A new geochemical approach to estimate the distribution of air pollutants from natural and anthropogenic sources: examples from Solfatara Crater (Campi Flegrei, Southern Italy) and Mt. Amiata Volcano (Siena, Central Italy)

Volcanic and geothermal systems significantly contribute to the input of volatile contaminants, such as mercury and hydrogen sulfide, into the atmosphere. Mercury has a strong environmental impact. In the atmosphere the prevalent elemental form is Hg0 (~98 %), whose main physical-chemical features are: high volatility, low solubility and chemical inertness. Hydrogen sulfide (H2S), one of the most abundant gas compounds in volcanic fluids, is highly poisoning and corrosive and unpleasantly smells of rotten eggs. Measurements of Hg0 and H2S concentrations in air are commonly performed by means of passive samplers. However, real-time measurements, coupled with monitoring of local atmospheric conditions, are strongly recommended for a reliable reconstruction of the dispersion dynamics once such contaminants are discharged in air. In this paper, a new real-time measurement method for Hg0 and H2S is presented. 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) were used for Hg0 and H2S measurements, respectively. These instruments were synchronized and set to high-frequency acquisition. Measurements were carried out along transects at an average speed <10 km/h. GPS data and meteorological parameters (wind direction and intensity) were also recorded. The proposed method was applied in two different sites, characterized by natural (Solfatara Crater, Campi Flegrei, Southern Italy) and anthropogenic (Mt. Amiata Volcano, Siena, Central Italy) emissions. With this highly efficient and effective approach, a reliable and reproducible Hg0 and H2S distribution in air was provided, allowing to identify and characterize the gas sources from such different environments. At Solfatara, the distribution of Hg0 and H2S concentrations, the highest values being measured close to the fumarolized areas (>60 ng/m3 and >2,100 \u3bcg/m3, respectively), suggests that these gases were discharged from both fumaroles and diffuse degassing from the crater bottom. At Mt. Amiata, the maximum Hg0 and H2S concentrations (>100 ng/m3 and >35 \u3bcg/m3, respectively) were recorded close to the geothermal power plants of Piancastagnaio. According to detailed dot-maps constructed on the basis of the measured values, as expected, wind was the main environmental parameter able to control the behavior and the dispersion halo of the Hg0- and H2S-rich plumes emitted from the contaminant sources