Variation of SO2 and CO2 in the volcanic gas plumes of Santa Ana and San Miguel volcanoes: an overview by real-time measurements

Volcanic degassing plays an important role in a volcano’s behavior. Going from large emissions at craters and fumaroles, to invisible degassing at vents and soil; a volcano releases H2O, CO2, SO2, HCl, HF, H2S, CO, H2, HBr, HI, Hg and noble gases.

SO2 emissions are considered a basic monitoring tool, mainly measured by remote-sensing techniques. The Differential Optical Absorption Spectroscopy (DOAS) is a well-established method currently used to regularly measure volcanic SO2 emission rates with about 80 scanning DOAS operating in 37 volcanoes within the framework of the global "Network for Observation of Volcanic and Atmospheric Change" (NOVAC) (Galle et al., 2010). Typically, SO2fluxes are often combined with in-situ gas measurements of SO2 and other volatiles (CO2, H2S), to evaluate the degassing regime. In-situ sampling can be made by collecting the gases directly in evacuated flasks or solution-filled bottles (alkaline traps), or by sampling with a multi-sensor instrument (MultiGAS) that enables real-time measurements of several gases at once (Aiuppa et al. 2005b; Shinohara 2005, Roberts et al. 2017).

Santa Ana and San Miguel are the most active volcanoes in El Salvador, with an average SO2 emission rate of 220 and 326 t/d, respectively during 2018. Both volcanoes arise along the Central American Volcanic Arc – CAVA, where the magmatism, fundamentally basaltic, is related to the convergence of the Caribbean Plate and the subducting Cocos Plate (Leeman and Carr 1995). Also, Santa Ana and San Miguel are part of the NOVAC group since 2008 with just a few published gas data (Rodriguez et al. 2004, Cartagena et al. 2004, Olmos et al. 2007, Colvin et al. 2013, Laiolo et al. 2017). The most recent studies were performed by Granieri et al. 2015 and Hasselle et al. 2019. The first, reported CO2/SO2, HCl/SO2 and HF/SO2 mass ratios (0.95, 0.13 and 0.016, respectively) measured at San Miguel volcano in early 2014; while the second, presented CO2/SO2, H2S/SO2 and H2O/SO2 ratios (<3-37.9, 0.03-0.1 and 32-205, respectively), measured in 2017-2018 at Santa Ana’s crater lake and rim.

In this study, we present an SO2 long-time data series (2008-2018) for San Miguel and Santa Ana obtained from the DOAS stations of each volcano, and complement with data collected during regular monitoring (2018-2020) and field campaigns in El Salvador (2019 and 2020) by means of MultiGAS devices. The aim of the study is to extend the characterization of these two volcanoes in El Salvador and the establishment of SO2 and CO2 baselines and inventories for them.