The detection of bromine monoxide (BrO) in the plume of Soufrière Hills volcano (Bobrowski et al., 2003) using ultraviolet Differential Optical Absorption Spectroscopy (40 DOAS) provided the first direct evidence for reactive halogen formation in volcanic plumes. BrO chemistry is known\ud to have significant effects on tropospheric oxidants, as has been observed in other locations (polar troposphere, marine, salt plains (von Glasow and Crutzen, 2007)). This knowledge led to speculation that ozone may be destroyed in volcanic plumes through reactive halogen chemistry\ud 45 (Bobrowski et al., 2003), and highlighted the need to understand the impact of volcanic plume chemistry on the troposphere (as discussed by von Glasow et al., this issue). Here we present studies using a new model, PlumeChem that simulates the reactive halogen chemistry of volcanic plumes. Model simulations of BrO formation in the downwind plume are presented and compared to observations from several volcanoes. The impacts of reactive halogen chemistry on volcanic 50 NOx speciation and on tropospheric ozone depletion are investigate
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