As volcanoes can be significant emitters of halogens , it has been hypothesized that bromine chemistry, more specifically the so-called "bromine explosion", could cause GEM oxidation in volcanic plumes . The "bromine explosion", regularly observed at polar sunrise , is an autocatalytic and heterogeneous photochemical reaction mechanism that causes the rapid generation of bromine radicals , one of the most important GEM oxidants in the atmosphere . Von Glasow (2010) suggested that such in-plume GEM oxidation could significantly reduce the net contribution of volcanism to the atmospheric Hg burden by shortening the atmospheric lifetime of volcanic Hg as well as background Hg entrained during plume dilution. If GEM can indeed be efficiently scavenged by other plume constituents, GEM concentrations in a volcanic plume could conceivably fall below the atmospheric background if the in-plume scavenging outweighs volcanic emission. However, perhaps due to the difficulty of obtaining Hg measurements in an aged volcanic plume, such an event has not yet been reported.