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Building a Global Modeling Capability for Mercury with GEOS-CHEM

Noelle Eckley Selin

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GEOS-CHEM meeting

6 April 2005

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THE MERCURY CYCLE: CURRENT

Wet & Dry

Deposition

2600

ATMOSPHERE

5000

SURFACE SOILS

1,000,000

OCEAN

289,000

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Net

Wet & Dry

Deposition

1900

Net Oceanic

Evasion

1500

Net burial

200

Land emissions

1600

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Quantities in Mg/year

Uncertainty ranges in parentheses

Adapted from Mason & Sheu, 2002

Anthropogenic

Emissions

2400

Extraction from deep reservoirs

2400

River

200

(1800-3600)

(700-3500)

(1680-3120)

(1680-3120)

(1300-2600)

(700-3500)

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Hg0

1.7 ng/m3

Gaseous

Phase

Aqueous

Phase

Hg0

Henry’s Constant

0.11 M/atm

Particulate �Phase

Oxidation

Hg2+

10-200 pg/m3

HgP

1-100 pg/m3

Hg2+

k=8.7(+/-2.8) x 10-14 cm3 s-1 (Sommar et al. 2001)

k=9.0(+/-1.3) x 10-14 cm3 s-1(Pal & Ariya 2004)

k=3(+/-2) x 10-20 cm3 s-1 (Hall 1995)

Reported rate constants up to k=1.7 x 10-18 cm3 s-1

Henry’s Constant

1.4x106 M/atm

OH

O3

Oxidation

HO2

?

Reduction

SO3

k=1.1-1.7 x 104 M-1 s-1 (Pehkonen & Lin 1998)

Shouldn’t occur (Gårdfeldt & Jonsson 2003)

k=0.0106 (+/- 0.0009) s-1 (vanLoon et al. 2000)

Occurs only where high sulfur, low chlorine

Oxalate?

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What does this mean for global modeling?

  • Use observations from latitudinal gradient, seasonal cycles, and short-term variability to constrain uncertainties
  • GEOS-CHEM simulation
    • Oxidation reactions: “best guess” from the published literature
    • Aqueous reduction: photochemically mediated, fixed reaction rate, proportional to [OH](g)

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ATMOSPHERE

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Hg0

4260

Hg(II)

(trop.)

280

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Via OH:10236

Dry Deposition

Ocean Emissions

Land (Natural) Emissions

Anthropogenic Emissions

Land Re-emissions

Hg(P)

2

​

775

204

Via O3: 2377

1500

1446

500

2000

Dry Deposition

Wet Deposition

Wet Deposition

1041

5327

191

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MERCURY BUDGET IN GEOS-CHEM

Inventories in Mg

Rates in Mg/yr

k=8.7 x 10-14 cm3 s-1

k=3 x 10-20 cm3 s-1

τ = 0.77 yr

τ = 7 days

τ = 3.5 days

Net ox: 5489

Reduction

7124

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Emissions for GEOS-CHEM Hg Model

500

Land Primary Emissions

1500

Re-emission of previous anthropogenic from land

2200

Anthropogenic

2000

Ocean

Amount (Mg)

Category

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Model vs. Measurements

+

  • Most measurements indicate low levels on average (5-20 pg/m3), with some spikes, diurnal variation
  • Model has high average levels in some regions

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  • Continental sources in South America?
  • Cruise data in the Pacific – inconsistent with Okinawa
  • Underestimate of concentrations over the oceans

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RGM

TGM

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Comparing Model with Measurements: Hemispheric Average TGM

  • Ratio of NH/SH in measurements: 1.49 +/- 0.12 (Temme et al. 2003)
  • Northern hemisphere average measurement ≈ 1.7 ng/m3
  • Northern hemisphere average GEOS-CHEM: 1.62 ng/m3
  • Ratio of NH/SH in GEOS-CHEM simulation: 1.5
    • Shows that Hg lifetime in GEOS-CHEM is realistic

Temme et al. 2003: black dots

GEOS-CHEM: red line

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Data from Dan Jaffe (Univ. of Washington)

GEOS-CHEM captures the day-to-day variation in Hg0 at Okinawa.

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RGM is high by an order of magnitude.

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However, we do capture diurnal variation in RGM…without oxidation by halogens!

RGM/10 at Okinawa vs. Measurements

With ‘background’ subtracted off…

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Banic et al. 2003: aircraft measurements of

Hg0 over Ontario

Vertical Profiles of Mercury

GEOS-CHEM:Vertical profile over Ontario

GEOS-CHEM: Latitudinal average Hg0

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Hg(II) in the stratosphere?

Supported by preliminary aircraft

Measurements…

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Future plans for GEOS-CHEM mercury simulation

  • New land re-emission parameterization
  • Examination of Arctic “Mercury Depletion Events”
  • Changes in mercury transport pathways and budget under changing climate?
  • Potential for application of inverse modeling?

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