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MERCURY IN THE ENVIRONMENT

MERCURY IN THE ENVIRONMENT. Daniel J. Jacob. Electronic structure of mercury. Mass number = 80: 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 4f 14 5s 2 5p 6 5d 10 6s 2. Complete filling of subshells gives Hg(0) a low melting point, volatility

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MERCURY IN THE ENVIRONMENT

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  1. MERCURY IN THE ENVIRONMENT Daniel J. Jacob

  2. Electronic structure of mercury Mass number = 80: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 • Complete filling of subshells gives Hg(0) a low melting point, volatility • Two stable oxidation states: Hg(0) and Hg(II)

  3. BIOGEOCHEMICAL CYCLING OF MERCURY ATMOSPHERE Hg (gas) combustion industry mining volcanoes erosion deposition re-emission SOIL OCEAN burial SEDIMENTS DEEP EARTH

  4. Global mercury deposition has roughly tripled since preindustrial times RISING MERCURY IN THE ENVIRONMENT Dietz et al. [2009]

  5. HUMAN EXPOSURE TO MERCURY IS MAINLY FROM FISH CONSUMPTION Tuna is the #1 contributor Mercury biomagnification factor State fish consumption advisories EPA reference dose (RfD) is 0.1 μg kg-1 d-1 (about 2 fish meals per week)

  6. MERCURY CYCLING INVOLVES CHEMICAL TRANSFORMATIONS elemental mercury VOLATILE mercuric compounds WATER-SOLUBLE ATMOSPHERE Hg(0) Hg(II) oxidation deposition re-emission Hg(0) Hg(II) reduction SURFACE RESERVOIRS (Ocean, Land) microbes MeHg Methylmercury TOXIC

  7. Atmospheric transport of Hg(0) takes place on global scale Implies global-scale transport of anthropogenic emissions Anthropogenic Hg emission (2006) Mean Hg(0) concentration in surface air: circles = observed, background = GEOS-Chem model Transport around northern mid-latitudes: 1 month Hg(0) lifetime = 0.5-1 year Transport to southern hemisphere: 1 year Streets et al. [2009]; Soerensen et al. [2010]

  8. LOCAL POLLUTION INFLUENCE FROM EMISSION OF Hg(II) High-temperature combustion emits both Hg(0) and Hg(II) 60% Hg(0) GLOBAL MERCURY POOL photoreduction 40% Hg(II) NEAR-FIELD WET DEPOSITION Hg(II) concentrations in surface air: circles = observed, background=model MERCURY DEPOSITION “HOT SPOT” Large variability of Hg(II) implies atmospheric lifetime of only days against deposition Thus mercury is BOTH a global and a local pollutant! Selin et al. [2007]

  9. Atmospheric redox chemistry of mercury Older models X X OH, O3, Cl, Br Hg(II) Hg(0) X ? HO2(aq) • Oxidation of Hg(0) by OH or O3 is endothermic • Oxidation by Cl and Br may be important: • No viable mechanism identified for atmospheric reduction of Hg(II) Goodsite et al., 2004; Calvert and Lindberg, 2005; Hynes et al., UNEP 2008; Ariya et al., UNEP 2008

  10. GOME-2 BrO columns Bromine chemistry in the atmosphere Inorganic bromine (Bry) O3 hv BrNO3 Br BrO Halons hv, NO OH HBr HOBr Stratospheric BrO: 2-10 ppt CH3Br Thule Stratosphere Tropopause (8-18 km) Troposphere TroposphericBrO: 0.5-2 ppt CHBr3 CH2Br2 OH Bry Satellite residual [Theys et al., 2011] debromination BrO column, 1013 cm-2 deposition Sea salt industry plankton

  11. TROPOSPHERIC BROMINE CHEMISTRYsimulated in GEOS-Chem global chemical transport model GEOS-Chem Observed Vertical profiles of short-lived bromocarbonsat northern mid-latitudes CHBr3 440 Gg a-1 CH2Br2 62 Gg a-1 Mean tropospheric concentrations (ppt) 0.09 0.6 0.3 hv, OH BrNO3 CHBr3 Br BrO 14 days OH including HBr+HOBr on aerosols HBr CH2 Br2 HOBr 91 days Sea salt 1.4 0.9 debromination industry OH CH3Br deposition plankton 1.1 years Parrella et al. [2012]

  12. GEOS-Chem global model of mercury 3-D atmospheric simulation coupled to 2-D surface ocean and land reservoirs GEOS-Chem 3-D atmospheric chemical transport model (CTM) • 2-D surface reservoirs • ocean mixed layer • vegetation Anthropogenic and natural emissions Hg(0)+Br ↔ Hg(I) → Hg(II) • Long-lived reservoirs • deep ocean • soil

  13. MERCURY WET DEPOSITION FLUXES,2004-2005 Circles: observations Background: GEOS-Chem model tropopause Scavenging of Hg(II)-rich air from upper troposphere Model contribution from North American anthropogenic sources Model contribution from external sources updraft FLORIDA SCAVENGING BY DEEP CONVECTION Selin and Jacob [2008]

  14. Historical inventory of global anthropogenic Hg emissions Large past (legacy) contribution from N. American and European emissions; Asian dominance is a recent phenomenon Streets et al. , 2011

  15. 1977-2010 surface air trend of Hg(0) over the Atlantic Ocean • 1990-2010 data from ship cruises show 50% decrease over North Atlantic, no significant trend over South Atlantic • Long-term observations at continental sites in N America and Europe also show 1990-2010 decrease though not as strong as over North Atlantic Sørensenet al., submitted

  16. GEOS-Chem simulation of Hg(0) 1990-2010 trends in surface air Global 3-D atmospheric model coupled to 2-D surface ocean and land models Forced by Streets emission trends Forced by observed subsurface Atlantic trends ng m-3 a-1 • Observations in the subsurface North Atlantic show a 80% decrease from 1990 • to 2010 [Mason et al., 2012], which can explain the observed trends in surface air • This must reflect a large decline in Hg inputs to the North Atlantic Ocean over • the 1970-2010 period. Sørensenet al., submitted

  17. Decreasing Hg input to subsurface North Atlantic, 1970-20001. Atmospheric deposition explanation 1970 2000 Hg(0) Hg(0) Br Br marine boundary layer Hg(0) Hg(II) Hg(0) Hg(II) fast slow ocean mixed layer subsurface ocean (down to thermocline) • Hg deposition to ocean is driven by MBL oxidation of Hg(0) by Br atoms • MBL ozone ~doubled during 1970-2000; Br concentrations would have correspondingly decreased (Br/BrO photochemical equilibrium) O3 BrO Br h Sørensenet al., submitted

  18. Decreasing Hg input to subsurface North Atlantic, 1970-20002. Coastal margin explanation 1970 2000 Disposed Hg-containing commercial products incineration Hg(II) Hg Hg wastewater, leaching Hg • Secondary wastewater treatment and phase-out of Hg from commercial products would have decreased the Hg input to the subsurface N Atlantic Sørensenet al., submitted

  19. Disposal of Hg in commercial products:a missing component of the Hg biogeochemical cycle? Global source of commercial Hg peaked in 1970 Streets et al. [2011] and Hannah Horowitz (Harvard)

  20. 7-box model with 7 coupled ODEs dm/dt= s(t) – km where s is primary emission • Loss rate constants k specified from best knowledge Global biogeochemical model for mercury Primary emissions Model is initialized at natural steady state, forced with historical anthropogenic emissions for 2000 BC – present; % present-day enrichments are indicated Amos et al., submitted

  21. Time scale for dissipation of an atmospheric emission pulse Reservoir fraction Pulse gets transferred to subsurface ocean within a few years and stays there ~100 years, maintaining a legacy in the surface ocean Amos et al., submitted

  22. Global source contributions to Hg in present-day surface ocean emissions pre-1850 natural • Human activity has increased 7x Hg content of the surface ocean • Half of this human influence is from pre-1950 emissions • N America, Europe and Asia share similar responsibilities for anthropogenic Hg in surface ocean ROW former USSR N America S America Europe Asia Amos et al., submitted

  23. Negotiations to be completed by 2013 Looking toward the future: UNEP global treaty for Hg Effect of zeroing global anthropogenic emissions by 2015 • Zeroing anthropogenic emissions would decrease ocean Hg by 30% by 2100, while keeping emissions constant would increase it by 40% • Elevated Hg in surface ocean will take centuries to fix; the only thing we can do in short term is prevent it from getting worse. Amos et al., submitted

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