First Estimates of Volcanic Mercury Emissions from Three Alaska Eruptions

Volcanoes are a natural source of mercury (Hg), which is emitted as gaseous elemental Hg(0) and may be oxidized to reactive gaseous Hg(II) leading to adsorption onto the surface of ash particles or into the aqueous phase. Total volcanic Hg emissions and Hg speciation, reaction rates, and pathways within volcanic plumes are not well characterized, but total global volcanic Hg emissions are estimated to be ~70 t a-1 for passively degassing volcanoes and ~630 t a-1 for actively erupting sources. This knowledge gap is partially due to the logistical challenges of collecting gas and ash samples during active eruptions. To better understand volcanic Hg emissions, we analyzed particulate phase Hg from 258 bulk ash samples from three recent Alaska eruptions: Augustine (2006), Redoubt (2009), and Spurr (1992). These samples were analyzed using direct combustion atomic absorption spectroscopy to identify potential variation in Hg concentrations on ash with distance from the vent, between volcanoes, and between eruptive events. In total, the samples represent distances from 3 - 350 km and were subcategorized by eruptive sequence event. These ash concentrations were then combined with total eruptive masses of ashfall to estimate the amount of Hg deposited through this pathway. We found an order of magnitude difference in the average concentration of Hg on ash samples and particulate Hg emission among the three volcanoes. We also found no statistically significant difference in Hg concentration with downwind distance or for different events from the same eruption. Our results suggest that volcanic Hg emissions can vary significantly among different volcanoes, but not from different events during the same eruption. Our results also suggest that conversion of gaseous to particulate Hg does not vary significantly within volcanic plumes at least for distances of 3 350 km. We find minimum Hg emissions in the particulate phase from our target eruptions of 0.2 t Hg for Augustine (2006), 1.5 t Hg for Redoubt (2009), and 10.4 t for Spurr (1992). These results may provide evidence for an influence of regional geology on Hg emissions from volcanoes and agree with the orders of magnitude difference in gaseous Hg concentration measured from persistently degassing volcanoes at other locations.