Volcanic sulfate aerosol is emitted from the vents of many active volcanoes, but its origin has not yet been explained. We report the first measurements of the isotopic compositions of near-vent volcanic aerosol sulfate and use the combined sulfur and oxygen isotope systemics of the aerosol and magma at Masaya volcano, Nicaragua, to draw preliminary conclusions about the production mechanism of near-source volcanic sulfate. The sulfate aerosol does not display a significant mass-independent oxygen or sulfur isotopic signature, which suggests that primary volcanic aerosol does not contribute to the larger mass-independent anomalies found in some ambient atmospheric aerosols and as preserved in ice cores. The oxygen isotope composition of the sulfate appeared to become heavier on the particle filters with increasing amount of sample collected and suggests that the δ<sup>18</sup>O value of the sulfate aerosol is < 8‰. The sy=ulfur isotopic composition of the sulfate did not vary with sample loading on the filters and had a mean δ<sup>34</sup>S value of 7.7 ± 0.8‰, similar to that of the magma (6.6 ± 0.2‰). The low Δ<sup>17</sup>O value, the δ<sup>18</sup>O value of the magma (6.6 ± 0.3‰) and the high δ<sup>18</sup>O value of atmospheric oxygen (23.5‰) suggest that known atmospheric oxidation pathways at ambient temperatures are not the major routes of production for this sulfate. Instead the isotopic systematics of aerosol and magma are consistent with sulfate production either by high-temperature equilibrium of the magmatic gas mixture with small amounts of atmospheric oxygen or by direct emission of SO₄²⁻ from the magma.The full-text of this article is not currently available in ORA, but you may be able to access the article via the publisher copy link on this record page. Citation: Mather, T. A., J. R. McCabe, V. K. Rai, M. H. Thiemens, D. M. Pyle, T. H. E. Heaton, H. J. Sloane, and G. R. Fern (2006), Oxygen and sulfur isotopic composition of volcanic sulfate aerosol at the point of emission, J. Geophys. Res., 111, D18205
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