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Bromine and chlorine chemistry in volcanic plumes

Subject Area Mineralogy, Petrology and Geochemistry
Term from 2010 to 2015
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 183111571
 
Final Report Year 2016

Final Report Abstract

During the DFG projects the chemistry of bromine and chlorine in volcanic plumes was investigated as well as steps forwards were undertaken in the understanding if BrO/SO2 ratios could be used as an additional precursor for volcanic eruption forecast. Halogens released to the atmosphere by volcanoes are thought predominantly to be degassed as hydrogen halides, which should be thermodynamically favored over other halogen species as equilibrium model data suggest. Nevertheless, there is conversion to more reactive halogen species such as molecular halogens, halogen atoms and halogen oxides once the plume begins to mix with ambient air and is still at high temperature (the so called "source region"). As a result of the project we know now that reactive bromine as well as chlorine chemistry is happening in volcanic plumes even faster than assumed before the start of the project. At Mt Etna the conversion of the emitted bromine and chlorine to the max. percentage of BrO and OClO on total volcanic halogen degassing amount is reached on a time scale of < 3 minutes, which corresponds to less than 2 km from the source by considering a wind speed of 10 m/s. The high amount of data collected validate the earlier assumption that the BrO/SO2 ratio, once reached its maxima, remain constant over a plume age up to > 30 minutes. This is an important finding to make it a feasible volcanic activity precursor. Measurements of volcanic OClO are still rare and most reports are merely based on a few data points. Our results provide new, very detailed insights into the chemical evolution of reactive chlorine species in the volcanic plume. For the first time, the formation of OClO could be observed in a volcanic plume. Furthermore, mean concentrations of the order of several hundred ppt (ClO and OClO) up to several ppb (BrO) could be estimated much higher than in other environments and upper limits for IO, OIO and OBrO were determined. Keeping in mind that most of the reactive Cl and reactive Br originates from the initially emitted HCl and HBr, our findings strongly suggest that the oxidation of chloride is much weaker compared to the bromide equivalent, making remote sensing determination of HCl downwind of a volcano still a valid tracer to study variations of volcanic chlorine emissions. This is mainly due to the fact that any potential Cl release mechanisms are likely less efficient compared to bromine. Moreover, once formed, the Cl radicals in the plume will rapidly react with CH4, which may even cause a significant depletion of CH4 in the plume. We estimated CH4 lifetimes reduced to 47 days down to 14 h, due to the influence of volcanic chlorine. These lifetimes are more than 2 orders of magnitude shorter than the average atmospheric lifetime of CH4. The variations of BrO/SO2 ratios simultaneously with observations of volcanic activity changes have been now confirmed on several volcanoes since the beginning of the project. There are also further indications that BrO/SO2 ratio variation behave somehow similar to the variations in the CO2/SO2. Indicating that bromine might indeed a sign of relatively deep degassing, but this has still to be proven. Some of the work initiated and started during the project is still continuing and will lead to more knowledge in the future.

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