ercury emissions from active volcanoes are of interest for estimating the volcanogenic contribution of Hg to the atmosphere and for monitoring volcanic activity. The global Hg flux from natural sources is poorly known and considerable uncertainty still exists with respect to the complex physical and chemical reactions taking place in the atmosphere among the various Hg forms. The environmental hazard of Hg have been widely demonstrated for its high toxicity. Previous estimates from volcanic plumes are based on limited measurements and poorly determined speciation of Hg in volcanic emissions which is crucial for making reliable estimates of volcanic Hg source strengths and for predicting atmospheric residence times and deposition mechanisms. In this contribution we present Hg data for few active volcanoes outlining the importance of Hg flux from volcanic sources. Measurements involved collection of both gas and particulate Hg forms and sporadic determinations of divalent Hg in the near-vent plume of eight active volcanoes. Hg emission rate was then evaluated by using Hg/SO2 mass ratios in combination with the SO2 flux from each volcano. Results indicate very high Hg0(g) concentrations in the plume (range 84-400 ng m-3) consistently above background levels, reflecting the importance of persistent open-conduit degassing volcanoes as Hg sources of global relevance in the atmosphere. Estimates of particulate and divalent Hg forms in the plume of Mt. Etna and Masaya volcanoes suggest that Hg is mainly transported in the gas phase as Hg0(g), with a mean Hg(p)/Hg(g) spanning from 0.01 to 0.05 by mass, and accounting both forms from 1% to 5% of the total gaseous mercury, respectively. Mean estimated Hg/SO2 ratios in the plume of the investigated volcanoes range from 1.5×10-5 (Mt. Etna, Sicily; Masaya, Nicaragua; Myakejima, Japan; Yasur, Vanuatu Islands) to 4.8×10-6 (Vulcano Isl., Aeolian Arc; Bembow, Mbwelesu, Niritatin, at Vanuatu Islands). At Vulcano island, a quiescent volcano in Solfatara stage, estimated Hg/SO2 ratios in the plume were on average in qualitative agreement with the Hg/SO2 ratio directly measured in the fumarole (mean plume and fumarole ratios being 1.09×10-6 and 2.9×10-6, respectively), where the factor 2 difference between plume and fumarole compositions provides evidence for fast Hg chemical processing the plume. Field measurements of SO2 fluxes, combined with the Hg/SO2 ratios, indicate a cumulative Hg flux of 22 t yr-1 from only three volcanoes (Etna, Myake-jima and Masaya), pointing out that global budget computations quoted by some authors (~1 and 35-57 t yr-1) are likely significantly under-estimated. On the basis of a time-averaged volcanic Hg inventory of 700-800 t yr-1, degassing from Mt. Etna, Masaya and Myake-jima volcanoes contribute each for about 1% of yearly global Hg emissions, and their cumulative emissions represent about 30% of Hg being yearly contributed by passive degassing volcanoes at global scale (75 t yr-1).
|Stato di pubblicazione||Published - 2008|