1. Global Transport of Mercury (Hg) Compounds Noelle Eckley EPS Second Year Symposium 22-23 September 2003 Photo: AMAP & Geological Museum, Copenhagen

2. Outline Introduction: What is mercury pollution and why is it an important issue? Scientific questions and research methods Mercury in the atmosphere: an overview Preliminary model results and evaluation Future research plans

3. Why are we interested in mercury transport? Mercury (Hg) is a global environmental pollutant –Current atmospheric concentrations are 3x higher than in pre-industrial times –Accumulates in food webs as methyl mercury; risk to humans & environment Fish consumption advisories Arctic pollution problem Regional, national and international policy interest –National regulation by EPA, new proposals under “Clear Skies” initiative –UNEP Governing Council (2/2003): agreed that further international policy action needed, but action was blocked by U.S. UNEP will revisit issue in 2005

4. Historical Record of Mercury from Ice Core Data Pre-industrial concentrations indicate natural source Episodic volcanic input Mining emerges Industrialization, and recent decrease Source: USGS

5. Scientific Questions & Research Methods What are the processes influencing the transport and fate of mercury in the atmosphere? How does mercury reach the Arctic environment? What pathways are important in the Arctic atmosphere? How do pathways and concentrations change over time? Will mercury transport be influenced by global climatic changes? What is the relative importance of natural vs. anthropogenic sources in controlling deposition in different regions? Method: Model global transport and chemistry of mercury species using GEOS-CHEM model

6. Mercury in the Atmosphere MERCURY SPECIES Elemental Mercury (Hg 0 ): –Predominant form in the atmosphere (98%) –Relatively insoluble Divalent Mercury (Hg(II)): –Primarily as HgCl 2 in the atmosphere –Very soluble –Undergoes Wet and Dry Deposition Particulate Mercury (Hg P ) MEASUREMENTS Total Gaseous Mercury (TGM) = Hg 0 +Hg(II) (g) Reactive Gaseous Mercury (RGM) = Hg(II) (g) Particulate Mercury (Hg P ) Typical concentrations: TGM: 1.7 ng m -3 (NH) RGM:10-200 pg m -3 Hg P : 1-100 pg m -3

7. Mercury Atmospheric Cycling Oxidation reactions in the gas phase: –Hg 0 + OH  Hg(II) k=8.7(+/-2.8) x 10 -14 cm 3 s -1 (Sommar et al. 2001) (?) –Hg 0 + O 3  Hg(II) k=3(+/-2) x 10 -20 cm 3 s -1 (Hall 1995) Wet and dry deposition of Hg(II), Hg P Other reactions (not included in model): aqueous chemistry; Hg P chemistry

8. GEOS-CHEM Hg Budget: Comparison with other models Shia et al. 1999 Seigneur et al. 2001 Lamborg et al. 2001 Bergan et al. 1999GEOS-CHEM Anthropogenic21402106.6260721502220 Natural Sources Total (incl. reemission) 4000 180539004000 Total Deposition615042126281 Wet Deposition28605330 Dry Deposition3290951 Residence Time (yr)1.71.131.81 (fixed)0.4 Total Amount in Atmosphere 52152569 Production of Hg25426

10. Comparing Model with Measurements: Longitudinal Average TGM GEOS-CHEM underestimates TGM concentrations in the Southern hemisphere and overestimates the interhemispheric gradient Lamborg et al. 2002 GEOS-CHEM

12. Future work: Next Steps Model improvements: Hg P chemistry, dry deposition; Hg(II) production mechanisms and rates Evaluation of pathways and source identification Modeling Arctic behavior Evaluating multimedia behavior of mercury; linking sources to effects through modeling

13. Acknowledgments Advisor: Prof. Daniel J. Jacob Rokjin Park, Bob Yantosca, other postdocs and graduate students of the Jacob group Funding sources: NSF Graduate Research Fellowship; Harvard University Committee on the Environment