THE ATMOSPHERIC CYCLE OF MERCURY AND THE ROLE OF COAL-BASED EMISSIONS Noelle Eckley Selin Harvard University Department of Earth and Planetary Sciences Atmospheric Chemistry Modeling Group Environmental Defense Science Day 11 May 2006

THE MERCURY CYCLE: CURRENT 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 ( ) ( ) ( ) ( ) ( )

HISTORICAL RECORD OF MERCURY Pre-industrial concentrations indicate natural source Episodic volcanic input Mining emerges Industrialization, and recent decrease Source: USGS ICE CORE FROM WYOMING

MERCURY IN THE ATMOSPHERE 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 COAL- FIRED POWER PLANTS

Hg(0) 4500 (3900) Hg(II) 860 (300) OH:12000 Dry deposition Land (primary) emission Anthropogenic emission Land re-emission Hg(P) 1.9 (1.9) O 3 : Dry deposition Wet deposition Inventories in Mg (Troposphere in parentheses) Rates in Mg/yr hv (cloud):8300 Ocean emission MERCURY BUDGET IN GEOS-CHEM

MERCURY: ANTHROPOGENIC SOURCES 2000 Global Emissions Inventory Activity (GEIA) inventory [Pacyna et al. 2005] Global Totals: 1990: 2143 Mg 1995: 2317 Mg 2000: 2190 Mg Source & Continent breakdown, 1995 inventory [Pacyna and Pacyna 2002]

U.S. EMISSIONS OF TOTAL HG Decreases in emissions since 1990 Policy successes: regulation of municipal waste combustors and medical waste incinerators Coal is the major remaining Hg source U.S. and Europe emissions have declined in the last decade; global total has held steady since 1990 because of increases from developing countries

OXIDATION AND REDUCTION: SCIENTIFIC QUESTIONS Seasonal variation of TGM Measurements, GEOS-Chem model, OH only, O 3 only RGM measurements at Okinawa, Japan Measurements, GEOS-Chem model, Seasonal variation shows influence of photochemical oxidation coupled with reduction Diurnal variation consistent with photochemical oxidation [Selin et al. 2006, JGR, submitted] But is bromine involved?

A HIGH-ALTITUDE RGM SOURCE? Thick line: Hg(0), Thin line: Hg(II) Mercury with altitude in GEOS-Chem model Measurements of RGM at Mt. Bachelor, Oregon (2.7 km) show elevated levels relative to surface measurements mean 43 pg m -3 [Swartzendruber et al. 2006, JGR, submitted] GEOS-Chem simulated Hg(II)+Hg(P) compared with measurements

DEPOSITION: LOCAL VS. GLOBAL SOURCES Two patterns of mercury wet deposition over the U.S. (background=model, dots=measured) 1)Latitudinal gradient (higher in warm, sunny, wet places, e.g. Florida, Texas). From oxidation of global pool of Hg(0) and subsequent rainout 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. 2006] % 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

FUTURE SCENARIOS: CYCLING OF “NEW” VS. “OLD” HG IN LAND-OCEAN-ATMOSPHERE SYSTEM Soils: Large pool of mercury & Potential sink for atmospheric Hg Emissions are sensitive to temperature, solar radiation, precipitation Re-emission estimates: 5-10% of deposited mercury re-emits over a year [Schlüter 2000; Hintelmann et al. 2002]; “new” mercury may be more available for re-emission Ocean: 75% of source is “re-emission” according to GEOS-Chem model [Strode et al. 2006] “New” mercury may be preferentially transformed into methyl mercury [METAALICUS study (Mercury Experiment To Assess Atmospheric Loading In Canada and the United States); ACME study (Aquatic Cycling of Mercury in the Everglades)] Vegetation: can be a significant mercury source [Lindberg et al. 1998]

LINKS BETWEEN HG AND CLIMATE CHANGE AMAP, 2003 Incoming solar radiation Precipitation (Rain/Snow) Ice Cover and gas exchange Air transport patterns

MESSAGES FOR POLICY Role of anthropogenic and natural sources in mercury cycle –We know: more Hg is being mobilized than ever before –Re-mobilization of Hg from soil and ocean are major uncertainties in the global budget, and may be significant in future climates –New Hg may act differently from old, and this may be a source of optimism Difference between Hg(0) and Hg(II) and significance for regional and global contamination –We know: Hg(II) and Hg(P) are associated with regional deposition; Hg(0) is a global problem –Neither an international treaty nor domestic regulation alone will solve the problem – need for a multi-scale approach

COAUTHORS AND ACKNOWLEDGMENTS D.J. Jacob, R.J. Park, R.M. Yantosca, C. Holmes (Harvard) S. Strode, L. Jaegle, D. Jaffe (University of Washington) U.S. National Science Foundation Atmospheric Chemistry Program U.S. Environmental Protection Agency STAR Research Fellowship U.S. EPA Intercontinental Transport of Air Pollutants (ICAP) program

Extra slides follow

COULD THE HG(0) OXIDANT BE BROMINE? AMAP, 2003 Lifetime of Hg(0) against oxidation by Br [Holmes et al. 2006, GRL, submitted] Time series of Hg(0) at Zeppelin (Arctic), Spring 2000 [Berg et al. 2003] Bromine implicated in Arctic “Mercury Depletion Events” Some evidence of rapid oxidation in marine boundary layer But could it be globally important?