Reactive bromine chemistry in Mount Etna's volcanic plume: the influence of total Br, high-temperature processing, aerosol loading and plume–air mixing

摘要

Volcanic emissions present a source of reactive halogens to the troposphere,through rapid plume chemistry that converts the emitted HBr to more reactiveforms such as BrO. The nature of this process is poorly quantified, yet isof interest in order to understand volcanic impacts on the troposphere, and infervolcanic activity from volcanic gas measurements (i.e. BrO / SO₂ ratios).Recent observations from Etna report an initial increase and subsequentplateau or decline in BrO / SO₂ ratios with distance downwind.We present daytime PlumeChem model simulations that reproduce and explain thereported trend in BrO / SO₂ at Etna including the initial rise andsubsequent plateau. Suites of model simulations also investigate theinfluences of volcanic aerosol loading, bromine emission, and plume–airmixing rate on the downwind plume chemistry. Emitted volcanic HBr isconverted into reactive bromine by autocatalytic bromine chemistry cycleswhose onset is accelerated by the model high-temperature initialisation.These rapid chemistry cycles also impact the reactive bromine speciationthrough inter-conversion of Br, Br₂, BrO, BrONO₂, BrCl, HOBr.We predict a new evolution of Br speciation in the plume. BrO, Br₂, Brand HBr are the main plume species near downwind whilst BrO and HOBr arepresent further downwind (where BrONO₂ and BrCl also make up a minorfraction). BrNO₂ is predicted to be only a relatively minor plumecomponent.The initial rise in BrO / SO₂ occurs as ozone is entrained into the plumewhose reaction with Br promotes net formation of BrO. Aerosol has a modestimpact on BrO / SO₂ near-downwind (< ~6 km,~10 min) at the relatively high loadings considered. Thesubsequent decline in BrO / SO₂ occurs as entrainment of oxidantsHO₂ and NO₂ promotes net formation of HOBr and BrONO₂, whilstthe plume dispersion dilutes volcanic aerosol so slows the heterogeneousloss rates of these species. A higher volcanic aerosol loading enhancesBrO / SO₂ in the (> 6 km) downwind plume.Simulations assuming low/medium and high Etna bromine emissions scenariosshow that the bromine emission has a greater influence on BrO / SO₂ furtherdownwind and a modest impact near downwind, and show either complete orpartial conversion of HBr into reactive bromine, respectively, yielding BrOcontents that reach up to ~50 or ~20%of total bromine (over a timescale of a few 10 s of minutes).Plume–air mixing non-linearly impacts the downwind BrO / SO₂, as shown bysimulations with varying plume dispersion, wind speed and volcanic emissionflux. Greater volcanic emission flux leads to lower BrO / SO₂ ratios neardownwind, but also delays the subsequent decline in BrO / SO₂, and thusyields higher BrO / SO₂ ratios further downwind. We highlight theimportant role of plume chemistry models for the interpretation of observedchanges in BrO / SO₂ during/prior to volcanic eruptions, as well as forquantifying volcanic plume impacts on atmospheric chemistry. Simulated plumeimpacts include ozone, HO_x and NO_x depletion, the latterconverted into HNO₃. Partial recovery of ozone occurs with distancedownwind, although cumulative ozone loss is ongoing over the 3 hsimulations.