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 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 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). Her...