Volcanoes emit halogens into the atmosphere that undergo complex chemical cycling in plumes and cause destruction of ozone. We present a case study of the Mount Etna plume in the summer of 2012, when the volcano was passively degassing, using aircraft observations and numerical simulations with a new 3D model "WRF-Chem Volcano" (WCV), incorporating volcanic emissions and multi-phase halogen chemistry. Measurements of SO.sub.2 - an indicator of plume intensity - and ozone were made in the plume a few tens of kilometres from Etna, revealing a strong negative correlation between ozone and SO.sub.2 levels. From these observations, using SO.sub.2 as a tracer species, we estimate a mean in-plume ozone loss rate of 1.3x10.sup.-5 molecules of O.sub.3 per second per molecule of SO.sub.2 . This value is similar to observation-based estimates reported very close to Etna's vents, indicating continual ozone loss in the plume up to at least tens of kilometres downwind. The WCV model is run with nested grids to simulate the plume close to the volcano at 1 km resolution. The focus is on the early evolution of passively degassing plumes aged less than 1 h and up to tens of kilometres downwind. The model is able to reproduce the so-called "bromine explosion": the daytime conversion of HBr into bromine radicals that continuously cycle in the plume. These forms include the radical BrO, a species whose ratio with SO.sub.2 is commonly measured in volcanic plumes as an indicator of halogen ozone-destroying chemistry. The species BrO is produced in the ambient-temperature chemistry, with in-plume BrO / SO.sub.2 ratios on the order of 10.sup.-4 mol/mol, similar to those observed previously in Etna plumes. Wind speed and time of day are identified as non-linear controls on this ratio. Sensitivity simulations confirm the importance of near-vent radical products from high-temperature chemistry in initiating the ambient-temperature plume halogen cycling. Heterogeneous reactions that activate bromine also activate a small fraction of the emitted chlorine; the resulting production of chlorine radical Cl strongly enhances the methane oxidation and hence the formation of formaldehyde (HCHO) in the plume. Modelled rates of ozone depletion are found to be similar to those derived from aircraft observations. Ozone destruction in the model is controlled by the processes that recycle bromine, with about three-quarters of this recycling occurring via reactions between halogen oxide radicals. Through sensitivity simulations, a relationship between the magnitude of halogen emissions and ozone loss is established. Volcanic halogen cycling profoundly impacts the overall plume chemistry in the model, notably hydrogen oxide radicals (HO.sub.x ), nitrogen oxides (NO.sub.x ), sulfur, and mercury chemistry. In the model, it depletes HO.sub.x within the plume, increasing the lifetime of SO.sub.2 and hence slowing sulfate aerosol formation. Halogen chemistry also promotes the conversion of NO.sub.x into nitric acid (HNO.sub.3). This, along with the displacement of nitrate out of background aerosols in the plume, results in enhanced HNO.sub.3 levels and an almost total depletion of NO.sub.x in the plume. The halogen-mercury model scheme is simple but includes newly identified photo-reductions of mercury halides. With this set-up, the mercury oxidation is found to be slow and in near-balance with the photo-reduction of the plume. Overall, the model findings demonstrate that halogen chemistry has to be considered for a complete understanding of sulfur, HO.sub.x, reactive nitrogen, and mercury chemistry and of the formation of sulfate particles in volcanic plumes.