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GLOBAL MODELING OF MERCURY WITH Br AS ATMOSPHERIC OXIDANT

GLOBAL MODELING OF MERCURY WITH Br AS ATMOSPHERIC OXIDANT. Chris D. Holmes and Daniel J. Jacob. and funding from EPRI and NSF. RISING MERCURY IN THE ENVIRONMENT. Mercury in polar bear fur. Wyoming ice core. Dietz et al., 2006. US fish consumption advisories (EPA). Schuster et al., 2002.

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GLOBAL MODELING OF MERCURY WITH Br AS ATMOSPHERIC OXIDANT

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  1. GLOBAL MODELING OF MERCURYWITH Br AS ATMOSPHERIC OXIDANT Chris D. Holmes and Daniel J. Jacob and funding from EPRI and NSF

  2. RISING MERCURY IN THE ENVIRONMENT Mercury in polar bear fur Wyoming ice core Dietz et al., 2006 US fish consumption advisories (EPA) Schuster et al., 2002 EPA, 2007

  3. ANTHROPOGENIC PERTURBATION: fuel combustion waste incineration mining THE MERCURY CYCLE: MAJOR PROCESSES oxidation (~1 y) 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

  4. ATMOSPHERIC REDOX CHEMISTRY OF MERCURY X X Standard models Br, Cl OH, O3 Hg(0) Hg(II) ? X HO2 (aq) Calvert and Lindberg, AE 2005 Hynes et al., UNEP 2008 • Oxidation of Hg(0) by OH or O3 is endothermic • Oxidation by Br and Cl may be important: • Oxidation by NO3, BrO, O3 (aq) is probably negligible • Atmospheric reduction of Hg(II) is hypothetical

  5. ARCTAS-A aircraft campaign (April 2008) showed ubiquitous MDEs over sea ice MERCURY DEPLETION EVENTS (MDEs) IN ARCTIC SPRING • MDEs are confined to below 0.5 km altitude, occur concurrently with ODEs and in presence of soluble bromide • Mercury depletion is consistent with Hg + Br Hg(0) vs. O3 in near-surface data Mao et al., Kim et al., submitted

  6. DIURNAL CYCLE OF REACTIVE GASEOUS MERCURY (RGM) IN MARINE BOUNDARY LAYER Early a.m. rise, midday peak suggests Br chemistry, deposition via sea salt uptake MBLbudget Subtropical Pacific cruise data Observed [Laurier et al., 2003] Model Hg(0)+BrModel Hg(0)+OH Model predicts that ~80% of Hg(II) in MBL should be in sea salt: sea-salt aerosol Br, OH Br Hg(0) HgBrX HgBr HgCl32-, HgCl42- T kinetics from Goodsite et al. [2004] deposition Holmes et al. [2009]

  7. WHAT DO ATMOSPHERIC DATA TELL US ABOUT GLOBAL Hg(0) OXIDATION? • Atmospheric Hg lifetime against deposition must be ~ 1 year • Observed variability of Hg(0) • Oxidant must be photochemical • Observed late summer minimum at northern mid-latitudes • Oxidant must be in gas phase and present in stratosphere • Hg(II) increase with altitude, Hg(0) depletion in stratosphere Oxidation by Br atoms can satisfy these constraints [Holmes et al., 2006] …WHAT DO ATMOSPHERIC DATA TELL US ABOUT GLOBAL Hg(II) REDUCTION? • If it happens at all it’s mostly in lower troposphere (clouds?) • RGM increase with altitude, Hg(0) depletion in stratosphere

  8. TROPOSPHERIC BROMINE CHEMISTRYsimulated in GEOS-Chem global chemical transport model GEOS-Chem Observed Northern mid-latitudes profiles of short-lived bromocarbons CHBr3 440 Gg a-1 CH2Br2 62 Gg a-1 Mean tropospheric concentrations (ppt) In GEOS-Chem 0.09 0.8 0.2 hv, OH BrNO3 CHBr3 Br BrO 14 days OH HBr CH2 Br2 HOBr 91 days Bry 5.0 1.5 debromination industry Sea salt OH CH3Br deposition 1.1 years plankton Justin Parrella, in prep.

  9. (2006) GEOS-Chem MODEL OF ATMOSPHERIC MERCURY • Global 3-D atmospheric simulation driven by GEOS meteorological data and coupled to 2-D dynamic surface ocean and land reservoirs • Hg(0) oxidation by Br [Donohoue et al., 2005; Goodsite et al., 2004; Balabanov et al. [2005] • Compare to previous model with Hg(0) oxidation by OH and ozone Streets et al. [2009] Holmes et al., in prep.

  10. SPECIFICATION OF Br CONCENTRATIONS IN GEOS-Chem Hg MODEL Zonal mean concentrations (ppt) from bromocarbons + hv, OH simulated by TOMCAT (troposphere) and GMI (stratosphere) with standard gas-phase chemistry Add 1 ppt BrO in MBL 5 ppt in Arctic spring BL

  11. PREFERENTIAL REGIONS FOR Hg(0) OXIDATION Annual zonal mean oxidation rates Hg(0) lifetime against oxidation 0.45 years 0.30 years Add aqueous-phase photoreduction of Hg(II) in cloud tuned to yield Hg lifetime against deposition of 0.9 years Holmes et al., in prep.

  12. Total gaseous mercury (TGM); model is 2006-2008 annual mean MODEL EVALUATION AGAINST SURFACE TGM DATA Hg+Br simulation • Unbiased at land sites (r2=0.88 for Hg+Br, r2 = 0.87 forHg+OH/O3) • Underestimate over N Atlantic is corrected in most recent GEOS-Chem version by using observed subsurface ocean concentrations (Soerensen et al., in prep.) • Hg+Br model has steeper latitudinal gradient model: Hg + Br Hg + OH/O3 Holmes et al., in prep.

  13. SEASONAL VARIATION OF TGM 15 sites 3 sites • Both models reproduce late summer minimum at northern mid-latitudes • Summer maximum at Cape Point is due to ocean emission • Only Hg+Br model can simulate polar spring depletion, summer rebound • Only Hg+Br model can simulate high-RGM subsidence events over Antarctica Holmes et al., in prep.

  14. VERTICAL PROFILES OF TGM • Uniform in troposphere, dropping in stratosphere • Arctic spring observations show much faster drop in stratosphere than elsewhere – underestimate of halogen oxidants? Holmes et al., in prep.

  15. WET DEPOSITION FLUX PATTERNS MDN and EMEP annual means (2006-2008) Observations as symbols, model as background Seasonal variation Hg+Br model • Hg +Br simulation is too low over Gulf of Mexico in summer – missing Br source in subtropics? • Model is too high at northerly sites in winter – insufficient scavenging by snow? Holmes et al., in prep.

  16. Oxidation by Br causes greater deposition to SH oceans MODEL DEPOSITION PATTERNS DEPEND ON OXIDANT Annual total Hg(II) deposition flux Hg+Br Hg+OH/O3 Environmental implications depend on cycling through land and ocean reservoirs; Development of a fully coupled atmosphere-ocean-land model is underway Holmes et al., in prep.

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