Volcanic plume chemistry is complex, particularly because conditions within a plume can differ greatly from typical atmospheric conditions, for example with regards to temperature, the concentration of radicals, particle loading, acidity, and sulfur and halogen contents. As the initial plume cools and mixes with background air, the speciation of atmospheric Hg likely evolves. Indeed, while emitted Hg is mainly in the form of GEM when it exits the vent , partial oxidation of volcanic GEM has been observed to occur within minutes upon plume cooling and dilution .