Mercury is a potent neurotoxin, which enters remote ecosystemsprimarily through atmospheric deposition. In the polar atmosphere, gaseous elemental mercury is oxidized to a highly reactive form of mercury, which is rapidly removed from the atmosphere by deposition. These atmospheric mercury-depletion events are caused by reactive halogens, such as bromine, which are released from sea-ice surfaces. Reactive halogens also exist at temperate and low latitudes, but their influence on mercury in the atmosphere outside polar regions has remained uncertain. Here we show that bromine can oxidize gaseous elemental mercury at mid-latitudes, using measurements of atmospheric mercury, bromine oxide and other trace gases over the Dead Sea, Israel. We observed some of the highest concentrations of reactive mercury measured in the Earth's atmosphere. Peaks in reactive mercury concentrations coincided with the near-complete depletion of elemental mercury, suggesting that elemental mercury was the source. The production of reactive mercury generally coincided with high concentrations of bromine oxide, but was also apparent at low levels of bromine oxide, and was observed at temperatures of up to 45°C. Using a chemical box model, we show that bromine species were the primary oxidants of elemental mercury over the Dead Sea. We suggest that bromine-induced mercury oxidation may be an important source of mercury to the world's oceans.
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