[1] Recent measurements of BrO, NO x , and near-source sulfate in volcanic plumes suggest that volcanic vents might not simply act as point sources of emissions into the troposphere, but may also act as hightemperature reaction sites where mixtures of magmatic and ambient atmospheric gases may combine, giving new and previously unexpected reaction products. The detection of such species demands that a more complex model be developed for the interaction of volcanoes and atmospheres. We show that general thermodynamic models can be applied successfully to volcanic gas equilibria by comparing the results from HSC Chemistry with those from two volcanic gas equilibrium models (Solvgas and Gasmix). Using a thermodynamic model optimized for volcanic gas chemistry (C-O-S-H-F-Cl-Br-I-N-Ar speciation), we show that the volume ratio of atmospheric gas to magmatic gas in a high-temperature mixture is an important parameter of the volcanic plume chemistry, and our results suggest that even small amounts of air (a few % for an H 2 O-rich magmatic gas) in the high-temperature mixture are sufficient to yield elevated levels of reactive nitrogen, halogen (Cl, Br, and I), and sulfur species within the volcanic plume. Further modifications of the plume chemistry may also occur due to low-temperature reactions, and chemical schemes for the modification of halogen (Cl, Br, I), nitrogen, and sulfur chemistry are suggested, within the constraints imposed by recent measurements.
[1] We present results from a campaign in March 2009 to assess the current state of emissions from Masaya Volcano, Nicaragua. These results constitute one of the most comprehensive inventories to date of emissions from an active volcano and update the exceptional record of emissions from Masaya. Results from open-path Fourier transform infrared spectroscopy and filter packs demonstrate that, in terms of H 2 O, SO 2 , CO 2 , HCl, and HF (molar H 2 O/SO 2 = 63, CO 2 /SO 2 = 2.7, SO 2 /HCl = 1.7, SO 2 /HF = 8.8), the 2009 gas composition was highly comparable to that from the 1998 to 2000 period, indicating stability of the shallow magma system. This continuity extends to certain aerosol species (molar SO 2 /SO 4 2− = 190, Na + /SO 4 2− = 0.68, K + /SO 4 2− = 0.71, Ca 2+ /SO 4 2− = 1.6 × 10 −2 , Mg 2+ /SO 4 2− = 3.6 × 10 −3 ) and, to a lesser extent, the heavy halogens (i.e., molar HCl/HBr = 2.4 × 10 3 , HCl/HI = 5.0 × 10 4 ). In contrast to an earlier study at Masaya, we did not detect HNO 3 . SO 2 fluxes were low (690 Mg d −1 ), suggesting that Masaya was close to the minimum of its degassing cycle. By combining compositional results with the SO 2 flux, we estimate a total volatile flux of 14,000 Mg d −1 . This rate is consistent with 1−4 wt% volatile loss from a convective magma flux of 17,000-4000 kg s −1 . These results will allow for a better understanding of degassing processes at Masaya and other basaltic volcanoes.
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