Impact on air quality of carbon and sulfur volatile compounds emitted from hydrothermal discharges: the case study of Pisciarelli (Campi Flegrei, South Italy)

Volcanoes are to be regarded as natural sources of air pollutants. Climatic and environmental forcing of large volcanic eruptions are well known, although species emitted during periods of quiescence or hydrothermal activity can also be highly impactful. Based on compositional and isotopic data, a survey on the spatial distribution in air of CO2, CH4, H2S and SO2 emitted from the fumarolic field of Pisciarelli (Campi Flegrei, South Italy), a hydrothermal area where degassing activity has significantly increased since 2009, was carried out. The aims of this study were (i) to assess the impact on air quality of these natural manifestations and (ii) inquire into the behavior of the selected species once released in air, and their possible use as tracers to distinguish natural and anthropogenic pollutant sources. Keeling plot analysis of CO2 and CH4 isotopes revealed that the hydrothermal area acts as a net source of CO2 in air, whilst CH4 originated mainly from anthropogenic activities. Approaching the urban area, anthropogenic CO2 sources increased and, at distances greater than 800 m from the Pisciarelli field, they prevailed over the hydrothermal signal. While hydrothermal CO2 simply mixed with that in the atmospheric background, hydrothermal H2S was affected by oxidation processes. Accordingly, SO2 generated by oxidation of hydrothermal H2S was recognized in the air near the hydrothermal emissions, whereas anthropogenic SO2 was measured at the measurement sites located within the urban area. Noteworthy, in the proximity of a geothermal well, whose drilling was in progress during our field campaign, the H2S concentrations have reached values up to 3 orders of magnitude higher than the urban background, claiming the attention of the local authorities.