Abstract
Abstract. Masaya (Nicaragua, 12.0∘ N, 86.2∘ W; 635 m a.s.l.) is one of the few volcanoes hosting a lava lake, today. This study has two foci: (1) discussing the state of the art of long-term SO2 emission flux monitoring with the example of Masaya and (2) the provision and discussion of a continuous data set on volcanic gas data with a large temporal coverage, which is a major extension of the empirical database for studies in volcanology as well as atmospheric bromine chemistry. We present time series of SO2 emission fluxes and BrO/SO2 molar ratios in the gas plume of Masaya from March 2014 to March 2020 – covering the three time periods (1) before the lava lake appearance, (2) a period of high lava lake activity (November 2015 to May 2018), and (3) after the period of high lava lake activity. For these three time periods, we report average SO2 emission fluxes of (1000±200), (1000±300), 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. Our SO2 emission flux retrieval is based on a comprehensive investigation of various aspects of spectroscopic retrievals, the wind conditions, and the plume height. We observed a correlation between the SO2 emission fluxes and the wind speed in the raw data. We present a partial correction of this artefact by applying dynamic estimates for the plume height as a function of the wind speed. Our retrieved SO2 emission fluxes are on average a factor of 1.4 larger than former estimates based on the same data. Further, we observed different patterns in the BrO/SO2 time series: (1) an annual cyclicity with amplitudes between 1.4 and 2.5×10-5 and a weak semi-annual modulation, (2) a step increase by 0.7×10-5 in late 2015, (3) a linear trend of 1.4×10-5 per year from November 2015 to March 2018, and (4) a linear trend of -0.8×10-5 per year from June 2018 to March 2020. The step increase in 2015 coincided with the lava lake appearance and was thus most likely caused by a change in the magmatic system. We suggest that the 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 −0.47) but, in contrast to that, neither a correlation with the ozone mixing ratio (+0.21) 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 transformation of bromide Br− solved in aerosol particles to atmospheric BrO.
Highlights
We derived semi-continuous time series of the differential slant column densities of SO2 and bromine monoxide (BrO) via multi-axis differential optical absorption spectroscopy (MAX-DOAS) applied to UV spectra of the diffuse solar irradiation recorded by the NOVAC stations (e.g. Edmonds et al, 2003; Galle et al, 2003; Bobrowski et al, 2003)
(3) The SO2 emission fluxes were only relatively weakly correlated with the daily average of the SO2 differential slant column densities (dSCDs) (+0.35). This can be explained by the two processes which presumably predominantly control the variability in the SO2 dSCDs: on the one hand, strong long-term variations in the SO2 emission flux should manifest proportionally in the long-term means of the SO2 dSCDs, but on the other hand, the variability of the SO2 dSCDs in the plume centre is significantly controlled by the horizontal plume dispersion and the wind speed
This study contributes to three independent fields of research: (1) a comprehensive discussion of a reliable retrieval of SO2 emission fluxes from ground-based remote sensing data, (2) a data set for the bromine chemistry in volcanic gas plumes with extraordinary temporal coverage and resolution, and (3) an investigation of the BrO/SO2 molar ratio as a proxy for magmatic processes
Summary
Volcanic gas emissions consist predominantly of water (H2O), followed in abundance by carbon dioxide (CO2) and sulfur dioxide (SO2) as well as by a large number of trace gases such as halogen halides (Giggenbach, 1996; Aiuppa, 2009; Oppenheimer et al, 2014; Bobrowski and Platt, 2015).Monitoring the magnitude or chemical composition of volcanic gas emissions can help to forecast volcanic eruptions (e.g. Carroll and Holloway, 1994; Oppenheimer et al, 2014). Volcanic gas emissions consist predominantly of water (H2O), followed in abundance by carbon dioxide (CO2) and sulfur dioxide (SO2) as well as by a large number of trace gases such as halogen halides (Giggenbach, 1996; Aiuppa, 2009; Oppenheimer et al, 2014; Bobrowski and Platt, 2015). SO2 emission fluxes, carbon-to-sulfur ratios, and halogen-tosulfur ratios turned out to be powerful tools enabling the detection of events of magma influx at depth and, respectively, the arrival of magma in shallow zones of the magmatic system F. Dinger et al.: SO2 and bromine emissions of Masaya volcano from 2014 to 2020 et al, 2005; Aiuppa et al, 2005; Burton et al, 2007; Bobrowski and Giuffrida, 2012). Monitoring of volcanic gas emissions allows a quantification of the global volcanic volatile emission fluxes (e.g. Carn et al, 2017; Fischer et al, 2019): it is an important tool for the validation of satellite data (e.g. Theys et al, 2019), provides empirical data on the impact of volcanoes on the chemistry in the local atmosphere (e.g. Bobrowski and Platt, 2015), and is one of the rare possibilities to gain information about the interior of the Earth (e.g. Oppenheimer et al, 2014, https://deepcarbon.net/, last access: 9 June 2021)
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