Model Validation of Passive Gas Dispersion: Examples from La Solfatara (Campi Flegrei, Italy) and Caldeiras Da Ribeira Grande (São Miguel Island, Azores)

The emission of volcanic gases can occur during both eruptive and quiescent stages of volcanic activity, affecting air quality in the surrounding areas and threatening human health when the concentrations exceed species-specific thresholds. In this regard, quantitative studies of model validation are essential before applying a simulator for probabilistic volcanic hazard assessment. Here, we provide a model validation aimed at testing the accuracy in providing realistic values of CO2 concentration at two active volcanic sites affected by persistent passive gas dispersion: La Solfatara (a maar crater within Campi Flegrei caldera, Italy) and Caldeiras da Ribeira Grande located in the north flank of Fogo volcano (São Miguel Island, Azores). We used published and original CO2 flux data as input for numerical simulations run through VIGIL, an open-source workflow for parallel simulations and probabilistic output using two Eulerian models, which account for the passive and gravity-driven gas transport, respectively. At Solfatara, we compared a 1-month-long simulation during June 2020 with CO2 concentration acquired by the INGV measurement station at 4 m from the ground in a selected point close to Pisciarelli vent: Our results showed a good correlation between the daily simulated and observed averages of CO2 concentrations. At Caldeiras da Ribeira Grande, we quantified the CO2 concentration at 43 tracking points, each referring to a specific acquisition (in space and time) during 13 selected days in July 2021. The comparison between the 1-month-long simulation and the observed data provided acceptable accordance. In both cases, we noted that the daily averaged concentrations provided by the model do not exceed the gas hazardous threshold limits. However, for shorter timescales (hours), a higher data acquisition rate is needed for future investigation.