LETTERS PUBLISHED ONLINE: 28 NOVEMBER 2010 | DOI: 10.1038/NGEO1018 Bromine-induced oxidation of mercury in the mid-latitude atmosphere Daniel Obrist 1 * , Eran Tas 2,3 , Mordechai Peleg 2 , Valeri Matveev 2 , Xavier Faïn 1 , David Asaf 2 and Menachem Luria 2 Mercury is a potent neurotoxin, which enters remote ecosystems primarily through atmospheric deposition 1,2 . 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 3,4 . These atmospheric mercury-depletion events are caused by reactive halogens, such as bromine, which are released from sea-ice surfaces 5,6 . Reactive halogens also exist at temperate and low latitudes 7,8 , 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. Atmospheric mercury depletion events (AMDE; ref. 3) have been described in many Arctic, sub-Arctic and Antarctic sites, where they lead to pulses of increased mercury deposition 9,10 and are estimated to increase mercury loads to the Arctic by 120–300 Mg each year 11,12 . AMDE are caused by reactive halogens 13 and accompanied by low levels of ozone (O 3 ), which is catalytically destroyed by halogens 14 . Reactive halogens, however, are not limited to the polar atmosphere and occur at temperate locations such as over salt lakes and in the marine boundary layer 7,8,15,16 . The degree to which reactive halogens cause conversion of elemental mercury, Hg(0), to oxidized mercury, Hg(ii), under non-freezing conditions at temperate and low latitudes is unclear. We report results from measurements on the shore of the Dead Sea, Israel, where we simultaneously quantified the main forms of atmospheric mercury, Hg(0) and gaseous and particulate- bound Hg(ii), bromine oxide (BrO), O 3 and auxiliary variables. We measured atmospheric mercury by means of cold-vapour atomic fluorescence spectrometry and BrO using long-path differential optical absorption spectroscopy (LP-DOAS), and quantified other trace gases (including O 3 ) and meteorology during two measurement campaigns in summer and winter. Time series 1 Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada, 89512, USA, 2 The Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel, 3 Department of Atmospheric Chemistry, Max-Planck-Institute for Chemistry, Mainz 55128, Germany. *e-mail: daniel.obrist@dri.edu. of measurements (Fig. 1a,b) showed daytime Hg(ii) enrichment to levels as high as 136 ppqv, among the highest Hg(ii) levels observed in the Earth’s atmosphere 4 . High daytime Hg(ii) levels occurred frequently and exceeded rural background levels (generally below 10 ppqv) on 26 of 29 days in the summer and 8 of 15 days in the winter. Hg(ii) enhancements were accompanied by strong depletions of Hg(0), down to 22 ppqv, which is below 10% of the global tropospheric background concentration, resulting in strong inverse correlations between the two (Fig. 1c), providing clear evidence for direct atmospheric conversion of Hg(0) to Hg(ii). Most Hg(ii) occurred in gaseous form, with only minor contributions of Hg(ii) bound to particulates. Hg(ii) production and Hg(0) depletion temporally coincided with high BrO levels and depletion of O 3 (Fig. 1b and 2a), with high-resolution temporal data (5 min) demonstrating exact alignment of Hg(0) and O 3 depletions (Fig. 2b) and indicating that Hg(0) depletion and O 3 destruction were highly related. Observed enhancements of BrO and corresponding O 3 depletions agree with well-characterized intensive reactive bromine chemistry that occurs in the local Dead Sea atmosphere and causes significant catalytic destruction of O 3 first described here outside the high latitudes 7,8,17 . The ‘bromine explosion’ mechanism 18 , induced by the high bromide level and low pH of the Dead Sea water, has been suggested as a key process for production of atmospheric BrO (refs 8,19). Observed oxidation of Hg(0) to Hg(ii), in the presence of high BrO levels and O 3 destruction, shows all characteristics of AMDE previously only described in the high latitudes, and provides evidence of strong temperate-zone AMDE on an almost daily basis. We modelled temporal patterns of halogen species, ozone and Hg(0) under typical summertime conditions (7 June 2009) using a heterogeneous chemical box model (MECCA; ref. 20), which accounts for 204 gas-phase, 292 aqueous-phase and 275 heterogeneous reactions, including 53 reactions involving mercury (Supplementary Section). When the full available bromine chemistry was implemented in the model (BASE scenario; Fig. 3a), it accurately simulated corresponding Hg(0) and O 3 depletions during the build-up of reactive bromine, which at the Dead Sea generally occurs near midday and in the afternoon 8,17 . Sensitivity analyses using stepwise elimination of bromine reactions from the BASE scenario showed that using only BrO x (=Br + BrO; named ‘only BrO x ’ in Fig. 3b) as oxidants for mercury can account for most of the observed AMDE. The use of atomic Br alone (‘only Br’), a compound not directly measured, showed similar Hg(0) depletion to BrO x , indicating that the effect of Br predominates over BrO (Fig. 3b). Model results also indicate that iodine species, which at the Dead Sea have been measured as iodine oxide (IO) NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION | www.nature.com/naturegeoscience 1 © 2010 Macmillan Publishers Limited. All rights reserved.