SO<sub>2</sub> and BrO emissions of Masaya volcano from 2014–2020

Abstract

Abstract. Masaya volcano (Nicaragua, 12.0° N, 86.2° W, 635 m a.s.l.) is one of the few volcanoes hosting a lava lake, today. We present continuous time series of SO2 emission fluxes and BrO / SO2 molar ratios in the gas plume of Masaya from March 2014 to March 2020. This study has two foci: (1) discussing the state of the art of long-term SO2 emission flux monitoring on the example of Masaya and (2) the provision and discussion of a continuous dataset on volcanic gas data unique in its temporal coverage, which poses a major extension of the empirical data base for studies on the volcanologic as well as atmospheric bromine chemistry. Our SO2 emission flux retrieval is based on a comprehensive investigation of various aspects of the spectroscopic retrievals, the wind conditions, and the plume height. Our retrieved SO2 emission fluxes are on average a factor of 1.4 larger than former estimates based on the same data. We furthermore observed a correlation between the SO2 emission fluxes and the wind speed when several of our retrieval extensions are not applied. We make plausible that such a correlation is not expected and present a partial correction of this artefact via applying dynamic estimates for the plume height as a function of the wind speed (resulting in a vanishing correlation for wind speeds larger than 10 m/s). Our empirical data set covers the three time periods (1) before the lava lake elevation, (2) period of high lava lake activity (December 2015–May 2018), (3) after the period of high lava lake activity. For these three time periods, we report average SO2 emission fluxes of 1000 ± 200 t d−1, 1000 ± 300 t d−1, and 700 ± 200 t d−1 and average BrO / SO2 molar ratios of (2.9 ± 1.5) × 10−5, (4.8 ± 1 : 9) ×10−5, and (5.5 ± 2–6) × 10−5. These variations indicate that the two gas proxies provide complementary information: the BrO / SO2 molar ratios were susceptible in particular for the transition between the two former periods while the SO2 emission fluxes were in particular susceptible for the transition between the two latter time periods. We observed an extremely significant annual cyclicity for the BrO / SO2 molar ratios (amplitudes between 1–4–2–6 × 10−5) with a weak semi-annual modulation. We suggest that this cyclicity might be a manifestation of meteorological cycles. We found an anti-correlation between the BrO / SO2 molar ratios and the atmospheric water concentration (correlation coefficient of −47 %) but in contrast to that neither a correlation with the ozone mixing ratio (+21 %) nor systematic dependencies between the BrO / SO2 molar ratios and the atmospheric plume age for an age range of 2–20 min after the release from the volcanic edifice. The two latter observations indicate an early stop of the autocatalytic partial transformation of bromide Br− solved in aerosol particles to atmospheric BrO. Further patterns in the BrO / SO2 time series were (1) a step increase by 0.7 × 10−5 in late 2015, (2) a linear trend of 1.2 × 10−5 per year from December 2015 to March 2018, and (3) a linear trend of −0.8 × 10−5 per year from June 2018 to March 2020. The step increase in 2015 coincided with the 55 elevation of the lava lake and was thus most likely caused by a change in the magmatic system. The linear trend between late 2015 and early 2018 may indicate the evolution of the magmatic gas phase during the ascent of juvenile gas-rich magma whereas the linear trend from June 2018 on may indicate a decreasing bromine abundance in the magma.

Wind Speed as a Dominant Source of Periodicities in Reported Emission Rates of Volcanic SO2

“…Gas accumulation in the crater has been proposed to account for correlations between wind speed and SO 2 emission rate at Masaya volcano (Dinger et al., 2021), but it seems unlikely that it could account for the common periodicities identified here, owing to the range of volcano types and surface activity. Despite explosive emissions creating poor measurement conditions, SO 2 emission rates used in this study span episodes of both explosive activity and passive degassing (Figure A1).…”

“…For measurements conducted under ideal conditions, it is well‐known that plume speeds are the main source of error in SO 2 emission rates (Arellano et al., 2021) and correlations have been observed between SO 2 emission rate and wind speed at several volcanoes (e.g., Dinger et al., 2021). The fact that these correlations translate into periodicities implies a systematic error in the calculation of SO 2 emission rates.…”

“…This range in reported periodicities may reflect the behavior and state of the system, but also depends on the instrumental techniques and therefore sampling rate used to study volcanic gases. Periodicities have been observed in time series of diffuse gas emanations of radon (Aumento, 2002; Cigolini et al., 2009) and CO 2 (Viveiros et al., 2015), as well as in the relative concentrations of plume‐gas species recorded by electrochemical sensors (Moussallam et al., 2017) and ground‐based remote sensing techniques, including Fourier‐Transform Infrared (FTIR; Allard et al., 2016) and Differential Optical Absorption Spectroscopy (DOAS; Dinger et al., 2018, 2021; Warnach et al., 2019). The emission rate of SO 2 as a plume‐gas is an important parameter for forecasting activity and monitoring ongoing eruptions for the world's most active volcanoes.…”

Quantifying BrO and SO2 distributions in volcanic plumes—Recent advances in imaging Fabry-Pérot interferometer correlation spectroscopy

“…The BrO/SO 2 ratio is commonly observed in many volcanic plumes, for instance by the NOVAC network (e.g., Luebcke et al, 2014;Dinger et al, 2021). Since SO 2 can serve as a quasiconservative tracer for dilution this ratio can give important insights in plume chemistry (von Glasow et al, 2009) but can be also related to volcanic activity (e.g., Warnach et al, 2019).…”

Combined direct-sun ultraviolet and infrared spectroscopies at Popocatépetl volcano (Mexico)

“…Among the halogen secondary species, BrO was measured for the first time in a volcanic plume at Soufrière Hills (Montserrat) by Bobrowski et al (2003), and then at a number of other volcanoes (Oppenheimer et al, 2006;Bobrowski and Platt, 2007;Kern et al, 2009;Heue et al, 2011;Rix et al, 2012;Donovan et al, 2014;Gliß et al, 2015;Dinger et al, 2018;Warnach et al, 2019;Dinger et al, 2021) including Popocatépetl (Boichu et al, 2011;Bobrowski and Giuffrida, 2012;Platt and Bobrowski, 2015;Fickel and Delgado Granados, 2017). Recent models succeeded in reproducing some observations of plume compositions at different plume ages (Rüdiger et al, 2021) and provide valuable information about the lifetimes and conversion rates of the primary (HCl, HF, HBr) and secondary (BrO, ClO, OClO, BrOH, Br 2 , BrCl, BrONO 2 ) halogen species.…”