Reactive bromine chemistry in Mt. Etna's volcanic plume: the influence of total Br, high temperature processing, aerosol loading and plume-air mixing
Authors: Roberts T. J. , Martin R. S. , Jourdain L. 
Published: March 2014

Abstract. Volcanic emissions present a source of reactive halogens to the troposphere, through rapid plume chemistry that converts the emitted HBr to more reactive forms such as BrO. The nature of this process is poorly quantified, yet is of interest to understand volcanic impacts on the troposphere, and infer volcanic activity from volcanic gas measurements (i.e. BrO / SO2 ratios). Recent observations from Etna report an initial increase and subsequent plateau or decline in BrO / SO2 ratios with distance downwind. We present daytime PlumeChem model simulations that reproduce and explain the reported trend in BrO / SO2 at Etna including the initial rise and subsequent plateau. Through suites of model simulations we also investigate the influences of volcanic aerosol loading, bromine emission, and plume-air mixing rate on the downwind plume chemistry. Emitted volcanic HBr is converted into reactive bromine by autocatalytic bromine chemistry cycles whose onset is accelerated by the model high-temperature initialisation. These rapid chemistry cycles also impact the reactive bromine speciation through inter-conversion of Br, Br2, BrO, BrONO2, BrCl, HOBr. Formation of BrNO2 is also discussed. We predict a new evolution of Br-speciation in the plume, with BrO, Br2, Br and HBr as the main plume species in the near downwind plume whilst BrO, and HOBr are present in significant quantities further downwind (where BrONO2 and BrCl also make up a minor fraction). The initial rise in BrO / SO2 occurs as ozone is entrained into the plume whose reaction with Br promotes net formation of BrO. Aerosol has a modest impact on BrO / SO2 near-downwind (< 6 km) at the relatively high loadings considered. The subsequent decline in BrO / SO2 occurs as entrainment of oxidants HO2 and NO2 promotes net formation of HOBr and BrONO2, whilst the plume dispersion dilutes volcanic aerosol so slows the heterogeneous loss rates of these species. A higher volcanic aerosol loading enhances BrO / SO2 in the (> 6 km) downwind plume. Simulations assuming low/medium and high Etna bromine emissions scenarios show the bromine emission has a greater influence on BrO / SO2 further downwind and a modest impact near downwind, and show either complete or partial conversion of HBr into reactive bromine, respectively, yielding BrO contents that reach up to ∼50% or ∼20% of total bromine (over a timescale of a few 10's of minutes). Plume-air mixing (which in our model with fixed plume dimensions is inversely related to the volcanic emission flux) non-linearly impacts the downwind BrO / SO2. A slower rate of plume-air mixing (or greater volcanic emission flux) leads to lower BrO / SO2 ratios near downwind, but also delays the subsequent decline in BrO / SO2, thus yields higher BrO / SO2 ratios further downwind. We highlight the important role of plume chemistry models for the interpretation of observed changes in BrO / SO2 during/prior to volcanic eruptions, as well as for quantifying volcanic plume impacts on atmospheric chemistry. Simulated plume impacts include ozone, HOx and NOx depletion, the latter converted into HNO3. Partial recovery of ozone concentrations occurs with distance downwind (as BrO concentrations decline), although cumulative ozone loss is ongoing over the 3 h simulations. We suggest plume BrNO2 may be less prevalent than previous model predictions. We highlight additional reactions for BrNO2 (and alternative pathways via BrONO) which likely reduce in-plume BrNO2 prevalence. We also highlight uncertainty in volcanic NOx emissions that might be lower than previously assumed (i.e., equilibrium NOx), due to the slow rate of N2 oxidation. The atmospheric : magmatic gas ratio, VA : VM, in equilibrium model representations of the near vent plume is presently poorly defined. Using a revised equilibrium model methodology, lower VA : VM become suitable (e.g. VA : VM = 98 : 2, 95 : 5), which also yield a lower estimate for volcanic NOx, although uncertainties to such equilibrium model representations of near-vent plume chemistry and especially NOx formation are emphasized.