ATMOSPHERIC HYDROGEN SULPHIDE CONCENTRATIONS IN FUMAROLIC AREAS OF THE SOUTH AEGEAN VOLCANIC ARC (GREECE)

Abstract

Volcanic and geothermal areas are one of the major natural sources of environmentally relevant gases to the atmosphere. Hydrogen sulphide (H2S) is a toxic gas, which is rather always present in geothermal fluids. Like carbon dioxide, being a gas that is denser than atmospheric air, it can accumulate in topographic depressions and enclosures sometimes reaching concentrations (500-1000 ppm) lethal to humans and animals. It has a characteristic odor of rotten eggs to which the human smell is very sensible (odor threshold as low as 10 ppb for very sensible persons). The south Aegean volcanic arc, which is related to the subduction of the African plate beneath the Eurasian plate, comprises many active or extinct volcanic centres. Most of them are characterised by the presence of fumarolic areas. The peculiarity and beauty of such manifestations attracts each year thousands of tourists many of which go very close to the emission vents. In the present study we measured the atmospheric concentrations and dispersion pattern of naturally emitted H2S at four fumarolic areas (Thiochoma – Sousaki, Fyriplaka – Milos. Nea Kameni – Santorini, Lakki plain – Nisyros). Measurements were made with a network of passive samplers positioned at about 1.5 m above the ground, which gave time-integrated values for periods ranging from days to weeks. Values ranged from 0.1 to 2050 ppb at Sousaki, from 0.8 to 33.8 ppb for Milos, from 0.1 to 5.1 ppb for Santorini and from 1.6 to 1240 ppb for Nisyros. At all the fumatolic areas concentrations decreased exponentially with increasing distances from the source. The highest values were recorded at distances of about 5-20 m from the fumarolic vents. The striking difference between the highest values measured at Sousaki and Nisyros with respect to Milos and Santorini depends on two main factors. The first factor is a higher H2S emission rate at the former two sites (stronger total gas emission and higher H2S concentrations). The second factor is the geomorphology of the same two sites that limits the gas exchange with the open atmosphere. In fact, while the fumarolic area of Sousaki is in a narrow ravine and that of Nisyros is within the calderic depression of the island, the fumarolic areas of Milos and Santorini are well exposed to free atmospheric circulation. Nevertheless also the highest measured concentrations, although disconfortable, does not have adverse effects on human health.