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 in order 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. <br><br> We present daytime <i>PlumeChem</i> model simulations that reproduce and explain the reported trend in BrO / SO₂ at Etna including the initial rise and subsequent plateau. Suites of model simulations 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. <br><br> We predict a new evolution of Br speciation in the plume. BrO, Br₂, Br and HBr are the main plume species near downwind whilst 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. <br><br> 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, ~10 min) 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. <br><br> Simulations assuming low/medium and high Etna bromine emissions scenarios show that 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). <br><br> Plume–air mixing non-linearly impacts the downwind BrO / SO₂, as shown by simulations with varying plume dispersion, wind speed and volcanic emission flux. Greater volcanic emission flux leads to lower BrO / SO₂ ratios near downwind, but also delays the subsequent decline in BrO / SO₂, and 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, HO_x and NO_x depletion, the latter converted into HNO₃. Partial recovery of ozone occurs with distance downwind, although cumulative ozone loss is ongoing over the 3 h simulations.