1. MERCURY IN THE ENVIRONMENT
Daniel J. Jacob

2. Electronic structure of mercury
Mass number = 80: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2
Complete filling of subshells gives Hg(0) a low melting point, volatility
Two stable oxidation states: Hg(0) and Hg(II)

3. BIOGEOCHEMICAL CYCLING OF MERCURY
ATMOSPHERE
Hg (gas)
combustion
industry
mining
volcanoes
erosion
deposition
re-emission
SOIL
OCEAN
burial
SEDIMENTS
DEEP EARTH

4. RISING MERCURY IN THE ENVIRONMENT
Global mercury deposition has roughly tripled since preindustrial times
Dietz et al. [2009]

5. HUMAN EXPOSURE TO MERCURY IS MAINLY FROM FISH CONSUMPTION
Tuna is the #1 contributor
Mercury biomagnification factor
State fish consumption advisories
EPA reference dose (RfD) is 0.1 μg kg-1 d-1 (about 2 fish meals per week)

6. MERCURY CYCLING INVOLVES CHEMICAL TRANSFORMATIONS
elemental mercury
VOLATILE
mercuric compounds
WATER-SOLUBLE
ATMOSPHERE
Hg(0)
Hg(II)
oxidation
deposition
re-emission
Hg(0)
Hg(II)
reduction
SURFACE RESERVOIRS
(Ocean, Land)
microbes
MeHg
Methylmercury
TOXIC

7. Atmospheric transport of Hg(0) takes place on global scale
Implies global-scale transport of anthropogenic emissions
Anthropogenic Hg emission (2006)
Mean Hg(0) concentration in surface air:
circles = observed, background = GEOS-Chem model
Transport around
northern mid-latitudes:
1 month
Hg(0) lifetime = year
Transport to southern hemisphere: 1 year
Streets et al. [2009]; Soerensen et al. [2010]

8. LOCAL POLLUTION INFLUENCE FROM EMISSION OF Hg(II)
High-temperature combustion emits both Hg(0) and Hg(II)
60%
Hg(0)
GLOBAL MERCURY POOL
photoreduction
40%
Hg(II)
NEAR-FIELD
WET DEPOSITION
Hg(II) concentrations in surface air:
circles = observed, background=model
MERCURY DEPOSITION
“HOT SPOT”
Large variability of Hg(II) implies
atmospheric lifetime of only days
against deposition
Thus mercury is BOTH a global and a local pollutant!
Selin et al. [2007]

9. Atmospheric redox chemistry of mercury
Older models X X
OH, O3,
Cl, Br
Hg(0)
Hg(II) X ?
HO2(aq)
Oxidation of Hg(0) by OH or O3 is endothermic
Oxidation by Cl and Br may be important:
No viable mechanism identified for atmospheric reduction of Hg(II)
Goodsite et al., 2004; Calvert and Lindberg, 2005;
Hynes et al., UNEP 2008; Ariya et al., UNEP 2008

10. Bromine chemistry in the atmosphere
GOME-2 BrO columns
Inorganic bromine (Bry)
Thule O3 hv Br
BrO
BrNO3
Halons
hv, NO OH
HBr
HOBr
Stratospheric BrO: 2-10 ppt
CH3Br
Stratosphere
Tropopause (8-18 km)
Troposphere
Tropospheric BrO: ppt
CHBr3
CH2Br2 OH
Bry
Satellite residual
[Theys et al., 2011]
debromination
BrO column, 1013 cm-2
deposition
Sea salt
industry
plankton

11. Mean tropospheric concentrations (ppt)
TROPOSPHERIC BROMINE CHEMISTRY simulated in GEOS-Chem global chemical transport model
GEOS-Chem
Observed
CHBr3
440 Gg a-1
CH2Br2
62 Gg a-1
Vertical profiles of short-lived bromocarbons at northern mid-latitudes
CH3Br OH
1.1 years
Mean tropospheric concentrations (ppt)
CH2 Br2 OH
91 days Br
BrO
BrNO3
CHBr3
hv, OH
14 days
including
HBr+HOBr
on aerosols
HBr
HOBr
Sea salt
debromination
industry
deposition
plankton
Parrella et al. [2012]

12. GEOS-Chem global model of mercury
3-D atmospheric simulation coupled to 2-D surface ocean and land reservoirs
GEOS-Chem 3-D atmospheric
chemical transport model (CTM)
2-D surface reservoirs
ocean mixed layer
vegetation
Anthropogenic and
natural emissions
Hg(0)+Br ↔ Hg(I) → Hg(II)
Long-lived reservoirs
deep ocean
soil

13. MERCURY WET DEPOSITION FLUXES, 2004-2005
Circles: observations
Background: GEOS-Chem model
tropopause
Model contribution
from North American
anthropogenic sources
Scavenging
of Hg(II)-rich air
from upper
troposphere
Model contribution
from external sources
updraft
FLORIDA
SCAVENGING BY
DEEP CONVECTION
Selin and Jacob [2008]

14. Historical inventory of global anthropogenic Hg emissions
Large past (legacy) contribution from N. American and European emissions;
Asian dominance is a recent phenomenon
Streets et al. , 2011

15. 1977-2010 surface air trend of Hg(0) over the Atlantic Ocean
data from ship cruises show 50% decrease over North Atlantic, no significant trend over South Atlantic
Long-term observations at continental sites in N America and Europe also show decrease though not as strong as over North Atlantic
Sørensen et al., submitted

16. GEOS-Chem simulation of Hg(0) 1990-2010 trends in surface air
Global 3-D atmospheric model coupled to 2-D surface ocean and land models
Forced by
Streets emission trends
ng m-3 a-1
Forced by
observed subsurface Atlantic trends
Observations in the subsurface North Atlantic show a 80% decrease from 1990
to 2010 [Mason et al., 2012], which can explain the observed trends in surface air
This must reflect a large decline in Hg inputs to the North Atlantic Ocean over
the period.
Sørensen et al., submitted

17. Decreasing Hg input to subsurface North Atlantic, 1970-2000 1
Decreasing Hg input to subsurface North Atlantic, Atmospheric deposition explanation
Hg(0)
Hg(0)
marine
boundary layer Br Br
Hg(0)
Hg(II)
Hg(0)
Hg(II)
fast
slow
ocean
mixed layer
subsurface ocean
(down to thermocline)
Hg deposition to ocean is driven by MBL oxidation of Hg(0) by Br atoms
MBL ozone ~doubled during ; Br concentrations would have correspondingly decreased (Br/BrO photochemical equilibrium) O3 Br
BrO h
Sørensen et al., submitted

18. Decreasing Hg input to subsurface North Atlantic, 1970-2000 2
Decreasing Hg input to subsurface North Atlantic, Coastal margin explanation
Disposed Hg-containing
commercial
products
incineration
Hg(II) Hg Hg
wastewater,
leaching Hg
Secondary wastewater treatment and phase-out of Hg from commercial products would have decreased the Hg input to the subsurface N Atlantic
Sørensen et al., submitted

19. Disposal of Hg in commercial products: a missing component of the Hg biogeochemical cycle?
Global source of commercial Hg peaked in 1970
Streets et al. [2011] and Hannah Horowitz (Harvard)

20. Global biogeochemical model for mercury
7-box model with 7 coupled ODEs dm/dt = s(t) – km where s is primary emission
Loss rate constants k specified from best knowledge
Primary
emissions
Model is initialized at natural steady state, forced with historical anthropogenic emissions for 2000 BC – present; % present-day enrichments are indicated
Amos et al., submitted

21. Time scale for dissipation of an atmospheric emission pulse
Reservoir fraction
Pulse gets transferred to subsurface ocean within a few years and stays there ~100 years, maintaining a legacy in the surface ocean
Amos et al., submitted

22. Global source contributions to Hg in present-day surface ocean
emissions
Human activity has increased 7x Hg content of the surface ocean
Half of this human influence is from pre-1950 emissions
N America, Europe and Asia share similar responsibilities for anthropogenic Hg in surface ocean
Europe
Asia
N America
S America
former
USSR
ROW
pre-1850
natural
Amos et al., submitted

23. Looking toward the future: UNEP global treaty for Hg
Negotiations to be completed by 2013
Effect of zeroing global anthropogenic
emissions by 2015
Zeroing anthropogenic emissions would decrease ocean Hg by 30% by 2100, while keeping emissions constant would increase it by 40%
Elevated Hg in surface ocean will take centuries to fix; the only thing we can do in short term is prevent it from getting worse.
Amos et al., submitted