[1] Recent investigations report mixing ratios of BrO gas reaching 1 ppb in plume 4 - 7 km downwind of the summit of Soufriere Hills volcano, Montserrat. The detection of BrO in volcanic plumes is potentially important evidence of halogen-catalyzed tropospheric ozone destruction and suggests that volcanoes either directly emit BrO or emit bromine species that are rapidly converted to reactive bromine in volcanic plumes. Species distribution models constrained by volcanic gas and condensate analytical data for arc, rift, and hot spot volcanoes demonstrate that shallow magma degassing generates volcanic gases with ppb to ppm levels of reactive radicals Br, Cl, H, and HO but no significant BrO or ClO. The conversion of volcanic Br and Cl by reaction with ozone in gas-phase catalytic reaction cycles during plume transport can lead to secondary BrO and ClO at mixing ratios of a ppt and higher, possibly approaching a ppb for especially halogen-rich volcanic gases, several kilometers downwind of degassing sources. In general, however, ppb BrO and ClO levels in volcanic plumes several kilometers downwind probably require near-vent, high-temperature reaction of magmatic gases with air and/or in-plume heterogeneous chemical processes involving aerosols during plume transport. These processes oxidize bromine and chlorine in HBr and HCl, giving rise to increased levels of reactive Br, Cl, and HO, in addition to NOx. They can also produce significant amounts of the photochemically active precursors Br-2, BrCl, and Cl-2, which are photolyzable to reactive Br and Cl. The precursors may build up in nighttime volcanic plumes and accelerate ozone destruction and BrO and ClO production after sunrise. Downwind conversion of reactive Br, Cl, H, and HO from volcanic gas sources by catalytic reaction chains to HOBr and HOCl may trigger heterogeneous chemical processes, which are probably essential for sustaining ozone destruction in volcanic plumes. Gas-phase reactions of HBr and HCl with HO in volcanic plumes might also boost BrO and ClO downwind. Thus in-plume processes that generate reactive halogens may contribute significantly to ozone destruction in volcanic plumes. Enhanced levels of HOx and NOx in volcanic emissions may trigger nonhalogen catalytic cycles depleting ozone in tropospheric volcanic plumes.
