MERCURY IN THE AIR

1. Daniel J. Jacob MERCURY IN THE AIR with Harvard Team-Hg: Helen Amos, Bess Corbitt, Jenny Fisher, Hannah Horowitz, Chris Holmes (now at UC Irvine), Justin Parrella, AsifQureshi, Noelle Selin (now at MIT), Anne Soerensen, Elsie Sunderland and funding from NSF, EPRI, EPA

2. ANTHROPOGENIC PERTURBATION: fuel combustion waste incineration mining Biogeochemical cycling of Hg oxidation (~months) Hg(II) Hg(0) highly water-soluble reduction volcanoes erosion ATMOSPHERE volatilization deposition SOIL/OCEAN oxidation particulate Hg Hg(II) Hg(0) biological uptake reduction uplift burial SEDIMENTS

3. Anthropogenic perturbation to the global Hg cycle GEOS-Chem model natural atmosphere + present-day human enhancement Primary emissions x7 Atmospheric deposition x3 Surface ocean x3 Soil +15% Deep ocean + 15% Selin et al. [2008]; Selin [2009]

4. 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]

5. By contrast, emitted Hg(II) can be deposited close to point of emission High-temperature combustion emits both Hg(0) and Hg(II) 60% Hg(0) GLOBAL MERCURY POOL photoreduction 40% Hg(II) Hg(II) concentrations in surface air: circles = observed, background=model NEAR-FIELD DEPOSITION MERCURY DEPOSITION “HOT SPOT” Large variability of Hg(II) implies atmospheric lifetime of only days against deposition Observed Hg(II) ≡ reactive gaseous mercury (RGM) + particle-bound mercury (PBM) Selin et al. [2007]

6. Atmospheric redox chemistry of mercury:what laboratory studies and kinetic theory tell us 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

7. 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 Working hypothesis: Br atoms could provide the dominant global Hg(0) oxidant • 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

8. Atmospheric composition of Hg(II)? precipitating cloud aqueous aerosol/cloud Hg2+ oxidation X- Hg(0) HgXY Cl- Y- gas-aerosol partitioning HgCl2, others? wet deposition dry deposition SURFACE • Hg(II) salts produced by Hg(0) oxidation may change composition during cycling through aerosols/clouds • HgCl2 (KH = 1.4x106 M atm-1) is expected to be an important component because of ubiquitous Cl- - but there may be others (organics?)

9. Reactive gaseous mercury (RGM) and particle-bound mercury (PBM) at several North American sites fitted to a gas-aerosol equilibrium constant K Observed gas-aerosol partitioning of Hg(II) PM2.5 ≡ fine particulate matter Rutter and Schauer [2007] Hg(II) appears to have semi-volatile behavior; partitions into gas phase when air is warm and clean, in aerosol when air is cold and polluted. Amos et al. [in prep]

10. Observed RGM diurnal cycle suggests Br chemistry, deposition via sea salt uptake Special case of Hg(II) uptake by sea salt Box model budget for marine boundary layer (MBL) Subtropical Pacific cruise data Observed [Laurier et al., 2003] Model Hg(0)+BrModel Hg(0)+OH Box model predicts that ~80% of Hg(II) in MBL should be in sea salt aerosol: sea-salt aerosol Br, OH Br Hg(0) HgBrX HgBr HgCl32-, HgCl42- T kinetics from Goodsite et al. [2004] deposition Holmes et al. [2009]

11. 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

12. 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. [in prep]

13. Model vs. observed troposphericBrO columns Theys et al. [2011] satellite residuals GEOS-Chem model • Observations show similar BrO in both hemispheres, increasing with latitude and with winter/spring max • Model is biased low but captures some of the latitudinal/seasonal features Parrella et al. [in prep]

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

15. Sensitivity of Hg deposition to oxidation mechanism Annual mean Hg(0) oxidation rates in GEOS-Chem with Br or OH/O3 as oxidants Hg(0) = 6 months Hg(0) = 3.7 months Effect on annual mean GEOS-Chem Hg deposition fluxes Maximum sensitivity is over the Southern Ocean Holmes et al. [2010]

16. Annual mean 2007-2009 MDN data (circles) and GEOS-Chem model (background) Mercury wet deposition fluxes over US, 2007-2009 Seasonal variation • Summer peak along Gulf Coast reflects deep convective scavenging of Hg(II) from upper troposphere • Very low winter values at northern latitudes reflect inefficient scavenging by snow • Reduction of emitted Hg(II) is necessary to avoid model maximum in Northeast Amos et al., in prep.

17. Quantifying source-receptor relationships for mercury:the grasshopper effect Atmosphere  = 6 months Hg effective = 9 months LAND OCEAN Surface reservoirs  ~ months Hg(II) Hg(0) Hg(II) Hg(0) Intermediate reservoirs  ~ decades legacy legacy GEOS-Chem influence functions for anthropogenic source regions Extratropical NH Tropical NH SH g m-2 Mg-1 Effective atmospheric lifetime is sufficiently short for hemispheric signatures; future growth of Indian emissions is likely to lead to S shift in ocean deposition Corbitt et al., submitted

18. New anthropogenic inputs to the world’s oceans • Asian emissions are so large that they account for >50% of new anthropogenic inputs to all open oceans • N American emissions influence N Atlantic, European emissions influence Arctic Corbitt et al., submitted

19. Legacy anthropogenic sources account for over 50%of mercury deposited to the oceans Source attribution of present-day Hg deposition to world’s oceans (GEOS-Chem) Legacy source is highest in North Atlantic: past Hg(II) emissions from N. America? Atmospheric Hg(0) data in March-May (circles) compared to GEOS-Chem (background) Soerensen et al. [2010], Corbitt et al., submitted

20. Historical inventory of global anthropogenic Hg emissions • Large legacy contribution from N. American and European emissions; Asian dominance is a recent phenomenon • Time integrals of global emissions imply that legacy reservoirs are not globally enriched relative to the surface Streets et al. , submitted

21. Observed decrease of total gaseous Hg (TGM) since 1996 20-38% worldwide decrease Slemr et al. [2011] • Explanation by decline of legacy emissions would imply much higher past emissions than in Streets et al. historical inventory • Faster atmospheric oxidation of Hg(0) may be an alternate explanation • - Increasing Br? • - A missing anthropogenic source would help simulation of troposphericBrO • - Increasing Cl? • - could reflect increase in CFC replacement products after Montreal Protocol • - could also help explain the leveling of atmospheric methane

22. Effect of climate change on mercury in the Arctic Ocean Atmospheric Hg depletion events (AMDEs) associated w/ice leads Hg(0) Br bromine Sea salt Hg(II) deposition light SEA ICE ICE LEAD ARCTIC OCEAN summer rebound • Summer rebound in atmospheric observations cannot be explained by snow re-emission; suggests external input to Arctic Ocean (Arctic rivers runoff?) • Implies in turn that Arctic Ocean is supersaturated relative to the atmosphere • Changing river runoff and shrinking sea ice in future climate could greatly affect Hg levels in Arctic Ocean AMDEs Composite obs at Arctic sites GEOS-Chem: standard with Arctic rivers runoff Fisher et al., in prep.