THE ATMOSPHERIC CYCLE OF MERCURY AND THE ROLE OF COAL-BASED EMISSIONS

1. 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

2. THE MERCURY CYCLE: CURRENT ATMOSPHERE 5000 (3x pre-industrial) Anthropogenic Emissions 2400 Land emissions 1600 Wet & Dry Deposition 2600 Wet & Dry Deposition 1900 Oceanic Evasion 1500 (1680-3120) (700-3500) (1800-3600) (1300-2600) (700-3500) SURFACE SOILS 1,000,000 OCEAN 289,000 Extraction from deep reservoirs 2400 Rivers 200 (1680-3120) Quantities in Mg/year (106 g, or metric tonnes) Uncertainty ranges in parentheses Adapted from Mason & Sheu, 2002 Net burial 200

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

4. MERCURY IN THE ATMOSPHERE REACTIVE GASEOUS MERCURY (RGM) TOTAL GASEOUS MERCURY (TGM) GAS PHASE AQUEOUS PHASE Oxidation OH, O3, Br(?) Hg(II) Hg(0) SOLID PHASE VERY SOLUBLE RELATIVELY INSOLUBLE ATMOSPHERIC LIFETIME: ABOUT 1 YEAR TYPICAL LEVELS: 1.7 ng m-3 Reduction Photochemical aqueous (?) Hg(II) Hg(P) LIFETIME: DAYS TO WEEKS TYPICAL LEVELS: 1-100 pg m-3 DRY AND WET DEPOSITION EMITTED BY COAL- FIRED POWER PLANTS ECOSYSTEM INPUTS

5. MERCURY BUDGET IN GEOS-CHEM Hg(0) 4500 (3900) OH:12000 Hg(II) 860 (300) Hg(P) 1.9 (1.9) O3:2400 hv (cloud):8300 200 700 1300 1500 500 2800 Anthropogenic emission 4700 Land re-emission 190 10 Ocean emission 2100 Land (primary) emission Dry deposition Wet deposition Inventories in Mg (Troposphere in parentheses) Rates in Mg/yr Wet deposition Dry deposition

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

7. 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

8. OXIDATION AND REDUCTION: SCIENTIFIC QUESTIONS Seasonal variation of TGM Measurements, GEOS-Chem model, OH only, O3 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 But is bromine involved? [Selin et al. 2006, JGR, submitted]

9. A HIGH-ALTITUDE RGM SOURCE? Mercury with altitude in GEOS-Chem model Thick line: Hg(0), Thin line: Hg(II) GEOS-Chem simulated Hg(II)+Hg(P) compared with measurements 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]

10. DEPOSITION: LOCAL VS. GLOBAL SOURCES • Two patterns of mercury wet deposition over the U.S. • (background=model, dots=measured) • Latitudinal gradient (higher in warm, sunny, wet places, e.g. Florida, Texas). From oxidation of global pool of Hg(0) and subsequent rainout • Near-source wet deposition of locally-emitted Hg(II) and Hg(P) (underestimated in GEOS-Chem) % 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 Measurements [Mercury Deposition Network, 2006]; GEOS-Chem [Selin et al. 2006]

11. FUTURE SCENARIOS: CYCLING OF “NEW” VS. “OLD” HG IN LAND-OCEAN-ATMOSPHERE SYSTEM Vegetation: can be a significant mercury source [Lindberg et al. 1998] 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)] 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

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

13. 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

14. 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

15. Extra slides follow

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