Rapid sensing of volcanic SO₂ fluxes using a dual ultraviolet camera system: new techniques and measurements at Southern Italian volcanoes

Volatiles carry crucial information on pre- to sin-eruptive processes at active volcanoes. Measurements of gas emission rates (crater plumes, fumaroles, diffuse soil degassing) therefore improve our understanding of degassing processes and subsurface magmatic and hydrothermal conditions, and contribute to eruption forecasting. Recent technological developments in spectroscopy have allowed, over the last 30 years, the remote sensing of magmatic volatile emissions from quiescent and erupting degassing volcanoes. These data-sets have contributed to discovering cyclic gas flux components due to periodic magma supply and replenishment in magma storage zones, and/or timescales of magma migration (and degassing) within the feeding conduit systems of volcanoes (chapter 2). In spite of these relevant achievements, a number of magmatic degassing processes have remained elusive to measure, as they occur at a faster rate than the time resolution of most available spectroscopic techniques. In this study, I take advantage of a novel technique - the UV camera (chapter 3) - to image SO2 emissions from the Italian volcanoes with improved high temporal resolution. The UV camera heralds the much awaited prospect of capturing transient (≤ tens of seconds) volcanic gas-driven phenomena, such as Strombolian explosions and puffing. Here, this technique has been updated to a new configuration (dual-camera system), which combines higher temporal resolution (0.5-1.2 Hz) and improved accuracy relative to the single-camera setup. During the first year of this PhD, the methodology has been extensively tested and improved, whilst developing a user-friendly control software (Vulcamera) and a calibration technique (in tandem DOAS-SO2 quartz cells calibration), which simplify instrument deployment, acquisition and data analysis (chapter 4). The results of the volcano applications of the UV camera are described in chapter 5. A first application (chapter 5.2) was focused on SO2 gas flux measurements at individual fumaroles from the La Fossa crater (Vulcano island, Italy) fumarolic field. There, the dual- UV camera technique allowed the simultaneous imaging of multiple-source emissions, discriminating between SO2 contributions from the four main fumarolic areas. The UV camera-derived individual fumarole SO2 fluxes have been used in tandem with MultiGASderived gas/SO2 molar ratios to accurately assess CO2, H2O, and H2S fluxes. Results highlight a factor ~2 increase in CO2 and H2O degassing during the La Fossa crater degassing/heating unrest event of November-December 2009. Bubbles nucleation (birth), coalescence (growth), outgassing and fragmentation (death), are stages of volatile's life within the magma. Our understanding of these processes mainly comes from modelling and textural studies. In this work, I have attempted to retrace part of the gas bubbles' life by measuring - at high rate - SO2 outgassing rates from two openvent volcanoes: Stromboli and Etna. On Stromboli (Chapter 5.3), the UV camera-derived data allowed the first simultaneous estimate of the SO2 flux contribution from the three main forms of degassing at Stromboli (passive degassing, 84-92 %; explosive degassing, 5-8 %; puffing, 3-8 %). The obtained high frequency SO2 flux time-series also revealed the existence of a periodic SO2 degassing pattern over timescales of minutes, modulated by rhythmic strombolian explosions. Also I report on systematic in tandem UV camera-geophysical observations. Among the key results, I provide experimental evidence for a positive correlation between seismic (very-long period; VLP) thermal, and gas (eruptive SO2 mass) signals irradiated by individual Strombolian explosions. During each strombolian event, onset of the SO2 flux emission systematically coincides with deflation of the conduit upon gas slug bursting during the explosion. At Mount Etna (Chapter 5.4), degassing mechanisms and rates have been studied during two field campaigns on Pizzi Dineri (northern rim of Valle del Bove), from which a clear view of the pulsate gas emissions (gas puffing) from the North-east crater was available. The >10 hour acquired SO2 flux time series highlighted a periodic degassing behaviour for this vent, with characteristic periods in the 60-250 s range. This allows deriving new constraints on model gas bubble distribution in a magmatic conduit. The data obtained here support a process of gas packaging into trains of discrete bubble-rich layers. This, coupled with time variations in ascent rate of individual gas bubble layers, may well account for the time-dependent periodicity of observed volcanic SO2 flux emissions.