Atmospheric impact of volcanic volatiles: trace elements in snow and bulk deposition samples at Mount Etna (Italy)

Vienna, Austria

Volcanoes represent an important natural source of several trace elements to the atmosphere.
For some species (e.g., As, Cd, Pb and Se) they may be the main natural source
and thereby strongly influencing geochemical cycles from the local to the global scale.
Mount Etna is one of the most actively degassing volcanoes in the world, and it is considered
to be, on the long-term average, the major atmospheric point source of many
environmental harmful compounds. Their emission occurs either through continuous
passive degassing from open-conduit activity or through sporadic paroxysmal eruptive
activity, in the form of gases, aerosols or particulate. To estimate the environmental
impact of magma-derived trace metals and their depositions processes, rainwater and
snow samples were collected at Mount Etna area. Five bulk collectors have been deployed
at various altitudes on the upper flanks around the summit craters of the volcano;
samples were collected every two week for a period of one year and analyzed
for the main chemical-physical parameters (electric conductivity and pH) and for major
and trace elements concentrations. Chemical analysis of rainwater clearly shows
that the volcanic contribution is always prevailing in the sampling site closest to the
summit crater (about 1.5 km). In the distal sites (5.5-10 km from the summit) and
downwind of the summit craters, the volcanic contribution is also detectable but often
overwhelmed by anthropogenic or other natural (seawater spray, geogenic dust) contributions.
Volcanic contribution may derive from both dry and wet deposition of gases
and aerosols from the volcanic plume, but sometimes also from leaching of freshly
emitted volcanic ashes. In fact, in our background site (7.5 km in the upwind direction)
volcanic contribution has been detected only following an ash deposition event.
About 30 samples of fresh snow were collected in the upper part of the volcano, during
the winters 2006 and 2007 to estimate deposition processes at high altitude during
cold periods. Some of the samples were collected immediately after a major explosive
event from the summit craters to understand the interaction between snow and fresh
erupted ash. Sulphur, Chlorine and Fluorine, are the major elements that prevailingly
characterize the volcanic contribution in atmospheric precipitation on Mount Etna, but
high concentrations of many trace elements are also detected in the studied samples.
In particular, bulk deposition samples display high concentration of Al, Fe, Ti, Cu,
As, Rb, Pb, Tl, Cd, Cr, U and Ag, in the site most exposed to the volcanic emissions:
median concentration values are about two orders of magnitude higher than those
measured in our background site. Also in the snow samples the volcanic signature is
clearly detectable and decreases with distance from the summit craters. Some of the
analysed elements display very high enrichment values with respect to the average
crust and, in the closest site to the summit craters, also deposition values higher than
those measured in polluted urban or industrial sites.