THE ATMOSPHERIC CYCLE OF THE ATMOSPHERIC CYCLE OF MERCURY AND THE ROLE OF MERCURY AND THE ROLE OF

COAL-BASED EMISSIONSCOAL-BASED EMISSIONS

Noelle Eckley SelinNoelle Eckley SelinHarvard UniversityHarvard University

Department of Earth and Planetary SciencesDepartment of Earth and Planetary SciencesAtmospheric Chemistry Modeling GroupAtmospheric Chemistry Modeling Group

Environmental Defense Science DayEnvironmental Defense Science Day11 May 200611 May 2006

THE MERCURY CYCLE: CURRENT

Wet & DryDeposition 2600

ATMOSPHERE5000

(3x pre-industrial)

SURFACE SOILS1,000,000 OCEAN

289,000

Wet & DryDeposition1900

Oceanic Evasion

1500

Net burial200

Land emissions1600

Quantities in Mg/year (106 g, or metric tonnes)Uncertainty ranges in parenthesesAdapted from Mason & Sheu, 2002

AnthropogenicEmissions 2400

Extraction from deep reservoirs2400

Rivers200

(1800-3600)(700-3500)(1680-3120)

(1680-3120)

(1300-2600)(700-3500)

HISTORICAL RECORD OF MERCURYHISTORICAL RECORD OF MERCURY

• Pre-industrial concentrations indicate natural source

• Episodic volcanic input

• Mining emerges• Industrialization, and

recent decrease

Source: USGS

ICE CORE FROM WYOMING

MERCURY IN THE ATMOSPHEREMERCURY IN THE ATMOSPHERE

Hg(0) Hg(II)Oxidation OH, O3, Br(?)

GAS PHASE

AQUEOUS PHASE

SOLID PHASE

TOTAL GASEOUS MERCURY (TGM)

DRY AND WET DEPOSITION

REACTIVE GASEOUS MERCURY (RGM)

RELATIVELY INSOLUBLE

ATMOSPHERIC LIFETIME: ABOUT 1 YEAR

TYPICAL LEVELS: 1.7 ng m-3

LIFETIME: DAYS TO WEEKS

TYPICAL LEVELS: 1-100 pg m-3

ReductionPhotochemical aqueous (?) Hg(II) Hg(P)

ECOSYSTEM INPUTS

VERY SOLUBLE

EMITTED BY COAL-EMITTED BY COAL-FIRED POWER PLANTSFIRED POWER PLANTS

Hg(0)4500

(3900)

Hg(II)860

(300)

OH:12000OH:12000

Dry depositionLand (primary) emission

Anthropogenic emissionLand re-emission

Hg(P)1.9

(1.9)

700700200200

OO33:2400:2400

15001500 1300130050050028002800

Dry depositionWet deposition

Wet deposition

47004700

21002100

190190 1010

Inventories in Mg(Troposphere in parentheses)Rates in Mg/yr

hv (cloud):8300

Ocean emission

MERCURY BUDGET IN GEOS-CHEM

MERCURY: ANTHROPOGENIC SOURCESMERCURY: ANTHROPOGENIC SOURCES2000 Global Emissions Inventory Activity (GEIA) inventory [Pacyna et al. 2005]

Global Totals:1990: 2143 Mg1995: 2317 Mg 2000: 2190 Mg

Source & Continent breakdown, 1995 inventory [Pacyna and Pacyna 2002]

U.S. EMISSIONS OF TOTAL HGU.S. EMISSIONS OF TOTAL HG

Decreases in emissions since 1990

Policy successes: regulation of municipal waste combustors andmedical waste incinerators

Coal is the major remaining Hg source

U.S. and Europe emissions have declined in the last decade; global total hasheld steady since 1990 because of increases from developing countries

OXIDATION AND REDUCTION: OXIDATION AND REDUCTION: SCIENTIFIC QUESTIONSSCIENTIFIC QUESTIONS

Seasonal variation of TGMMeasurements, GEOS-Chem model, OH only, O3 only

RGM measurements at Okinawa, JapanMeasurements, GEOS-Chem model,

Seasonal variation shows influence of photochemical oxidation coupled with reduction

Diurnal variation consistent with photochemical oxidation

[Selin et al. 2006, JGR, submitted]

But is bromine involved?

A HIGH-ALTITUDE RGM SOURCE?A HIGH-ALTITUDE RGM SOURCE?

Thick line: Hg(0), Thin line: Hg(II)Mercury with altitude in GEOS-Chem model

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]

GEOS-Chem simulated Hg(II)+Hg(P)compared with measurements

DEPOSITION: LOCAL VS. GLOBAL SOURCESDEPOSITION: LOCAL VS. GLOBAL SOURCESTwo patterns of mercury wet

deposition over the U.S.

(background=model, dots=measured)

1) Latitudinal gradient (higher in warm, sunny, wet places, e.g. Florida, Texas). From oxidation of global pool of Hg(0) and subsequent rainout

2) Near-source wet deposition of locally-emitted Hg(II) and Hg(P) (underestimated in GEOS-Chem)

Measurements [Mercury Deposition Network, 2006]; GEOS-Chem [Selin et al. 2006]

% contribution of North Americansources 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

FUTURE SCENARIOS: CYCLING OF “NEW” VS. FUTURE SCENARIOS: CYCLING OF “NEW” VS. “OLD” HG IN LAND-OCEAN-ATMOSPHERE “OLD” HG IN LAND-OCEAN-ATMOSPHERE

SYSTEMSYSTEM

Soils: Large pool of mercury &Potential sink for atmospheric Hg

Emissions are sensitive to temperature, solar radiation, precipitation

Re-emission 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

Ocean: 75% of source is “re-emission”according to GEOS-Chem model

[Strode et al. 2006]

“New” mercury may be preferentiallytransformed into methyl mercury

[METAALICUS study (Mercury Experiment To Assess Atmospheric Loading In Canada and the UnitedStates); ACME study (Aquatic Cyclingof Mercury in the Everglades)]

Vegetation: can be a significant mercury source [Lindberg et al. 1998]

LINKS BETWEEN HG AND CLIMATE CHANGELINKS BETWEEN HG AND CLIMATE CHANGE

AMAP, 2003

Incoming solar radiation

Precipitation(Rain/Snow)

Ice Cover and gas exchange

Air transport patterns

MESSAGES FOR POLICYMESSAGES 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

COAUTHORS AND ACKNOWLEDGMENTSCOAUTHORS 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

Extra slides followExtra slides follow

COULD THE HG(0) OXIDANT BE BROMINE?COULD THE HG(0) OXIDANT BE BROMINE?

AMAP, 2003

Lifetime of Hg(0) against oxidation byBr [Holmes et al. 2006, GRL, submitted]

Time series of Hg(0) at Zeppelin (Arctic),Spring 2000[Berg et al. 2003]

• Bromine implicated in Arctic “Mercury Depletion Events”

• Some evidence of rapid oxidation in marine boundary layer

• But could it be globally important?