Modelling reactive halogen formation and ozone depletion in volcanic plumes

In this paper, we present a purpose-built model, PlumeChem, to simulate the reactive halogen chemistry in plumes from persistently degassing volcanoes. The model includes photolytic, gas-phase and aqueous-phase chemical reactions and dispersion of the plume into the background atmosphere. We investigate how high temperature chemistry close to the vent (believed to enhance plume NO_x, HO_x, BrO_x, and ClO_x) affects BrO formation in the downwind plume. PlumeChem simulations initialised using output from thermodynamic calculations at 1000 °C with atmospheric:magmatic gas ratio = 0.1 (above the compositional discontinuity) are in good quantitative agreement with observations of BrO/SO₂, and also provide a mechanism whereby ClO and OClO may be formed in the plume. The model reproduces the observed magnitudes and rates of increase of BrO/SO₂ at several volcanoes. The HBr content of the initial plume is shown to be an important control in determining the BrO/SO₂ ratio and ozone depletion in the downwind plume. The model simulates conversion of NO_x to HNO₃ and destruction of ozone in the plume, providing an explanation for observed elevated HNO₃ in volcanic plumes and in agreement with an in-plume observation of depleted ozone. The predicted ozone depletion was locally and regionally significant for three different volcanic plume compositions. Therefore we present a new model of volcanic plume chemistry, supported by observations of volcanic BrO and HNO₃, which identifies HBr emission as a key determinant in ozone depletion by volcanoes.