The aim of this study is to elucidate atmospheric conditions and chemical pathways responsible for recently observed atmospheric mercury depletion events (AMDE) at the Dead Sea in Israel. Hypotheses include: (1) depletion of gaseous elemental mercury (GEM) at the Dead Sea is accompanied by increased levels of reactive gaseous mercury (RGM) and particulate forms (PHg), which would confirm that AMDE at the Dead Sea are analogous to those observed in the polar troposphere; (2) AMDE are correlated with high levels of reactive halogen compounds (RHC), specifically BrOx (Br + BrO); (3) even small concentrations of RHC may initiate GEM oxidation, with implications for other midlatitude marine boundary layer sites with lower RHC levels. These hypotheses will be tested by simultaneous measurement of speciated mercury, halogen oxides, nitrogen oxides, ozone, sulfate aerosols, fine particulates, and meteorological parameters. A modeling study will help identify the mechanisms for reactive halogen chemistry and mercury oxidation.
Knowledge of source and sink strengths of atmospheric mercury species is critical for regulatory agencies to protect humans and ecosystems from this potent toxin. The research will involve undergraduate and graduate students from the University of Nevada, Reno. A partnership will be initiated with a local (Nevada) school district to expose teachers and students to this research and expand a current collaboration with their Gifted and Talented Program.
Mercury is a potent neurotoxin, and its major inputs to remote ecosystems occur through atmospheric deposition. After being deposited, mercury can bioaccumulate through the food chain reaching high concentrations in fish, wildlife, and humans. In the polar atmosphere, the dominant gaseous elemental mercury is known to convert to highly reactive oxidized mercury, which then increases atmospheric deposition. These conversions, called Atmospheric Mercury Depletion Events, are caused by reactive gas-phase halogens, and the increased mercury deposition from this chemistry is linked to higher mercury loads - by hundreds of tons of mercury each year - to sensitive arctic environments. Reactive halogens also exist at temperate and low latitudes, but it has been unclear if and how they affect mercury in the non-polar atmosphere. In this study, we characterized the effects of reactive halogens on atmospheric mercury cycling under temperate conditions in the Dead Sea basin of Israel. Extensive measurements of atmospheric chemistry were conducted along the shore of the Dead Sea, one of the most saline water bodies in the world. In particular, Dead Sea water is highly enriched in bromide, which leads to high levels of gas-phase bromine species in the atmosphere. Hence, the Dead Sea atmosphere serves as an ideal natural laboratory to study effects of atmospheric halogens such as bromine on mercury cycling. Results from two extensive measurements campaigns - one conducted in summer 2009 and one in winter 2009/10 - showed near-complete conversion of elemental to oxidized mercury in the presence of high bromine oxide, producing among the highest observed natural levels of oxidized mercury in the Earth’s atmosphere. These Dead Sea Mercury Depletion Events occurred both in winter and in mid-summer under temperatures up to 45 degrees Celsius, showing that this peculiar chemistry is not limited to cold, polar areas. Modeling studies using comprehensive atmospheric chemical models suggest that reactive bromine species were responsible for Mercury Depletion Events (accounting for 85 to 90%); in particular, we suggest that bromine oxide is the dominant oxidant at the Dead Sea, as opposed to atomic bromine which is generally considered the main oxidant in Polar Regions. Importantly, mercury conversion started even at low bromine levels such as commonly are observed in the marine boundary layer. Hence, bromine-induced mercury oxidation may occur across the globe where atmospheric bromine is present making it a potentially important source of mercury deposition. Educational components of this study included: collaboration with a local High school and a chemistry teacher attending field measurements, interns visiting our institute, and students working with data collected during this project; post-doctoral mentoring provided to two post-doctoral follows who were fully involved in field measurements, data analysis, and publication writing; and undergraduate and graduate student mentoring with seven students directly working on this project. Outreach activities included: open house presentations; High school visits; publication of seven manuscripts, including one paper published in the prestigious journal ‘Nature Geoscience’; presentation of 18 conference talks and seminars; and press releases and webpage presentations of our results by the National Science Foundation and by our institute.
