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1. Aqueous Elemental Mercury Production versus Mercury Inventories in the Lake Michigan Airshed: Deciphering the Spatial and Diel Controls of Mercury Gradients in Air and Water

   https://doi.org/10.1021/acsestwater.0c00187  

   Lepak, Ryan F. ; Tate, Michael T. ; Ogorek, Jacob M. ; DeWild, John F. ; Peterson, Benjamin D. ; Hurley, James P. ; Krabbenhoft, David P. ( January 2020 , ACS ES&T Water)

   Atmospheric delivery of mercury (Hg) is important to the Upper Great Lakes, and understanding gaseous Hg exchange between surface water and air is critical to predicting the effects of declining mercury emissions. Speciated atmospheric Hg, dissolved gaseous Hg (DGM), and particulate and filter passing total Hg were measured on a cruise in Lake Michigan. Low mercury levels reflected pristine background conditions, especially in offshore regions. In the atmosphere, reactive and particle-associated fractions were low (1.0 ± 0.5%) compared to gaseous elemental Hg (1.34 ± 0.14 ng m–3) and were elevated in the urbanized southern basin. DGM was supersaturated, ranging from 17.5 ± 4.8 pg L–1 (330 ± 80%) in the main lake to 33.2 ± 2.4 pg L–1 (730 ± 70%) in Green Bay. Diel cycling of surface DGM showed strong Hg efflux during the day due to increased winds, and build-up at night from continued DGM production. Epilimnetic DGM is formed from photochemical reduction, while hypolimnetic DGM originates from biological Hg reduction. We found that DGM concentrations were greatest below the thermocline (30.8 ± 13.6 pg L–1), accounting for 68–92% of the total DGM in Lake Michigan, highlighting the importance of nonphotochemical reduction in deep stratified lakes.
2. Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna

   https://doi.org/10.1021/acs.est.7b06429  

   Madigan, Daniel J. ; Li, Miling ; Yin, Runsheng ; Baumann, Hannes ; Snodgrass, Owyn E. ; Dewar, Heidi ; Krabbenhoft, David P. ; Baumann, Zofia ; Fisher, Nicholas S. ; Balcom, Prentiss ; et al ( April 2018 , Environmental Science & Technology)
3. A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

   https://doi.org/10.5194/acp-17-6353-2017  

   Horowitz, Hannah M. ; Jacob, Daniel J. ; Zhang, Yanxu ; Dibble, Theodore S. ; Slemr, Franz ; Amos, Helen M. ; Schmidt, Johan A. ; Corbitt, Elizabeth S. ; Marais, Eloïse A. ; Sunderland, Elsie M. ( January 2017 , Atmospheric Chemistry and Physics)

   Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg⁰. Oxidation to water-soluble Hg^II plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg⁰∕Hg^II redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model (MITgcm), enabling a global 3-D representation of atmosphere–ocean Hg⁰∕Hg^II cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg⁰ oxidant and that second-stage HgBr oxidation is mainly by the NO₂ and HO₂ radicals. The resulting chemical lifetime of tropospheric Hg⁰ against oxidation is 2.7 months, shorter than in previous models. Fast Hg^II atmospheric reduction must occur in order to match the  ∼ 6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM ≡ Hg⁰+Hg^II(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase Hg^II–organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows thatmore »the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because Southern Hemisphere Hg mainly originates from oceanic emissions rather than transport from the Northern Hemisphere. The model reproduces the observed seasonal TGM variation at northern midlatitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but it does not reproduce the lack of seasonality observed at southern hemispheric marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM–ozone relationship indicative of fast Hg⁰ oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China with little bias (0–30%). It reproduces qualitatively the observed maximum in US deposition around the Gulf of Mexico, reflecting a combination of deep convection and availability of NO₂ and HO₂ radicals for second-stage HgBr oxidation. However, the magnitude of this maximum is underestimated. The relatively low observed Hg wet deposition over rural China is attributed to fast Hg^II reduction in the presence of high organic aerosol concentrations. We find that 80% of Hg^II deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for Hg^II reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO₂ and NO₂ for second-stage HgBr oxidation.

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4. Fifty years after its discharge, methylation of legacy mercury trapped in the Penobscot Estuary sustains high mercury in biota

   https://doi.org/10.1016/j.scitotenv.2018.06.060  

   Rudd, John W.M. ; Bodaly, R.A. ; Fisher, Nicholas S. ; Kelly, C.A. ; Kopec, Dianne ; Whipple, Chris ( November 2018 , Science of The Total Environment)
5. New Insights on Ecosystem Mercury Cycling Revealed by Stable Isotopes of Mercury in Water Flowing from a Headwater Peatland Catchment

   https://doi.org/10.1021/acs.est.7b04449  

   Woerndle, Glenn E. ; Tsz-Ki Tsui, Martin ; Sebestyen, Stephen D. ; Blum, Joel D. ; Nie, Xiangping ; Kolka, Randall K. ( January 2018 , Environmental Science & Technology)