Global Modeling of Mercury in the Atmosphere using the GEOS-CHEM model

Noelle Eckley, Rokjin Park, Daniel Jacob

30 January 2004

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 – Some recent decreases in emissions in

Europe, North America, emissions increasing in Asia

– Accumulates in food webs as methyl mercury; risk to humans & environment

• Fish consumption advisories• Arctic pollution problem

Regional, national, and international policy interest

• U.S. state and regional regulations, and some progress from pollution reduction co-benefits – further national action coming soon?

• UNEP Governing Council (2/2003): agreed that further international policy action needed. UNEP will revisit issue in 2005

Source: UNEP Global Mercury Assessment

Hg in the atmosphere:3 species: •elemental Hg (Hg0)•divalent Hg (Hg(II))• particulate Hg (HgP).

Hg0 reacts chemically with OH, O3 to form Hg(II)

HgII and HgP undergo wet and dry deposition

Measurements: Total Gaseous Mercury (TGM) = Hg0+Hg(II)(g)Reactive Gaseous Mercury (RGM) = Hg(II)(g)Particulate mercury (TPM)

Typical concentrations:TGM: 1.7 ng m-3 (NH)RGM:10-200 pg m-3 HgP: 1-100 pg m-3

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

GEOS-CHEM model

• Atmospheric chemistry and transport model, used extensively at Harvard and elsewhere for oxidant-aerosol chemistry

• 2x2.5 degree lon/lat resolution, 30 vertical layers, assimilated meteorology from NASA-GMAO

GEOS-CHEM mercury simulation

• 1995 anthropogenic emissions (GEIA 2002), land and ocean emissions (primary and re-emission)

• 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) • Realistic results at k=5.9 x 10-14 cm3 s-1 , lower end of

uncertainty

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

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

• Not included in model: aqueous chemistry; HgP

chemistry not yet included

GEOS-CHEM mercury simulation results

• Total Gaseous Mercury (TGM)

• Results in the range of expected values

Comparing Model with Measurements: Longitudinal Average TGM

• Basic agreement with hemispheric average concentrations and interhemispheric gradient

Lamborg et al. 2002 GEOS-CHEM

GEOS-CHEM MERCURY SIMULATION: COMPARISON WITH SITE MEASUREMENTS

Site Lat. Long. Measured Simulated (GEOS-CHEM)

Model Error

Alert, Nunavut, Canada 82.5 N 62.3 W 1.54 1.73 +11.7%

Zeppelin, Norway 78.5 N 11.5 E 1.56 1.88 +20.6%

Pallas, Finland 68 N 24 E 1.50 1.94 +29.3%

Rörvik, Sweden 57 N 25 E 1.53 1.98 +29.4%

Mace Head, Ireland 53.7 N 9.6 W 1.73 1.43 -17.3%

Delta, British Columbia, Canada

49.1 N 123.1 W 1.72 1.43 -16.7%

Cheeka Peak, Washington, USA

48.3 N 124.6 W 1.56 1.43 -8.0%

St. Andrews, New Brunswick, Canada

45.1 N 67.0 W 1.43 1.60 +12.0%

Kejimujik, Nova Scotia, Canada

44.4 N 65.2 W 1.45 1.60 +9.9%

Guiyang, China 26 N 106 E 9.00 5.78 -64.2%

Cape Point, South Africa 34.4 S 18.5 E 1.39 1.31 -5.2%

Simulated vs. measured TGM at selected sites, annual average ( ng/m3)

Comparison with US Mercury Deposition Network (MDN) measurements

Wet Deposition Hg, Model vs. Meas (ug/m2), 1998

0

5

10

15

20

25

30

0 5 10 15 20 25Measured

Mod

eled

(GE

OS

-CH

EM

)

R2=0.77

“Mercury Depletion Events” (MDEs) in the Arctic

• Episodic depletion of TGM at polar sunrise

• Correlates with Arctic O3 depletion events

• Mechanism: conversion to Hg(II) and subsequent deposition

• Proposed mechanism: reaction with BrO?

AMAP, 2002

Future work: Next Steps

• Model improvements: HgP chemistry

• Arctic behavior – testing the proposed mechanism for mercury depletion events (using GOME BrO columns)

• Tagged source simulation: how much Hg deposition comes from where?

• Resolving uncertainties in Hg chemistry (BrO in marine boundary layer?)

Acknowledgments

• Advisor: Prof. Daniel J. Jacob, Harvard University

• Collaborators: Dr. Rokjin Park, Bob Yantosca

• Funding sources: U.S. National Science Foundation Graduate Research Fellowship; Harvard University Committee on the Environment

Anthropogenic Sources

Source: Pacyna and Pacyna, 2002

Historical Record of Mercury from Ice Core

• Pre-industrial concentrations indicate natural source

• Episodic volcanic input

• Mining emerges• Industrialization, and

recent decrease

Source: USGS