CO2 soil flux at Vulcano (Italy): comparison between active and passive methods

Carbon dioxide soil flux has been used for many years to monitor Italian active volcanoes and both the active Dynamic Concentration (DCM) and the passive Accumulation Chamber (ACM) methods are employed. These two methods have been compared by means of 218 simultaneous flux measurements carried out in the La Fossa area of Vulcano Island, where a large variation of CO2 soil release occurs. Results indicate that DCM overestimates CO2 flux and is proportional to it only in high flux zones (flux higher than 100 gm−2 day−1). Using ACM fluxes and the Stefan–Maxwell equation, the measured CO2/depth curves in the soil could be reproduced. In high flux points CO2 is transported mostly by viscous flow up to a very shallow depth and then by diffusive flow, which is the dominant gas transport mechanism in low flux points. Carbon dioxide soil flux values are controlled by proximity to active gas releasing fractures, by changes in the barometric pressure and by variations in soil permeability.