Reactive bromine in volcanic plumes confines the emission temperature and oxidation of magmatic gases at the atmospheric interface.

The redox composition of volcanic gases relays substantial information about magmatic conditions and volcanic activity. Volcanic plume gas measurements are often interpreted assuming that magmatic gases are chemically inert on emission and near-source dilution in air. Conversely, many volcanic plumes contain high levels of bromine monoxide (BrO), which is produced by atmospheric oxidation of magmatic hydrogen bromide (HBr) emissions. We investigate the chemical kinetics of hot magmatic gases mixing with air. Our model reproduces and explains observations of volcanic plume BrO at Mt. Etna, evidencing that reduced gases [HBr, as well as carbon monoxide (CO) and hydrogen (H)] can oxidize at the hot magma-air interface. The extent of oxidation is controlled by the magmatic gas temperature. Observations of BrO and H in Mt. Etna plume indicate that magmatic gases enter air at several hundred kelvin below magmatic temperature, consistent with hypothesized decoupling of gas temperatures from magma prior to emission.