MERCURY IN THE ATMOSPHERE, BIOSPHERE, AND POLICY SPHERE: MERCURY IN THE ATMOSPHERE, BIOSPHERE, AND POLICY SPHERE: Constraints from a global 3D land-ocean-atmosphere model on mercury sources, cycling and deposition Noelle Eckley Selin Harvard University Department of Earth and Planetary Sciences Atmospheric Chemistry Modeling Group Princeton University 24 May 2007 Coauthors: D.J. Jacob, R.J. Park, R.M. Yantosca (Harvard) S. Strode, L. Jaegle, D. Jaffe, P. Swartzendruber (U. Washington)

MERCURY POLLUTION: A SCIENCE & POLICY PROBLEM Ice core record of deposition from Wyoming, USA [Schuster et al., ES&T 2002] Mercury in polar bear fur up 5-12X since 1890, [Dietz et al., ES&T 2006] States with fish mercury advisories [EPA, 2004] Mercury deposition has increased by 300% since industrialization Growing concern about human exposure through methylmercury in fish Particular concern in Arctic ecosystems due to bioaccumulation, human exposure

POLITICAL ACTIONS AND UNCERTAINTIES GLOBAL: 2002: Global Mercury Assessment: sufficient evidence to warrant international action 2003, 2005, 2007: UNEP Governing Council meetings reject proposed global mercury agreement. Mercury Programme and voluntary partnerships established. [Selin, Environment, 2005; Selin and Selin, RECIEL, 2006] U.S. : 2005: CLEAN AIR MERCURY RULE establishes a “cap and trade” approach to regulating mercury from coal-fired power plants REGIONAL: -U.S./Mexico/Canada regional action plan under Commission for Environmental Cooperation (1997,2000) -U.S./Canada/Europe/former Soviet Union countries agreement on heavy metals under Convention on Long Range Transboundary Air Pollution (1998) What are the relative contributions of global, regional and domestic sources to deposition? What is the impact of anthropogenic emissions (past & present) on the global mercury cycle?

Wet & Dry Deposition 2600 ATMOSPHERE 5000 (3x pre-industrial) SURFACE SOILS 1,000,000 OCEAN 289,000 Wet & Dry Deposition 1900 Oceanic Evasion 1500 Net burial 200 Land emissions 1600 Quantities in Mg/year (10 6 g, or metric tonnes) Uncertainty ranges in parentheses Adapted from Mason & Sheu, 2002 Anthropogenic Emissions 2400 Extraction from deep reservoirs 2400 Rivers 200 ( ) ( ) ( ) ( ) ( ) SCEINTIFIC UNCERTAINTIES: SOURCES AND SINKS

SCIENTIFIC UNCERTAINTIES: ATMOSPHERIC CHEMISTRY Hg(0) Hg(II)Oxidation OH, O 3, Br(?) GAS PHASE AQUEOUS PHASE SOLID PHASE TOTAL GASEOUS MERCURY (TGM) DRY AND WET DEPOSITION REACTIVE GASEOUS MERCURY (RGM) RELATIVELY INSOLUBLE ATMOSPHERIC LIFETIME: ABOUT 1 YEAR TYPICAL LEVELS: 1.7 ng m -3 LIFETIME: DAYS TO WEEKS TYPICAL LEVELS: pg m -3 Reduction Photochemical aqueous (?) Hg(II)Hg(P) ECOSYSTEM INPUTS VERY SOLUBLE EMITTED BY ANTHROPOGENIC SOURCES

CONSTRAINING POLICY-RELEVANT UNCERTAINTIES WITH A GLOBAL ATMOSPHERIC MODEL Mercury budget in GEOS-Chem Global, 3D tropospheric chemistry model (GEOS- Chem) simulation, 4x5 degree resolution [Selin et al. JGR 2007 (atmosphere); Strode et al. GBC 2007 (ocean)] Reproduces annual average concentration at 22 land-based sites, interhemispheric gradient Measured: 1.58 ± 0.19 ng/m3 Simulated: 1.63 ± 0.10 ng/m3 High Atlantic cruise data (enrichment from past decades emissions in North Atlantic?)

OXIDATION AND REDUCTION PROCESSES Seasonal variation of TGM is consistent with a photochemical oxidation of Hg(0) partially balanced by reduction of Hg(II) Observations GEOS-Chem No reduction (oxidation by OH) Diurnal variation of RGM (at Okinawa, Japan, measured by Jaffe et al. 2005) supports a photochemical source [Selin et al. JGR 2007] In most models (including GEOS-Chem) OH is the dominant Hg(0) oxidant. But the Hg+OH reaction may not occur [Calvert & Lindberg 2005] Could the dominant oxidant be Br? [Holmes et al. 2006] Observations GEOS-Chem

HIGH LEVELS OF RGM IN THE FREE TROPOSPHERE AND STRATOSPHERE Vertical profile of GEOS-Chem vs. measurements at Mt. Bachelor, Oregon (2.7 km) show elevated levels relative to surface [Swartzendruber et al. JGR 2006] ▲ =daytime ● (blue)=all ◊ = nighttime 800 mb Hg(II) fields show the influence of large-scale subsidence (contributes to high levels of Hg(II) deposition in the subtropics) [Selin et al. in prep for GBC]

DEPOSITION: LOCAL VS. GLOBAL SOURCES Two patterns of mercury wet deposition over the U.S. (background=model, dots=measured) 1)Latitudinal gradient (higher in the subtropics). From oxidation of global pool of Hg(0) and subsequent rainout; influence of subsidence. 2)Near-source wet deposition of locally-emitted Hg(II) and Hg(P) (underestimated in GEOS-Chem) Measurements [Mercury Deposition Network, 2006]; GEOS-Chem [Selin et al., JGR, 2007] % contribution of North American sources to total (wet + dry) deposition GEOS-Chem model U.S. mean: 20% Reflects influence of locally-deposited Hg(II) and Hg(P) in source regions

CONSTRAINING NATURAL AND RECYCLED SOURCES THROUGH A PRE-INDUSTRIAL MODEL Steady state assumption: -Soil Hg comes from the atmosphere (for about 90% of land area) -What goes down, must come up… GEOS-Chem (4x5) grid box Runoff: negligible Deposition = Evasion Soil volatilization: F(T, [Hg], solar radiation) Evapotranspiration: F([Hg], transp. rate) Prompt recycling: “New” Hg can be more easily reduced/emitted than resident Hg [Hintelmann et al. 2002] [Selin et al. in prep for GBC]  g m-2 y-1

EVALUATING MERCURY CYCLE AND LIFETIMES GEOS-Chem Pre-industrial Hg Cycle Quantities in Mg, Fluxes in Mg/y Hg is very long-lived in the soil (1000 y); however, the surface ocean recycles Hg efficiently (1 y) Recycling in the surface ocean more than doubles the effective atmospheric lifetime of emitted Hg Future work: coupling with intermediate/deep ocean reservoirs [Selin et al. in prep for GBC]

ESTIMATING THE ANTHROPOGENIC, RECYCLED AND NATURAL CONTRIBUTIONS TO DEPOSITION Anthropogenic Enrichment Factor (Present/Preindustrial Deposition) Deposition to the U.S.: 20% from North American anthropogenic emissions 22% from outside North America anthropogenic 26% from recycled anthropogenic emissions 32% natural [Selin et al. in prep for GBC]

TAKE-HOME MESSAGES FOR POLICY Domestic, regional, and global regulation are all important in addressing the mercury problem Hg(0), Hg(II) and Hg(P) emissions have different deposition patterns, and may need different regulatory strategies Need for better understanding of redox chemistry, and cycling in land & ocean reservoirs (will climate change have an effect?) Need for improved cross-scale governance vs. In the US, Florida and Ohio both see high deposition -- but the source patterns are very different