The continuous monitoring network of air gas concentration at Vulcano Porto (Aeolian Island, Italy)

Since September 2021, Vulcano Island was affected by a volcanic unrest crisis with an increase in fumarolic activity at La Fossa crater, seismic activity and ground deformations. A contemporary strong increase in the soil CO₂ flux was observed both at the base of La Fossa cone and in some zones of Vulcano Porto settlement. The gas hazard also increased because the massive gas plume (mainly CO₂ and SO₂) descended the crater rim, particularly during night, investing the resident areas. The first civil protection measures adopted to face the health risk at Vulcano Porto included the evacuation of the houses most exposed to the gas and the night time ban of all houses. The long-term gas risk reduction was then achieved with the establishment of a continuous monitoring network for air gas concentration, equipped with an alert system. Also the INGV volcanic gas monitoring network of Vulcano Porto was strengthened. An ad-hoc appointed scientific commission, chaired by DPC and composed by all national and local authorities expert of health, environmental and volcanic hazard, suggested the continuous monitoring of volcanic gases both outdoor and indoor. Two dedicated networks were then built, installed and managed by Arpa Sicilia. The outdoor Arpa network, operational since March 2022 with two mobile labs, since February 2023 includes 6 stations with continuous monitoring of air CO₂, H₂S and SO₂ concentration of which 1 for monitoring also CO, NO₂, PM, C₆H₆, O₃ concentration. The regulatory references of the limit values ​​and alarm thresholds are those provided for by Legislative Decree 13 August 2010, n. 155 - Implementation of Directive 2008/50/EC, as well as the values ​​identified by the WHO global air quality guidelines and in the ISTISAN Report 16/15. The recorded monitoring data show the occasional exceeding of the outdoor thresholds for CO₂, H₂S and SO₂. The pilot indoor network, started in December 2023, is fully operational since June 2024 and currently monitors 33 buildings selected on a voluntary basis. The alert thresholds for indoor monitoring have been identified in agreement with the Regional Department of Civil Protection and the ISS - Istituto Superiore di Sanità. Some indoor stations, located in the most critical sectors of Vulcano Porto (Faraglione and Camping Sicilia) show the recurrence of anomalous CO₂ values (and sporadically of H₂S), especially during night time. The anomalous degassing affecting Vulcano Porto slowly decreased returning in January 2025 to background values. In conclusion, we stress the importance of finally having a risk mitigation tool for volcanic gases at Vulcano Porto, which will allow addressing the gas hazard both in condition of ordinary degassing and in possible future unrest crises. The Vulcano Porto air gas concentration monitoring networks represent one of the few examples present on active volcanoes in Europe together with those of Sao Miguel (Azores, Portugal) and La Palma (Canary Islands, Spain).