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 Two stable oxidation states: Hg(0) and Hg(II)

Orbital energies vs. atomic number Energetic arrangement of orbitals is such that mercury (Z=80) has all its subshells filled

Biogeochemical cycle of mercury Hg(0) Hg(II) particulate Hg burial SEDIMENTS uplift volcanoes erosion oxidation Hg(0)Hg(II) reduction biological uptake ANTHROPOGENIC PERTURBATION: fuel combustion mining ATMOSPHERE OCEAN/SOIL VOLATILE WATER-SOLUBLE (months)

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

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)

Measuring Speciated Hg in the Atmosphere Landis et al., GEM, RGM, and PBM drawn in through inlet 2.RGM collected on KCl denuder, HgP collected on particulate filter, and GEM passes through to be analyzed on 2537A. 3.RGM and HgP collected for 1-2hr. 4.HgP filter heated to 800°C and analyzed on 2537A as GEM. 5. RGM is thermally desorbed from KCl denuder at 500°C and analyzed on 2537A as GEM.

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

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

Atmospheric redox chemistry of mercury: what laboratory studies and kinetic theory tell us Hg(0) Hg(II) OH, O 3, Oxidation of Hg(0) by OH or O 3 is endothermic HO 2 (aq) Older models Goodsite et al., 2004; Calvert and Lindberg, 2005; Hynes et al., UNEP 2008; Ariya et al., UNEP 2008 Oxidation by Cl and Br may be important: ? XX Cl, Br No viable mechanism identified for atmospheric reduction of Hg(II) X

Atmospheric redox chemistry of mercury: what field observations tell us Hg(0) lifetime against oxidation must be ~ months –Observed variability of Hg(0) Oxidant must be photochemical – Observed late summer minimum of Hg(0) at northern mid-latitudes – Observed diurnal cycle of Hg(II) Oxidant must be in gas phase and present in stratosphere –Hg(II) increase with altitude, Hg(0) depletion in stratosphere Oxidation in marine boundary layer is by halogen radicals, likely Br –Observed diurnal cycle of Hg(II) Oxidation can be very fast (hours-days) in niche environments during events –Boundary layer Hg(0) depletion in Arctic spring, Dead Sea from high Br If reduction happens at all it must be in the lower troposphere –Hg(II) increase with altitude, Hg(0) depletion in stratosphere Hg(II)/Hg(0) emission ratios may be overestimated in current inventories –Lower-than-expected Hg(II)/Hg(0) observed in pollution plumes –Weaker-than-expected regional source signatures in wet deposition data Working hypothesis: Br atoms could provide the dominant global Hg(0) oxidant

Halogen radical chemistry in troposphere: sink for ozone, NO x, VOCs, mercury X ≡ Cl, Br, I sea salt source organohalogen source radical cycling non-radical reservoir formation heterogeneous recycling

Bromine chemistry in the atmosphere Tropopause (8-18 km) Troposphere Stratosphere Halons CH 3 Br CHBr 3 CH 2 Br 2 Sea salt BrBrO BrNO 3 HOBrHBr O3O3 hv, NO hv OH Inorganic bromine (Br y ) Br y OH, h debromination deposition industryplankton Stratospheric BrO: 2-10 ppt Tropospheric BrO: ppt Thule GOME-2 BrO columns Satellite residual [Theys et al., 2011] BrO column, cm -2 VSLS

Mean vertical profiles of CHBr 3 and CH 2 Br 2 From NASA aircraft campaigns over Pacific in April-June Vertical profiles steeper for CHBr 3 (mean lifetime 21 days) than for CH 2 Br (91 days), steeper in extratropics than in tropics Parrella et al. [2012]

Global tropospheric Br y budget in GEOS-Chem (Gg Br a -1 ) SURFACE CHBr CH 2 Br 2 57 CH 3 Br Marine biosphere Sea-salt debromination (50% of 1-10 µm particles) STRATOSPHERE TROPOSPHERE 7-9 ppt Liang et al. [2010] stratospheric Bry model (upper boundary conditions) Br y 3.2 ppt Volcanoes (5-15) Deposition lifetime 7 days 1420 Sea salt is the dominant global source but is released in marine boundary layer where lifetime against deposition is short; CHBr 3 is major source in the free troposphere Parrella et al. [2012]

Tropospheric Br y cycling in GEOS-Chem Gg Br (ppt) Global annual mean concentrations in Gg Br (ppt), rates in Gg Br s -1 Model includes HOBr+HBr in aq aerosols with  = 0.2, ice with  = 0.1 Mean daytime BrO = 0.6 ppt; would be 0.3 ppt without HOBr+HBr reaction Parrella et al. [2012]

Zonal annual mean concentrations (ppt) in GEOS-Chem Br BrO Br y Br y is 2-4 ppt, highest over Southern Ocean (sea salt) BrO increases with latitude (photochemical sink) Br increases with altitude (BrO photolysis) Parrella et al. [2012]

Comparison to seasonal satellite data for tropospheric BrO [Theys et al., 2011] model TOMCAT has lower  =0.02 for HOBr+HBr than GEOS-Chem, large polar spring source from blowing snow HOBr+HBr reaction critical for increasing BrO with latitude, winter/spring NH max in GEOS-Chem (9:30 am) Parrella et al. [2012]

Effect of Br chemistry on tropospheric ozone Zonal mean ozone decreases (ppb) in GEOS-Chem Two processes: catalytic ozone loss via HOBr, NO x loss via BrNO 3 Global OH also decreases by 4% due to decreases in ozone and NO x Parrella et al. [2012]

Bromine chemistry improves simulation of 19 th century surface ozone Standard models without bromine are too high, peak in winter-spring; bromine chemistry corrects these biases Model BrO is similar in pre-industrial and present atmosphere (canceling effects) Parrella et al. [2012]

GEOS-Chem global mercury model Hg(II) vegetationocean mixed layer Hg(0) Hg(II)Hg(0) natural + legacy boundary conditions 3-D atmospheric simulation coupled to 2-D surface ocean and land reservoirs Gas-phase Hg(0) oxidation by Br atoms In-cloud Hg(II) photoreduction to enforce 7-month Hg lifetime against deposition soil Hg(0) + Br ↔ Hg(I) → Hg(II) surface reservoirs  ~ months stable reservoirs  ~ decades anthropogenic + geogenic primary emissions Kinetics from Goodsite et al. [2004], Donohoue et al. [2005]; Balabanov et al. [2005]

MERCURY WET DEPOSITION FLUXES, Circles: observations Background: GEOS-Chem model Model contribution from North American anthropogenic sources Model contribution from external sources Selin and Jacob [2008]

History 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

surface air trend of Hg(0) over the Atlantic Ocean data from ship cruises show a 50% decrease over North Atlantic, no significant trend over South Atlantic Surface ocean Hg in North Atlantic also show a 50% decrease for , while subsurface Hg shows a 80% decrease [Mason et al., 2012] Sørensen et al., submitted Cruises Mace Head Cruises Cape Point ocean mixed layer m thermocline (~1000 m) surface ocean subsurface ocean

GEOS-Chem simulation of Hg(0) trends in surface air Forced by Streets emission trends ng m -3 a -1 Simulation forced by 80% decrease in subsurface North Atlantic yields 50% decrease in surface air Hg(0) over , consistent with observations Subsurface decrease must reflect a large decline in Hg inputs to the North Atlantic over Sørensen et al., submitted Global 3-D atmospheric model coupled to 2-D surface ocean and land models, with subsurface ocean as boundary condition Forced by observed subsurface Atlantic trends

Decreasing Hg input to subsurface North Atlantic, Atmospheric deposition explanation Hg(0) Hg(II) Br Hg(0) Hg(II) Br marine boundary layer ocean mixed layer subsurface ocean (down to thermocline) Hg deposition is thought to be driven by MBL oxidation of Hg(0) by Br atoms MBL ozone ~doubled during ; Br concentrations would have correspondingly decreased (Br/BrO photochemical equilibrium) Sørensen et al., submitted fast slow Br BrO O3O3 h

Decreasing Hg input to subsurface North Atlantic, Disposal of Hg-containing commercial products Hg Disposed Hg- containing commercial products wastewater, leaching incineration Secondary wastewater treatment and phase-out of Hg from commercial products would have greatly decreased Hg input to the subsurface N Atlantic Hg(II) Hg Sørensen et al., submitted N AMERICA, EUROPE

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

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

7-box model with transfer rate constants Figure 1. Rate coefficients k ij (a -1 ) driving our 7-compartment global biogeochemical box model for Hg. k ij defines the first-order transfer from reservoir i to reservoir j as F ij =k ij m i, where F ij (Mg a -1 ) is the Hg flow from reservoir i to reservoir j and m i (Mg) is the mass of Hg in reservoir i.Values of k ij are derived from best estimates of present-day flows and masses from the literature and are assumed to be constant in time. The red arrow represents the external forcing by primary emissions (geogenic or anthropogenic) from the deep mineral reservoir. Geogenic emissions are constant (90 Mg a -1 ) and anthropogenic emissions are time-dependent.

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

Global source contributions to Hg in present-day surface ocean Human activity has increased 7x the 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 present-day surface ocean Amos et al., submitted Europe Asia N America S America former USSR ROW pre-1850 natural emissions

TOWARDS A GLOBAL MERCURY TREATY: Focus activity of United Nations Environmental Program (UNEP) February 2009: Governing Council of UNEP agrees on need for global legally binding instrument on mercury Goal is to complete negotiations by 2013 In US; Clean Air Mercury Rule (CAMR) to reduce power plant emissions was struck down by courts in 2008; new effort is underway CHALLENGES: How to regulate in the face of considerable uncertainty? How to account for legacy mercury from past US and European emissions? How to account for possible major effects of climate change?

Looking toward the future: UNEP global treaty for Hg Negotiations to be completed by 2013 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