THE ATMOSPHERIC CYCLE OF MERCURY AND THE ROLE OF COALBASED EMISSIONS

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THE ATMOSPHERIC CYCLE OF MERCURY AND THE ROLE OF
COAL-BASED EMISSIONS

• Noelle Eckley Selin
• Harvard University
• Department of Earth and Planetary Sciences
• Atmospheric Chemistry Modeling Group
• Environmental Defense Science Day
• 11 May 2006

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THE MERCURY CYCLE CURRENT
ATMOSPHERE 5000 (3x pre-industrial)
Anthropogenic Emissions 2400
Land emissions 1600
Wet Dry Deposition 2600
Wet Dry Deposition 1900
Oceanic Evasion 1500
(1680-3120)
(700-3500)
(1800-3600)
(1300-2600)
(700-3500)
SURFACE SOILS 1,000,000
OCEAN 289,000
Extraction from deep reservoirs 2400
Rivers 200
(1680-3120)
Quantities in Mg/year (106 g, or metric
tonnes) Uncertainty ranges in parentheses Adapted
from Mason Sheu, 2002
Net burial 200
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HISTORICAL RECORD OF MERCURY
ICE CORE FROM WYOMING

• Pre-industrial concentrations indicate natural
  source
• Episodic volcanic input
• Mining emerges
• Industrialization, and recent decrease

Source USGS
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MERCURY IN THE ATMOSPHERE
REACTIVE GASEOUS MERCURY (RGM)
TOTAL GASEOUS MERCURY (TGM)
GAS PHASE
AQUEOUS PHASE
Hg(II)
Oxidation OH, O3, Br(?)
Hg(0)
SOLID PHASE
VERY SOLUBLE
RELATIVELY INSOLUBLE ATMOSPHERIC LIFETIME
ABOUT 1 YEAR TYPICAL LEVELS 1.7 ng m-3
Reduction Photochemical aqueous (?)
Hg(II)
Hg(P)
LIFETIME DAYS TO WEEKS TYPICAL LEVELS
1-100 pg m-3
DRY AND WET DEPOSITION
EMITTED BY COAL- FIRED POWER PLANTS
ECOSYSTEM INPUTS
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MERCURY BUDGET IN GEOS-CHEM
Hg(0) 4500 (3900)
Hg(II) 860 (300)
OH12000
Hg(P) 1.9 (1.9)
O32400
hv (cloud)8300
200
700
1300
1500
500
2800
Anthropogenic emission
4700
Land re-emission
190
10
2100
Ocean emission
Land (primary) emission
Dry deposition
Wet deposition
Inventories in Mg (Troposphere in
parentheses) Rates in Mg/yr
Wet deposition
Dry deposition
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MERCURY ANTHROPOGENIC SOURCES
2000 Global Emissions Inventory Activity (GEIA)
inventory Pacyna et al. 2005
Source Continent breakdown, 1995 inventory
Pacyna and Pacyna 2002
Global Totals 1990 2143 Mg 1995 2317 Mg 2000
2190 Mg
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U.S. EMISSIONS OF TOTAL HG
Decreases in emissions since 1990
Policy successes regulation of municipal waste
combustors and medical waste incinerators
Coal is the major remaining Hg source
U.S. and Europe emissions have declined in the
last decade global total has held steady since
1990 because of increases from developing
countries
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OXIDATION AND REDUCTION SCIENTIFIC QUESTIONS
Seasonal variation of TGM Measurements, GEOS-Chem
model, OH only, O3 only

RGM measurements at Okinawa, Japan Measurements,
GEOS-Chem model,
Seasonal variation shows influence of
photochemical oxidation coupled with reduction
Diurnal variation consistent with photochemical
oxidation
But is bromine involved?
Selin et al. 2006, JGR, submitted
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A HIGH-ALTITUDE RGM SOURCE?
Mercury with altitude in GEOS-Chem model
Thick line Hg(0), Thin line Hg(II)
GEOS-Chem simulated Hg(II)Hg(P) compared with
measurements
Measurements of RGM at Mt. Bachelor, Oregon (2.7
km) show elevated levels relative to surface
measurements mean 43 pg m-3 Swartzendruber et
al. 2006, JGR, submitted
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DEPOSITION LOCAL VS. GLOBAL SOURCES

• Two patterns of mercury wet deposition over the
  U.S.
• (backgroundmodel, dotsmeasured)
• Latitudinal gradient (higher in warm, sunny, wet
  places, e.g. Florida, Texas). From oxidation of
  global pool of Hg(0) and subsequent rainout
• Near-source wet deposition of locally-emitted
  Hg(II) and Hg(P) (underestimated in GEOS-Chem)

contribution of North American sources to total
(wet dry) deposition GEOS-Chem model U.S.
mean 20 Reflects influence of
locally-deposited Hg(II) and Hg(P) in source
regions
Measurements Mercury Deposition Network, 2006
GEOS-Chem Selin et al. 2006
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FUTURE SCENARIOS CYCLING OF NEW VS. OLD HG
IN LAND-OCEAN-ATMOSPHERE SYSTEM
Vegetation can be a significant mercury source
Lindberg et al. 1998
Ocean 75 of source is re-emission according
to GEOS-Chem model Strode et al. 2006 New
mercury may be preferentially transformed into
methyl mercury METAALICUS study (Mercury
Experiment To Assess Atmospheric Loading In
Canada and the United States) ACME study
(Aquatic Cycling of Mercury in the Everglades)
Soils Large pool of mercury Potential sink for
atmospheric Hg Emissions are sensitive to
temperature, solar radiation, precipitation Re-em
ission estimates 5-10 of deposited mercury
re-emits over a year Schlüter 2000 Hintelmann
et al. 2002 new mercury may be more available
for re-emission
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LINKS BETWEEN HG AND CLIMATE CHANGE
Incoming solar radiation
Precipitation (Rain/Snow)
Air transport patterns
Ice Cover and gas exchange
AMAP, 2003
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MESSAGES FOR POLICY

• Role of anthropogenic and natural sources in
  mercury cycle
• We know more Hg is being mobilized than ever
  before
• Re-mobilization of Hg from soil and ocean are
  major uncertainties in the global budget, and may
  be significant in future climates
• New Hg may act differently from old, and this may
  be a source of optimism
• Difference between Hg(0) and Hg(II) and
  significance for regional and global
  contamination
• We know Hg(II) and Hg(P) are associated with
  regional deposition Hg(0) is a global problem
• Neither an international treaty nor domestic
  regulation alone will solve the problem need
  for a multi-scale approach

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COAUTHORS AND ACKNOWLEDGMENTS

• D.J. Jacob, R.J. Park, R.M. Yantosca, C. Holmes
  (Harvard)
• S. Strode, L. Jaegle, D. Jaffe (University of
  Washington)
• U.S. National Science Foundation Atmospheric
  Chemistry Program
• U.S. Environmental Protection Agency STAR
  Research Fellowship
• U.S. EPA Intercontinental Transport of Air
  Pollutants (ICAP) program

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Extra slides follow
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COULD THE HG(0) OXIDANT BE BROMINE?
Time series of Hg(0) at Zeppelin (Arctic), Spring
2000 Berg et al. 2003
Lifetime of Hg(0) against oxidation by Br Holmes
et al. 2006, GRL, submitted

• Bromine implicated in Arctic Mercury Depletion
  Events
• Some evidence of rapid oxidation in marine
  boundary layer

• But could it be globally important?

AMAP, 2003