Trans-Pacific Transport of Mercury

Sarah Strode, Lyatt Jaeglé, Dan Jaffe, Peter Weiss-Penzias, Phil Swartzendruber University of Washington

Noelle Eckley Selin, Chris Holmes, Daniel Jacob Harvard

• Can GEOS-Chem capture observed Hg lrt events?
• What can observed Hg/CO ratios tell us about Asian Hg emissions?

Hg emissions & long-range transport

• Hg⁰ lifetime ~1 year 🡪 global transport
• Approx. 1/3 of emissions anthropogenic: ~50% from Asia 🡪 underestimated?
• Long-range transport important for North American concentrations

Pacyna et al., 2005

Seigneur et al., 2004

Deposition to U.S.

Hg Emissions from Asia

• Land emission where geologic deposits
• Land reemissions follow deposition pattern
• Biomass burning scaled to CO
• Emissions+reemissions from slab ocean

Trans-Pacific Hg Transport

• Hg⁰ & CO transported across Pacific
• Hg^II at lower latitudes from Hg⁰ oxidation

March-May averages

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• Deposition largest over E. Asia & SW N. America

Asian anth CO at 700 hPa

Asian anth Hg⁰ at 700 hPa

Asian anth Hg^II at 700 hPa

Asian anth Hg^II deposition

Vertical View of Transport

• March-May, 28°-60°N average
• Asian Hg⁰ & CO trans-Pacific transport decreases w/ height
• Hg^II transport in upper troposphere
• Ocean reemission increases transport

Including ocean reemission of Asian Hg

Asia

N. Am

Asian FF CO

Asian anth Hg⁰

Asian anth Hg^II

Asian anth Hg⁰

Asian anth Hg^II

pressure

Timeseries at Okinawa, Spring 2004

• Asian antrhopogenic tracer dominates variability
• Asian land Hg tracer covaries with Asian anthro tracer

Obs from Jaffe et al. [2005]

CO obs and model

Hg⁰ obs and model

ALRT

event

Model

Asian anthro

Asian land

Constraining Asian Hg emissions

• Observed Hg⁰/CO enhancement ratio during Asian LRT events ~twice ratio expected for emissions [Jaffe et al., 2005]:
• Underestimate in anthropogenic Hg⁰ emission
• Covariance w/ Asian land emission/reemission
• Production of Hg⁰ during transport
• Can the model distinguish these possibilities?

Slope= .0053, .0041, .0027

r² = .84, .82, .84

Hg⁰ (ng/m³)

Obs from Jaffe et al. [2005]

• reduction of Hg^II emission adds ~20%
• increasing anth + decreasing land overestimates Hg⁰ conc.

Timeseries at Mount Bachelor, OR

CO underestimate in spring

Captures mean Hg but peaks underestimated

N. American land emissions are major contributor

Model statistics:

CO: mean model/obs = 0.89, r² = 0.35

Hg⁰: mean model/obs = 1.02, r² = 0.35

2004 obs and model: CO

2004 obs and model: Hg⁰

• Peak at 2.8 km a.s.l. – upslope/downslope flow
• Less variability than Okinawa

Obs from Weiss-Penzias et al. [2006]

2004 tracer contributions: Hg⁰

RGM at MBO, April-Aug. 2005

• Model captures timing of some events but underestimates magnitude
• Greater subsidence could give higher RGM
• Oxidation of global pool, not direct transport

Obs from Swartzendruber et al. [2006]

Conclusions & Future work

• Land emissions + reduction of Hg^II can explain most of the observed Hg/CO ratio: anthropogenic inventory may be ok
• Asian Hg⁰ reaches N. America by direct transport, while RGM descends from the high altitude global pool

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Future plans: biogeochemical Hg cycling

• More sophisticated ocean formulation
• Track Hg partitioning between soil, ocean, and atmospheric reservoirs thru time

Global GEOS-Chem Hg Budget

• Hg⁰ oxidation to Hg^II (RGM) by O₃ & OH; Hg^II reduction in clouds
• Coupled to slab ocean model w/ sea-air exchange, chemistry and mixing w/ deep ocean

22.4 Mmol Hg⁰

Land

Ocean mixed layer

Atmosphere

1.0

Wet & dry deposition

11.1

22.8

Ocean evasion

14.1

4.3 Mmol Hg^II

Oxidation

71.8

.01 Mmol Hg^P

Land emissions

Wet & dry deposition

1.0

Anthropogenic emissions

6.4

3.5

Reduction

41.4

10.0

Deep ocean

Hg⁰

Hg^II

Hg^nr