MERCURY IN THE AIR

Daniel J. Jacob

with Harvard Team-Hg: Helen Amos, Bess Corbitt, Jenny Fisher, Hannah Horowitz, Chris Holmes (now at UC Irvine), Justin Parrella, Asif Qureshi, Noelle Selin (now at MIT), Anne Soerensen, Elsie Sunderland

and funding from NSF, EPRI, EPA

Biogeochemical cycling of Hg

Hg(0) Hg(II)

particulate

Hg

burialSEDIMENTS

uplift

volcanoeserosion

oxidation (~months)

reduction

volatilization

Hg(0) Hg(II)oxidation

reduction

deposition

biologicaluptake

ANTHROPOGENIC PERTURBATION:fuel combustion

waste incinerationmining

highly water-soluble

ATMOSPHERE

SOIL/OCEAN

Anthropogenic perturbation to the global Hg cycle

Selin et al. [2008]; Selin [2009]

GEOS-Chem model natural atmosphere + present-day human enhancement

Primaryemissions x7 Atmospheric deposition x3

Soil +15%

Surface ocean x3

Deep ocean + 15%

Atmospheric transport of Hg(0) takes place on global scale

Anthropogenic Hg emission (2006)

Streets et al. [2009]; Soerensen et al. [2010]

Mean Hg(0) concentration in surface air:circles = observed, background = GEOS-Chem model

Transport around northern mid-latitudes:

1 month

Transport to southern hemisphere: 1 year

Implies global-scale transport of anthropogenic emissions

Hg(0) lifetime = 0.5-1 year

By contrast, emitted Hg(II) can be deposited close to point of emission

High-temperature combustion emits both Hg(0) and Hg(II)

Hg(0)

Hg(II)

60%

40%

GLOBAL MERCURY POOL

NEAR-FIELDDEPOSITION

photoreduction

MERCURY DEPOSITION“HOT SPOT”

Hg(II) concentrations in surface air:circles = observed, background=model

Large variability of Hg(II) impliesatmospheric lifetime of only days

against deposition

Selin et al. [2007]

Observed Hg(II) ≡ reactive gaseous mercury (RGM) + particle-bound mercury (PBM)

Atmospheric redox chemistry of mercury:what laboratory studies and kinetic theory tell us

Hg(0) Hg(II)OH, O3,

• Oxidation of Hg(0) by OH or O3 is endothermic

HO2(aq)

Older models

Goodsite et al., 2004; Calvert and Lindberg, 2005; Hynes et al., UNEP 2008; Ariya et al., UNEP 2008

• Oxidation by Cl and Br may be important:

, ,

Hg Br M HgBr M

HgBr X M HgBrX M X OH Br Cl

?

X X Cl, Br

• No viable mechanism identified for atmospheric reduction of Hg(II)

X

Atmospheric redox chemistry of mercury:what field observations tell us

• Hg(0) lifetime against oxidation must be ~ months– Observed variability of Hg(0)

• Oxidant must be photochemical– Observed late summer minimum of Hg(0) at northern mid-latitudes– Observed diurnal cycle of Hg(II)

• Oxidant must be in gas phase and present in stratosphere– Hg(II) increase with altitude, Hg(0) depletion in stratosphere

• Oxidation in marine boundary layer is by halogen radicals, likely Br– Observed diurnal cycle of Hg(II)

• Oxidation can be very fast (hours-days) in niche environments during events– Boundary layer Hg(0) depletion in Arctic spring, Dead Sea from high Br

• If reduction happens at all it must be in the lower troposphere– Hg(II) increase with altitude, Hg(0) depletion in stratosphere

• Hg(II)/Hg(0) emission ratios may be overestimated in current inventories– Lower-than-expected Hg(II)/Hg(0) observed in pollution plumes– Weaker-than-expected regional source signatures in wet deposition data

Working hypothesis: Br atoms could provide the dominant global Hg(0) oxidant

Atmospheric composition of Hg(II)?

Hg(0) HgXY

aqueous aerosol/cloud

HgCl2,

others?

Hg2+

X-

Cl-

SURFACE

precipitating cloud

wetdeposition

gas-aerosolpartitioning

drydeposition

Y-

• Hg(II) salts produced by Hg(0) oxidation may change composition during cycling through aerosols/clouds

• HgCl2 (KH = 1.4x106 M atm-1) is expected to be an important component because of ubiquitous Cl- - but there may be others (organics?)

oxidation

Observed gas-aerosol partitioning of Hg(II)Reactive gaseous mercury (RGM) and particle-bound mercury (PBM) at several North American sites fitted to a gas-aerosol equilibrium constant K

K [PBM]/PM2.5

[RGM]

Rutter and Schauer [2007]

Amos et al. [in prep]

Hg(II) appears to have semi-volatile behavior; partitions into gas phase when air is warm and clean, in aerosol when air is cold and polluted.

PM2.5 ≡ fine particulate matter

Special case of Hg(II) uptake by sea saltObserved RGM diurnal cycle suggests Br chemistry, deposition via sea salt uptake

Hg(0) HgBrBr

T

Br, OHHgBrX

sea-salt aerosol

HgCl32-, HgCl4

2-

deposition

Box model predicts that ~80% of Hg(II) in MBL should be in sea salt aerosol:

Holmes et al. [2009]

Observed [Laurier et al., 2003]Model Hg(0)+BrModel Hg(0)+OH

Subtropical Pacific cruise data

kinetics from Goodsite et al. [2004]

Box model budget for marine boundarylayer (MBL)

Bromine chemistry in the atmosphere

Tropopause (8-18 km)

Troposphere

Stratosphere

Halons

CH3Br

CHBr3

CH2Br2

Sea salt

Br BrO BrNO3

HOBrHBr

O3

hv, NO

hv

OH

Inorganic bromine (Bry)

Bry

OH

debrom

inatio

n

deposition

industry plankton

Stratospheric BrO: 2-10 ppt

Tropospheric BrO: 0.5-2 ppt

Thule

GOME-2 BrO columns

Satellite residual[Theys et al., 2011]

BrO

co

lum

n,

101

3 c

m-2

TROPOSPHERIC BROMINE CHEMISTRYsimulated in GEOS-Chem global chemical transport model

CHBr3 hv, OH

14 days

CH2 Br2

OH

91 days

CH3BrOH

1.1 years

Br BrO BrNO3

HOBrHBr including HBr+HOBr

on aerosols

deposition

Parrella et al. [in prep]

GEOS-ChemObserved

CHBr3

440 Gg a-1

CH2Br2

62 Gg a-1

Vertical profiles of short-lived bromocarbons at northern mid-latitudes

Sea salt debromination

0.09 0.6 0.3

1.4 0.9

Mean tropospheric concentrations (ppt)

plankton

industry

Model vs. observed tropospheric BrO columns

Theys et al. [2011] satelliteresidualsGEOS-Chem model

Parrella et al. [in prep]

• Observations show similar BrO in both hemispheres, increasing with latitude and with winter/spring max

• Model is biased low but captures some of the latitudinal/seasonal features

GEOS-Chem global mercury model

Hg(II)vegetation oceanmixed layer

Hg(0) Hg(II) Hg(0)

natural + legacy boundary conditions

• 3-D atmospheric simulation coupled to 2-D surface ocean and land reservoirs• Gas-phase Hg(0) oxidation by Br atoms (TOMCAT model)• In-cloud Hg(II) photoreduction to enforce 7-month Hg lifetime against deposition

soil

Hg(0) + Br ↔ Hg(I) → Hg(II)

surface reservoirs

~ months

stable reservoirs ~ decades

anthropogenic+ geogenicprimaryemissions

Kinetics from Goodsite et al. [2004],Donohoue et al. [2005]; Balabanov et

al. [2005]

Sensitivity of Hg deposition to oxidation mechanism

Annual mean Hg(0) oxidation rates in GEOS-Chem with Br or OH/O3 as oxidants

Hg(0) = 6 months Hg(0) = 3.7 months

Effect on annual mean GEOS-Chem Hg deposition fluxes

Maximum sensitivity is over the Southern OceanHolmes et al. [2010]

Mercury wet deposition fluxes over US, 2007-2009Annual mean 2007-2009 MDN data (circles)and GEOS-Chem model (background) Seasonal variation

• Summer peak along Gulf Coast reflects deep convective scavenging of Hg(II) from upper troposphere

• Very low winter values at northern latitudes reflect inefficient scavenging by snow

• Reduction of emitted Hg(II) is necessary to avoid model maximum in Northeast

Amos et al., in prep.

Quantifying source-receptor relationships for mercury:the grasshopper effect

Hg

Hg(II)LAND OCEAN

Hg(0) Hg(II) Hg(0)Surface reservoirs

~ months

Intermediate reservoirs ~ decades

Atmosphere

effective = 9 months = 6 months

g m-2 Mg-1

GEOS-Chem influence functions for anthropogenic source regionsExtratropical NH Tropical NH SH

Effective atmospheric lifetime is sufficiently short for hemispheric signatures; future growth of Indian emissions is likely to lead to S shift in ocean deposition

Corbitt et al., submitted

legacylegacy

New anthropogenic inputs to the world’s oceans

Corbitt et al., submitted

• Asian emissions are so large that they account for >50% of new anthropogenic inputs to all open oceans

• N American emissions influence N Atlantic, European emissions influence Arctic

Legacy anthropogenic sources account for over 50%of mercury deposited to the oceans

Source attribution of present-day Hg deposition to world’s oceans (GEOS-Chem)

Soerensen et al. [2010], Corbitt et al., submitted

Atmospheric Hg(0) data in March-May (circles)compared to GEOS-Chem (background)

Legacy source is highest in North Atlantic: past Hg(II) emissions from N. America?

Historical inventory of global anthropogenic Hg emissions

• Large legacy contribution from N. American and European emissions; Asian dominance is a recent phenomenon

• Time integrals of global emissions imply that legacy reservoirs are not globally enriched relative to the surface

Streets et al. , submitted

Observed decrease of total gaseous Hg (TGM) since 1996

• Explanation by decline of legacy emissions would imply much higher past emissions than in Streets et al. historical inventory

• Faster atmospheric oxidation of Hg(0) may be an alternate explanation - Increasing Br?

- A missing anthropogenic source would help simulation of tropospheric BrO - Increasing Cl? - could reflect increase in CFC replacement products after Montreal Protocol - could also help explain the leveling of atmospheric methane

20-38% worldwide decrease

Slemr et al. [2011]

Effect of climate change on mercury in the Arctic Ocean

Sea saltdeposition

bromineBr

Hg(0)

Hg(II)

SEA ICE ICE LEAD

ARCTIC OCEAN

light

Atmospheric Hg depletion events(AMDEs) associated w/ice leads

Composite obs at Arctic sitesGEOS-Chem: standard with Arctic rivers runoff

AMDEssummerrebound

• Summer rebound in atmospheric observations cannot be explained by snow re-emission; suggests external input to Arctic Ocean (Arctic rivers runoff?)

• Implies in turn that Arctic Ocean is supersaturated relative to the atmosphere

• Changing river runoff and shrinking sea ice in future climate could greatly affect Hg levels in Arctic OceanFisher et al., in prep.