Reactive bromine chemistry in Mount Etna’s volcanic plume: theinfluence of total Br, high-temperature processing, aerosol loadingand plume–air mixing

Abstract : Volcanic emissions present a source of reactivehalogens to the troposphere, through rapid plume chemistrythat converts the emitted HBr to more reactive forms such asBrO. The nature of this process is poorly quantified, yet is ofinterest in order to understand volcanic impacts on the troposphere,and infer volcanic activity from volcanic gas measurements(i.e. BrO / SO2 ratios). Recent observations fromEtna report an initial increase and subsequent plateau or declinein BrO / SO2 ratios with distance downwind.We present daytime PlumeChem model simulations thatreproduce and explain the reported trend in BrO / SO2 atEtna including the initial rise and subsequent plateau. Suitesof model simulations also investigate the influences of volcanicaerosol loading, bromine emission, and plume–air mixingrate on the downwind plume chemistry. Emitted volcanicHBr is converted into reactive bromine by autocatalyticbromine chemistry cycles whose onset is accelerated by themodel high-temperature initialisation. These rapid chemistrycycles also impact the reactive bromine speciation throughinter-conversion of Br, Br2, BrO, BrONO2, BrCl, HOBr.We predict a new evolution of Br speciation in the plume.BrO, Br2, Br and HBr are the main plume species near downwindwhilst BrO and HOBr are present further downwind(where BrONO2 and BrCl also make up a minor fraction).BrNO2 is predicted to be only a relatively minor plume component.The initial rise in BrO / SO2 occurs as ozone is entrainedinto the plume whose reaction with Br promotes net formationof BrO. Aerosol has a modest impact on BrO / SO2 neardownwind (<6 km, 10 min) at the relatively high loadingsconsidered. The subsequent decline in BrO / SO2 occursas entrainment of oxidants HO2 and NO2 promotes net formationof HOBr and BrONO2, whilst the plume dispersiondilutes volcanic aerosol so slows the heterogeneous loss ratesof these species. A higher volcanic aerosol loading enhancesBrO / SO2 in the (> 6 km) downwind plume.Simulations assuming low/medium and high Etna bromineemissions scenarios show that the bromine emission has agreater influence on BrO / SO2 further downwind and a modestimpact near downwind, and show either complete or partialconversion of HBr into reactive bromine, respectively,yielding BrO contents that reach up to 50 or 20% oftotal bromine (over a timescale of a few 10 s of minutes).Plume–air mixing non-linearly impacts the downwindBrO / SO2, as shown by simulations with varying plumedispersion, wind speed and volcanic emission flux. Greatervolcanic emission flux leads to lower BrO / SO2 ratiosnear downwind, but also delays the subsequent decline inBrO / SO2, and thus yields higher BrO / SO2 ratios furtherdownwind. We highlight the important role of plume chemistrymodels for the interpretation of observed changes inBrO / SO2 during/prior to volcanic eruptions, as well as forquantifying volcanic plume impacts on atmospheric chemistry.Simulated plume impacts include ozone, HOx and NOxdepletion, the latter converted into HNO3. Partial recovery ofozone occurs with distance downwind, although cumulativeozone loss is ongoing over the 3 h simulations.
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Submitted on : Monday, December 15, 2014 - 10:47:43 AM
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T.J. Roberts, R.S. Martin, L. Jourdain. Reactive bromine chemistry in Mount Etna’s volcanic plume: theinfluence of total Br, high-temperature processing, aerosol loadingand plume–air mixing. Atmospheric Chemistry and Physics, European Geosciences Union, 2014, 14, pp.11201-11219. ⟨insu-01095138⟩

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