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
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