Global Transport of Mercury (Hg) Compounds

Noelle EckleyEPS Second Year Symposium

22-23 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 (Hg0):

– Predominant form in the atmosphere (98%)

– Relatively insoluble

• Divalent Mercury (Hg(II)):– Primarily as HgCl2 in the

atmosphere– Very soluble– Undergoes Wet and Dry

Deposition

• Particulate Mercury (HgP)

MEASUREMENTS• Total Gaseous Mercury

(TGM) = Hg0+Hg(II)(g)

• Reactive Gaseous Mercury (RGM) = Hg(II)(g)

• Particulate Mercury (HgP)

Typical concentrations:

TGM: 1.7 ng m-3 (NH)RGM:10-200 pg m-3

HgP: 1-100 pg m-3

Mercury Atmospheric Cycling

• Oxidation reactions in the gas phase:– Hg0 + OH Hg(II)

• k=8.7(+/-2.8) x 10-14 cm3 s-1 (Sommar et al. 2001) (?)

– Hg0 + O3 Hg(II)• k=3(+/-2) x 10-20 cm3 s-1 (Hall 1995)

• Wet and dry deposition of Hg(II), HgP

• Other reactions (not included in model): aqueous chemistry; HgP chemistry

GEOS-CHEM Hg Budget: Comparison with other models

Shia et al. 1999

Seigneur et al. 2001

Lamborget al. 2001

Bergan et al. 1999

GEOS-GEOS-CHEMCHEM

Anthropogenic 2140 2106.6 2607 2150 2220

Natural Sources Total (incl. reemission)

4000 4000 1805 3900 4000

Total Deposition 6150 4212 6281

Wet Deposition 2860 5330

Dry Deposition 3290 951

Residence Time (yr) 1.7 1.13 1.8 1 (fixed) 0.4

Total Amount in Atmosphere

5215 2569

Production of Hg2 5426

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

TGM at Cape Point, measured vs. modeled

0

0.5

1

1.5

2

2.5

1 2 3 4 5 6 7 8 9 10 11 12

Month (1998 modeled, measured average)

TGM, ng/m3Measured

Modeled

TGM at Zeppelin, Measured vs Modeled

0

0.5

1

1.5

2

2.5

1 2 3 4 5 6 7 8 9 10 11 12

Month (1998 model; 2001 meas)

TGM (ng/m3) Measured

Modeled

Future work: Next Steps

• Model improvements: HgP 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