Behaviour of S-bearing compounds (H2S and SO2) emitted in air from natural emissions in hydrothermal-volcanic systems

The main sources of SO2 and H2S in air consist of (i) natural fluid emissions from active/quiescent volcanoes and (ii) anthropogenic activities. These gas compounds have a strong impact on air quality, since they are toxic and cli- mate forcing agents. Notwithstanding, the behaviour of these S-compounds in air is poorly known, since relatively scarce are the available thermody- namic data as well as those deriving from direct measurements. Hydro- gen sulphide is the main S-compounds of low-temperature emissions in hy- drothermal systems and, in the atmosphere, tends to be oxidized to SO2 by photochemical reactions. Oxidation processes are also affecting atmospheric SO2, which is the main S-compounds of high-temperature emissions in vol- canic systems, since about 65% is transformed to SO42− whilst the remaining 35% is removed by dry deposition. This PhD research project was aimed to provide insights into the chemical-physical processes affecting H2S and SO2 in plumes discharged from hydrothermal fluid emissions which con- trol the spatial distribution of these gases in air. The empirical approach carried out in Iceland, where a number of hydrothermal emissions related to different volcanic/geothermal systems occurs, followed a measurement strategy optimized during two first field campaign at Vulcano Island (Aeo- lian Archipelago) and at La Solfatara (Phlegrean Fields). Hydrogen sulphide and SO2 measurements in air were performed using a Thermo Scientific 450i Analyzer positioned at increasing distance along patterns downwind from the emission sources, in order to obtain a “snapshot” of the H2S and SO2 concentrations within the plume. A mathematical model of the spatial dis- tribution of the two air pollutants, coupled with a statistical elaboration of the measured data, was applied to the measured data to i) describe the evo- lution of H2S and SO2 within the plume with distance observed in the field, ii) determine the rate of loss of H2S and SO2 at increasing distances from the source, and iii) discriminate the effects of physical (i.e. dilution) and chem- ical (e.g., oxidation of H2S to SO2) mechanisms controlling the spatial and temporal dispersion of the S-bearing gases. The results show that dilution by air affect both H2S and SO2 at a lower extent with respect to the chemi- cal degradation processes. Simulations carried out using the mathematical model show that, at distances >100 meters from the emitting source, the H2S and SO2 concentrations were below (<5 ppb by vol.) those of the guidelines of WHO (World Health Organization). Remarkably, the reaction rates of ox- idation processes (i.e. homogeneous gas-phase reactions via OH radical) in air for both H2S and SO2 calculated by the model are faster than those sug- gested in literature. This implies that the H2S and SO2 removal mechanism from the plume mostly consist of heterogenous and multiphase reactions that, on their turn, depends on different variables, e.g., presence of water droplets and plume temperature. Different SO2/H2S ratios were measured at each geothermal system, as well as at the same system in the proximity of different gas emissions, suggesting that these SO2/H2S variations were not related to the deep magmatic source, but, more likely, to secondary shallow processes.