A new approach for the measurement of gaseous elemental mercury (GEM) and H2S in air from anthropogenic and natural sources: Examples from Mt. Amiata (Siena, Central Italy) and Solfatara Crater (Campi Flegrei, Southern Italy)

Real-time measurements of GEM and H2S discharged from natural and anthropogenic sources are a valuable tool to investigate the dispersion dynamics of these contaminants in air. In this study, a new approach to measure GEM and H2S concentrations in air, carried out by coupling a portable Zeeman atomic absorption spectrometer with high frequency modulation of light polarization (Lumex RA-915M) and a pulsed fluorescence gas analyzer (Thermo Scientific Model 450i), was applied to two distinct areas: (i) in the surroundings of Piancastagnaio (Siena, Central Italy), located in the eastern flanks of Mt. Amiata (a 200,000 years old volcano), where three geothermal plants are operating and whose exhaust gases are dispersed in the atmosphere after passing through the turbines and an abatement system to mitigate the environmental impact on air, and (ii) at Solfatara Crater (Campi Flegrei, Southern Italy), a volcanic apparatus characterized by intense hydrothermal activity. In 2014, seven GEM and H2S surveys were carried out in the two areas along pre-defined pathways performed by car at both the study sites. The lowest and highest recorded GEM and H2S concentrations at Piancastagnaio were up to 194 and 77 ng/m3, respectively, whilst at Solfatara Crater were up to 690 and 3392 μg/m3, respectively. Although the GEM concentrations at Piancastagnaio were lower than the limit value recommended by local regulations for outdoor environment (300 ng/m3), they were almost one order of magnitude higher than the GEM background both in Tuscany (~ 3.5 ng/m3) and Mt. Amiata (3–5 ng/m3), suggesting that the main source of GEM was likely related to the geothermal plants. At Solfatara Crater, the highest GEM values were recognized in proximity of the main fumarolic gas discharges. As far as the H2S concentrations are concerned, the guideline value of 150 μg/m3, recommended by WHO (2000), was frequently overcome in the study areas. Dot (in the surroundings of Piancastagnaio) and contour (at Solfatara Crater) maps for GEM and H2S concentrations built for each survey highlighted the important effects played by the meteorological parameters, the latter being measured by a Davis® Vantage Vue weather station. In particular, the GEM and H2S plumes were strongly affected by the wind speed and direction that were able to modify the dispersion of the two parameters in air in a matter of hours, indicating that the proposed analytical approach is able to produce a more realistic picture of the distribution of these air pollutants than that provided by using passive traps. Finally, the H2S/GEM ratio, calculated by normalizing the measured GEM and H2S concentrations to their highest values (nH2S/GEM), was used as a good proxy for the chemical-physical processes that these two gas species can suffer once emitted in the air. In particular, H2S resulted to be more affected by secondary processes than GEM, possibly related to photochemical oxidation reactions.