Quantifying volcanic gas emission rates from infrasound and SO2 cameras: potentials, limitations, and volcanological implications.

Volcanic degassing, a persistent manifestation of active volcanoes, provides crucial information on the dynamics of the magmatic feeding systems, and allows identifying the phases of volcanic unrest in the runup to volcanic eruptions. While thus determining volcanic degassing rates is a central topic in modern Volcanology, direct volcanic gas flux observations by classic spectroscopic techniques are challenged by (i) the need of adequate illumination (by sunlight) and clear weather conditions (ii) difficulties in robustly estimating plume speed velocity and transport direction, and (iii) a variety of optical and radiative transfer issues. Because of these, volcanic gas flux records are often sparse and incomplete, and affected by intrinsic noise that may prevent from fully resolving the gas emission changes associated with changing volcanic activity. To overcome such limitations, measuring the infrasound produced by the expansion of over-pressurized volcanic gas in the atmosphere, using infrasonic arrays, offers as a promising alternative/complementary tool to quantify and locate degassing at active volcanoes. Here, we report on 2-year long (April 2017—March 2019) period of combined measurements of the SO₂ flux and of volcano-acoustic emissions produced by regular mild persistent strombolian activity and passive degassing of Stromboli Volcano (Sicily, Italy). These were obtained by a permanent monitoring SO₂ camera and a five-element short-aperture (~300 m) infrasonic array. Our results highlight substantial temporal changes in degassing activity, that reflect the recurrent episodes of activations/inactivation of multiple distinct degassing sources within the crater area, as coherently tracked by SO₂ and infrasound together. A simple waveform modeling of the infrasonic record, assuming a monopole acoustical source, suggests that infrasonic degassing, comprising of explosive events and continuous puffing activity, dominates the total persistent degassing budget as tracked by the SO₂ camera.