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

Abstract

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 (CO₂ and CH₄) and sulfur (H₂S and SO₂) 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 CO₂ and CH₄ isotopes revealed that the hydrothermal area acts as a net source of CO₂ in air, whilst CH₄ originated mainly from anthropogenic sources. Approaching the urban area, anthropogenic sources of CO₂ 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, H₂S was possibly affected by oxidation processes. Therefore, SO₂ measured in the air near the hydrothermal emissions had a secondary origin, i.e. generated by oxidation of hydrothermal H₂S. Anthropogenic SO₂ 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 H₂S concentrations have reached values up to 3 orders of magnitude higher than the urban background, claiming the attention of the local authorities.

Keywords: Air quality; Carbon and sulfur volatile compounds; Carbon isotopes; Hydrothermal systems; Natural sources of pollutants..