Volcanic plume monitoring at Mount Etna by diffusive (passive) sampling

This paper reports the use of diffusive tubes in determining HF, HCl, and SO 2 in the volcanic plume of Mount Etna in an attempt to highlight the potential of this method in studying volcanoes. In a first application a network of 18 diffusive tubes was installed on Etna's flanks, aimed at evaluating the atmospheric dispersion of the volcanic plume on a local scale. Results showed a monotonic decrease in volatile air concentrations with distance from the craters (HF from 0.15 to <0.003 μmol m −3 , HCl from 2 to <0.01 μmol m −3 , and SO 2 from 11 to 0.04 μmol m −3 ), revealing the prevalently volcanic contribution. Matching of SO 2 /HCl and HCl/HF volatile ratios with contemporaneous measurements at the summit craters validated the use of diffusive tubes in tracing the chemical features of a volcanic plume from remote locations. A first tentative assessment of dry deposition rates of volcanogenic acidic gases was also made, yielding 2.5–74 t d −1 (SO 2 ), 0.6–17 t d −1 (HCl), and 0.02–0.6 t d −1 (HF) and revealing the potential environmental impact of gas emissions. In a second experiment, carried out during the recent October 2002 to February 2003 eruption of Etna, diffusive tubes provided a continuous record of the chemical composition of the eruptive plume from a “safe” distance of ∼1 km from the vents, thus considerably decreasing the risks involved in sampling. This highlighted a clear time decrease in SO 2 concentrations and SO 2 /HCl ratios, which was interpreted as due to progressive exhaustion of volatile degassing and eruption energy.