Bromine monoxide / sulphur dioxide ratios in relation to volcanological observations at Mt. Etna 2006–2009

Abstract

Abstract. Over a 3-yr period, from 2006 to 2009, frequent scattered sunlight DOAS measurements were conducted at Mt. Etna at a distance of around 6 km downwind from the summit craters. During the same period and in addition to these measurements, volcanic observations were made by regularly visiting various parts of Mt. Etna. Here, results from these measurements and observations are presented and their relation is discussed. The focus of the investigation is the bromine monoxide/sulphur dioxide (BrO / SO2) ratio, and its variability in relation to volcanic processes. That the halogen/sulphur ratio can serve as a precursor or indicator for the onset of eruptive activity was already proposed by earlier works (e.g. Noguchi and Kamiya 1963; Menyailov, 1975; Pennisi and Cloarec, 1998; Aiuppa et al., 2002). However, there is still a limited understanding today because of the complexity with which halogens are released, depending on magma composition and degassing conditions. Our understanding of these processes is far from complete, for example of the rate and mechanism of bubble nucleation, growth and ascent in silicate melts (Carroll and Holloway, 1994), the halogen vapour-melt partitioning and the volatile diffusivity in the melt (Aiuppa et al., 2009). With this study we aim to add one more piece to the puzzle of what halogen/sulphur ratios might tell about volcanic activities. Our data set shows an increase of the BrO / SO2 ratio several weeks prior to an eruption, followed by a decline before and during the initial phase of eruptive activities. Towards the end of activity or shortly thereafter, the ratio increases to baseline values again and remains more or less constant during quiet phases. To explain the observed evolution of the BrO / SO2 ratio, a first empirical model is proposed. This model suggests that bromine, unlike chlorine and fluorine, is less soluble in the magmatic melt than sulphur. By using the DOAS method to determine SO2, we actually observe most of the emitted sulphur of Mt.~Etna. Regarding bromine, however, we are aware that by determining only the bromine monoxide (BrO) radical we might just observe a small or even a variable fraction of the total emitted bromine, which is most probable originally in the form of HBr. Therefore, we present first studies to justify the assumption that, despite the disadvantage just mentioned, the BrO / SO2 ratio can nevertheless serve as a new parameter to indicate the state of a volcano, when measurements are conducted under certain, but rather convenient, conditions.

Implementation of electrochemical, optical and denuder-based sensors and sampling techniques on UAV for volcanic gas measurements: examples from Masaya, Turrialba and Stromboli volcanoes

“…The halogen chemistry is of especially great interest as BrO / SO 2 ratios in volcanic plumes are readily measurable by remote sensing UV spectrometry (e.g., Bobrowski et al, 2003;Lübcke et al, 2014) and have been discussed in recent years as another potential precursory observable parameter for volcanic activity changes. Although several studies observed decreases in the BrO / SO 2 ratio before eruptive phases (e.g., Lübcke et al, 2014) and lower ratios during periods of continuous activity (Bobrowski and Giuffrida, 2012), it is not yet clear whether magma-gas partitioning of bromine occurs before or after sulfur during the pressure drop associated with magma ascents (Dinger et al, 2018). Furthermore, BrO is not a directly emitted species; rather it is the product of complex heterogeneous chemistry in the volcanic plume involving reactions with magmatic gases with entrained air (e.g., Gerlach, 2004;Bobrowski et al, 2007).…”

“…Changes in the magma degassing behavior and/or the hydrothermal systems beneath volcanoes generally influence the gas composition and gas fluxes. Measuring the emitted gas composition can provide crucial information on understanding subsurface processes related to activity changes (e.g., Allard et al, 1991;Aiuppa et al, 2007;Bobrowski and Giuffrida, 2012;de Moor et al, 2016a;Liotta et al, 2017) and help to estimate fluxes of the geological carbon cycle (e.g., Burton et al, 2013;Mason et al, 2017) and tectonic processes controlling volcanic degassing (e.g., Aiuppa et al, 2017;de Moor et al 2017). In the field of volcanic monitoring, the observation of gas composition changes has become an important tool for detecting precursory processes for volcanic eruptions.…”

Periodicity in the BrO/SO<sub>2</sub> molar ratios in the volcanic gas plume of Cotopaxi and its correlation with the Earth tides during the eruption in 2015

“…The conversion rate or even the stationary state ratio between HBr and BrO may depend on the plume composition and the atmospheric conditions (meteorology) (Roberts et al, 2014) and thus on parameters like solar irradiance, air temperature, air pressure, or relative hu-35 midity. However, Bobrowski and Giuffrida (2012) have not found a correlation between BrO and the relative humidity or the wind speed, which poses the question if the bromine explosion in volcanic gas plumes is indeed significantly depending on the meteorological conditions or not.…”

“…Accordingly, even under the assumption that the HBr/SO 2 degassing ratio is purely pressure-dependent no quantitative deduction of the pressure change could be made based on BrO/SO 2 molar ratios. Further, it was even not yet possible to derive the sign of this correlation indirectly by empirical studies of BrO/SO 2 molar ratios simultaneously to pressure changes 15 (Bobrowski and Giuffrida (2012), Lübcke et al (2014)) because of the still limited data sets available. In the absence of an explicit interpretation of the BrO/SO 2 molar ratio, at least the reasoning that a change in BrO/SO 2 molar ratios is probably caused by a significant change in magmatic conditions appears to be nevertheless a reasonable hypothesis.…”

“…Because the pressure-dependency of the volatile solubility is different for each particular volatile species, among others, halogen to sulphur molar ratios in volcanic gas plumes are discussed as accessible and suitable proxies for the magma 20 pressure or the degassing depth, e.g. Edmonds et al (2001) for chlorine to sulphur ratio, Bobrowski and Giuffrida (2012) for bromine to sulphur ratio.…”