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

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

THE MERCURY CYCLE: MAJOR PROCESSES Hg(0) Hg(II) particulate Hg burial SEDIMENTS uplift volcanoes erosion oxidation (~1 y) reduction volatilization Hg(0) Hg(II) oxidation reduction deposition biological uptake ANTHROPOGENIC PERTURBATION: fuel combustion waste incineration mining highly water-soluble ATMOSPHERE SOIL/OCEAN

ATMOSPHERIC REDOX CHEMISTRY OF MERCURY Hg(II)Hg(0) OH, O 3 HO 2 (aq) X X X? Calvert and Lindberg, AE 2005 Hynes et al., UNEP 2008 Oxidation of Hg(0) by OH or O 3 is endothermic Oxidation by NO 3, BrO, O 3 (aq) is probably negligible Br, Cl Standard models Oxidation by Br and Cl may be important: Atmospheric reduction of Hg(II) is hypothetical

MERCURY DEPLETION EVENTS (MDEs) IN ARCTIC SPRING ARCTAS-A aircraft campaign (April 2008) showed ubiquitous MDEs over sea ice Hg(0) vs. O 3 in near-surface data 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 Mao et al., Kim et al., submitted

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 Hg(0) HgBr Br T Br, OH HgBrX sea-salt aerosol HgCl 3 2-, HgCl 4 2- deposition MBL budget Model predicts that ~80% of Hg(II) in MBL should be in sea salt: Holmes et al. [2009] Observed [Laurier et al., 2003] Model Hg(0)+Br Model Hg(0)+OH Subtropical Pacific cruise data kinetics from Goodsite et al. [2004]

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 …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 Oxidation by Br atoms can satisfy these constraints [Holmes et al., 2006]

TROPOSPHERIC BROMINE CHEMISTRY simulated in GEOS-Chem global chemical transport model CHBr 3 hv, OH 14 days CH 2 Br 2 OH 91 days CH 3 Br OH 1.1 years BrBrO BrNO 3 HOBr HBr Br y deposition Justin Parrella, in prep. GEOS-Chem Observed CHBr Gg a -1 CH 2 Br 2 62 Gg a -1 Northern mid-latitudes profiles of short-lived bromocarbons Sea salt debromination Mean tropospheric concentrations (ppt) In GEOS-Chem plankton industry

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 Holmes et al., in prep. (2006) Streets et al. [2009]

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

PREFERENTIAL REGIONS FOR Hg(0) OXIDATION Annual zonal mean oxidation rates Hg(0) lifetime against oxidation 0.45 years0.30 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.

MODEL EVALUATION AGAINST SURFACE TGM DATA Total gaseous mercury (TGM); model is annual mean Hg + Br Hg + OH/O 3 model: Unbiased at land sites (r 2 =0.88 for Hg+Br, r 2 = 0.87 forHg+OH/O 3 ) 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 Holmes et al., in prep. Hg+Br simulation

SEASONAL VARIATION OF TGM 15 sites3 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.

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.

WET DEPOSITION FLUX PATTERNS MDN and EMEP annual means ( ) Observations as symbols, model as background Seasonal variation 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. Hg+Br model

MODEL DEPOSITION PATTERNS DEPEND ON OXIDANT Hg+Br Hg+OH/O 3 Holmes et al., in prep. Annual total Hg(II) deposition flux Oxidation by Br causes greater deposition to SH oceans Environmental implications depend on cycling through land and ocean reservoirs; Development of a fully coupled atmosphere-ocean-land model is underway