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

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

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

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

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

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: pg m -3 Hg P : pg m -3

Mercury Atmospheric Cycling Oxidation reactions in the gas phase: –Hg 0 + OH  Hg(II) k=8.7(+/-2.8) x cm 3 s -1 (Sommar et al. 2001) (?) –Hg 0 + O 3  Hg(II) k=3(+/-2) x 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

GEOS-CHEM Hg Budget: Comparison with other models Shia et al Seigneur et al Lamborg et al Bergan et al. 1999GEOS-CHEM Anthropogenic Natural Sources Total (incl. reemission) Total Deposition Wet Deposition Dry Deposition Residence Time (yr) (fixed)0.4 Total Amount in Atmosphere Production of Hg25426

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

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

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