On long-term variations in the BrO/SO2 molar ratios in volcanic gas plumes
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Dinger, Florian
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Abstract
This dissertation reports, analyses, and compares multi-year time series of the sulphur dioxide (SO2) and bromine monoxide (BrO) slant column densities and the BrO/SO2 molar ratios in the gas plumes emitted by 6 volcanoes in Central and South America (Nevado del Ruiz, Cotopaxi, Masaya, Tungurahua, Galeras, Villarrica). The time series have been obtained from assessing remote-sensing data from the Network for Observation of Volcanic and Atmospheric Change (NOVAC) with differential optical absorption spectroscopy (DOAS). This data set exceeds 10 years of continuous data and thus provides not only a unique empirical foundation for studies on the atmospheric bromine chemistry but allows also for the first time for an investigation of long-term variations in the chemical composition in volcanic gas plumes. The observed long-term variations in the BrO/SO2 time series range from sudden changes, linear long-term trends, annual periodicities, and a fortnightly periodicity at Cotopaxi.
For Masaya, Nevado del Ruiz, and Cotopaxi, the BrO/SO2 time series have been compared with the meteorological conditions obtained from ECMWF simulation data. A strong anti-correlation between the BrO/SO2 molar ratios and the specific humidity has been observed at Masaya (coefficient of -42%, average humidity of 14 g/kg). No such correlation has been observed for Nevado del Ruiz or Cotopaxi (both with an average humidity of 4 g/kg). At Nevado del Ruiz, an asymptotic increase of the mean BrO/SO2 molar ratios depending on the atmospheric age of the volcanic gas plumes has been observed; the mean BrO/SO2 molar ratios there reached an equilibrium around 9 min after the release to the atmosphere and remained at this level for at least 60 min. In contrast, the mean BrO/SO2 molar ratios were the same for all observed plume ages at Masaya (2-20 min) and Cotopaxi (9-60 min).
A mean long-term BrO/SO2 molar ratio of about 4x10^-5 has been observed for 4 of the 5 reported low-latitude volcanoes. Decreased BrO/SO2 molar ratios correlated with enhanced volcanic activity on several time scales, for example, persistently lower BrO/SO2 molar ratios during multi-month phases of high activity at Nevado del Ruiz and Tungurahua and lower BrO/SO2 molar ratios at the starts of a multi-year eruption period when compared to the long-term means.
The Earth tides have been identified as the most likely cause for the fortnightly periodicity observed at Cotopaxi (correlation coefficient of 50%). A quantitative model has been developed to investigate a possible causal link between the Earth tides and volcanic degassing. The model describes the impact of the tide-induced gravity variations on the bubble coalescence rate in magmatic melt and suggests a tide-induced enhancement by up to a multiple of 10%. Another novel model investigates under which conditions the mechanical interaction between gas bubbles and tilted crystals in magmatic melt could cause an erection of the crystals or an enhanced bubble coalescence rate.