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[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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/ P S with extent of S degassing at Erta Ale, indicating negligible effect on fO 2 , and further suggesting that H 2 S is the dominant gas species exsolved from the S 2À -rich melt (i.e., no redistribution of electrons). High SO 2 /H 2 S observed in Erta Ale gas emissions is due to gas re-equilibration at low pressure and fixed fO 2. Sulfur budget considerations indicate that the majority of S injected into the systems is emitted as gas, which is therefore representative of the magmatic S isotope composition. The composition of the Masaya gas plume (þ4.8%) cannot be explained by fractionation effects but rather reflects recycling of high 34 S oxidized sulfur through the subduction zone.