MERCURY IN THE ATMOSPHERE, BIOSPHERE, MERCURY IN THE ATMOSPHERE, BIOSPHERE, AND POLICY SPHERE: AND POLICY SPHERE:

Constraints from a global 3D land-ocean-atmosphere model on mercury sources, cycling and deposition

Noelle Eckley SelinHarvard UniversityHarvard University

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

Princeton UniversityPrinceton University24 May 200724 May 2007

Coauthors: D.J. Jacob, R.J. Park, R.M. Yantosca (Harvard)S. Strode, L. Jaegle, D. Jaffe, P. Swartzendruber (U. Washington)

MERCURY POLLUTION: A SCIENCE & POLICY PROBLEMMERCURY POLLUTION: A SCIENCE & POLICY PROBLEM

Ice core record of deposition from Wyoming, USA[Schuster et al., ES&T 2002]

Mercury in polar bear fur up 5-12X since 1890, [Dietz et al., ES&T 2006]

States with fish mercury advisories [EPA, 2004]

Mercury deposition has increased by 300% since industrialization

Growing concern about human exposure through methylmercury in fish

Particular concern in Arctic ecosystems due to bioaccumulation, human exposure

POLITICAL ACTIONS AND UNCERTAINTIES

GLOBAL:2002: Global Mercury Assessment: sufficient evidence to warrant international action2003, 2005, 2007: UNEP Governing Council meetings reject proposed global mercury agreement. Mercury Programme and voluntary partnerships established.

[Selin, Environment, 2005; Selin and Selin, RECIEL, 2006]

U.S. :2005: CLEAN AIR MERCURY RULE establishes a “cap and trade” approach to regulating mercury from coal-fired power plants

REGIONAL:-U.S./Mexico/Canada regional action plan under Commission for Environmental Cooperation (1997,2000)-U.S./Canada/Europe/former Soviet Union countries agreement on heavy metals under Convention on Long Range Transboundary Air Pollution (1998)

• What are the relative contributions of global, regional and domestic sources to deposition?

• What is the impact of anthropogenic emissions (past & present) on the global mercury cycle?

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)

SCEINTIFIC UNCERTAINTIES: SOURCES AND SINKS

SCIENTIFIC UNCERTAINTIES: ATMOSPHERIC CHEMISTRYSCIENTIFIC UNCERTAINTIES: ATMOSPHERIC CHEMISTRY

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 EMITTED BY ANTHROPOGENIC ANTHROPOGENIC SOURCESSOURCES

CONSTRAINING POLICY-RELEVANT UNCERTAINTIES WITH A GLOBAL ATMOSPHERIC MODEL

Mercury budget in GEOS-Chem

Global, 3D tropospheric chemistry model (GEOS-Chem) simulation, 4x5 degree resolution

[Selin et al. JGR 2007 (atmosphere); Strode et al. GBC 2007 (ocean)]

Reproduces annual average concentration at 22 land-based sites, interhemispheric gradientMeasured: 1.58 ± 0.19 ng/m3Simulated: 1.63 ± 0.10 ng/m3

High Atlantic cruise data (enrichment from past decades emissions in North Atlantic?)

OXIDATION AND REDUCTION PROCESSES• Seasonal variation of TGM is consistent with a photochemical oxidation of Hg(0) partially balanced by reduction of Hg(II)

ObservationsGEOS-ChemNo reduction (oxidation by OH)

• Diurnal variation of RGM (at Okinawa, Japan, measured by Jaffe et al. 2005) supports a photochemical source

[Selin et al. JGR 2007]

• In most models (including GEOS-Chem) OH is the dominant Hg(0) oxidant.

• But the Hg+OH reaction may not occur [Calvert & Lindberg 2005]

•Could the dominant oxidant be Br? [Holmes et al. 2006]

ObservationsGEOS-Chem

HIGH LEVELS OF RGM IN THE FREE TROPOSPHERE AND STRATOSPHERE

Vertical profile of GEOS-Chem vs.measurements at Mt. Bachelor, Oregon (2.7 km) show elevated levels relative to surface [Swartzendruber et al. JGR 2006]

▲=daytime

● (blue)=all

◊ = nighttime

800 mb Hg(II) fields show the influence of large-scale subsidence (contributes to high levels of Hg(II) deposition in the

subtropics) [Selin et al. in prep for GBC]

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 the subtropics). From oxidation of global pool of Hg(0) and subsequent rainout; influence of subsidence.

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., JGR, 2007]

% 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

CONSTRAINING NATURAL AND RECYCLED SOURCES CONSTRAINING NATURAL AND RECYCLED SOURCES THROUGH A PRE-INDUSTRIAL MODELTHROUGH A PRE-INDUSTRIAL MODEL

Steady state assumption:-Soil Hg comes from the atmosphere (for about 90% of land area)-What goes down, must come up…

GEOS-Chem (4x5) grid box Runoff:

negligible

Deposition = Evasion

Soil volatilization: F(T, [Hg], solar radiation)

Evapotranspiration:F([Hg], transp. rate)

Prompt recycling: “New” Hg can be more easily reduced/emitted than resident Hg[Hintelmann et al. 2002]

[Selin et al. in prep for GBC]g m-2 y-1

EVALUATING MERCURY CYCLE AND LIFETIMESEVALUATING MERCURY CYCLE AND LIFETIMESGEOS-Chem Pre-industrial Hg Cycle

Quantities in Mg, Fluxes in Mg/y

Hg is very long-lived in the soil (1000 y); however, the surface ocean recycles Hg efficiently (1 y)

Recycling in the surface ocean more than doubles the effective atmospheric lifetime of emitted Hg

Future work: coupling with intermediate/deep ocean reservoirs

[Selin et al. in prep for GBC]

ESTIMATING THE ANTHROPOGENIC, RECYCLED AND NATURAL CONTRIBUTIONS TO DEPOSITION

Anthropogenic Enrichment Factor (Present/Preindustrial Deposition)

Table 1: Total deposition to the U.S. from natural, anthropogenic, and recycled emissions (Mg) Source Dry Hg(0) Wet Hg(II)+(P) Dry Hg(II)+(P) Total Natural 39 15 29 83 (32%) Primary Anthropogenic (North America)

8 14 30 52 (20%)

Primary Anthropogenic (Outside North America)

24 11 22 57 (22%)

Recycled Anthropogenic 29 14 25 68 (26%) Total 100 54 106 260 (100%)

Deposition to the U.S.:20% from North American anthropogenic emissions

22% from outside North America anthropogenic

26% from recycled anthropogenic emissions

32% natural

[Selin et al. in prep for GBC]

TAKE-HOME MESSAGES FOR POLICY

• Domestic, regional, and global regulation are all important in addressing the mercury problem

• Hg(0), Hg(II) and Hg(P) emissions have different deposition patterns, and may need different regulatory strategies

• Need for better understanding of redox chemistry, and cycling in land & ocean reservoirs (will climate change have an effect?)

• Need for improved cross-scale governance

vs.

In the US, Florida and Ohio both see high deposition -- but the source patterns are very different