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 currently to be regarded as natural sources of air pollutants. Climatic and environmental forcing of large volcanic eruptions are well known, although gases emitted through passive degassing during periods of quiescence or hydrothermal activity can also be highly dangerous for the environment and public health. Based on compositional and isotopic data, a survey on the spatial distribution in air of the main volatile compounds of carbon (CO and CH) and sulfur (HS and SO) emitted from the fumarolic field of Pisciarelli (Campi Flegrei, Pozzuoli, Naples), a hydrothermal area where degassing activity has visibly increased since 2009, was carried out. The main goals of this study were (i) to evaluate the impact on air quality of these natural manifestations and (ii) inquire into the behavior of the selected chemical species once released in air, and their possible use as tracers to distinguish natural and anthropogenic sources. Keeling plot analysis of CO and CH isotopes revealed that the hydrothermal area acts as a net source of CO in air, whilst CH originated mainly from anthropogenic sources. Approaching the urban area, anthropogenic sources of CO increased and, at distances greater than 800 m from the Pisciarelli field, they prevailed over the hydrothermal signal. While hydrothermal CO simply mixed with that in the atmospheric background, HS was possibly affected by oxidation processes. Therefore, SO measured in the air near the hydrothermal emissions had a secondary origin, i.e. generated by oxidation of hydrothermal HS. Anthropogenic SO was recognized only in the furthest measurement site from Pisciarelli. Finally, in the proximity of a geothermal well, whose drilling was in progress during our field campaign, the HS concentrations have reached values up to 3 orders of magnitude higher than the urban background, claiming the attention of the local authorities.