explosive eruptions has much lower contents (3 to 6 percent) of crystalline silica. High levels of cristobalite in respirable ash raise concerns about adverse health effects of long-term human exposure to ash from lava dome eruptions.

Masaya volcano (560 m a.s.l.), Nicaragua, resumed its degassing activity in mid-1993 with the continuous emission of SO 2 at rates increasing from 600 metric tons (t) day -1 (7.0 kg s -1 ) in 1995 to 1800 t day -1 (21.0 kg s -1 ) in 1999. The low-altitude gas plume is typically blown westward by the prevailing wind across the Masaya caldera and Las Sierras highlands, which are at a higher elevation than the gas vent. In this study, the areal distribution of atmospheric SO 2 concentrations was monitored within 44 km of the vent with a network of passive samplers. Measured SO 2 air concentrations ranged from <2 to 90 ppbv in 1998 and from <2 to 230 ppbv in 1999. The data suggest that the volcanic emissions influenced air quality across a 1,250-km 2 area downwind. Local topography exerts a strong control on plume dispersal, and hilltops are particularly prone to fumigation and thus, to high ambient SO 2 levels. In a zone 22 km 2 in size located within 15 km of the source, the response of vegetation to sustained exposure to high atmospheric dose of volcanic SO 2 and HF resulted in a strong reduction in the number of plant communities. A transition zone of somewhat indefinite boundary surrounds the devastated zones and exhibits vegetation damage in the form of leaf injury. In addition to the environmental impacts of the volcanic emissions, both shortand long-term public health hazards may exist in areas most exposed to the plume. The harmful effects of the volcanic emissions on cultivated vegetation could be diminished by using windbreaks made of gas-tolerant trees and shrubs such as Eugenia jambos, Brosimum utile and Clusia rosea. The current gas crisis at Masaya volcano provides an unique opportunity for investigating the atmospheric, environmental and medical impacts of volcanic gases and aerosols.

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[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.