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
ANTHROPOGENIC
PERTURBATION:
fuel combustion
waste incineration
mining
oxidation (~months)
Hg(II)
Hg(0)
reduction
volcanoes
erosion
volatilization
highly water-soluble
ATMOSPHERE
deposition
SOIL/OCEAN
Hg(0)
oxidation
reduction
uplift
particulate
Hg(II)
biological
uptake
Hg
burial
SEDIMENTS
Anthropogenic perturbation to the global Hg cycle
GEOS-Chem model natural atmosphere + present-day human enhancement
Primary
emissions x7
Atmospheric deposition x3
Surface ocean x3
Soil +15%
Deep ocean + 15%
Selin et al. [2008]; Selin [2009]
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 = 0.5-1 year
Transport to southern
hemisphere: 1 year
Streets et al. [2009];
Soerensen et al. [2010]
By contrast, emitted Hg(II) can be deposited close to point of emission
High-temperature combustion emits both Hg(0) and Hg(II)
60%
Hg(0)
GLOBAL MERCURY POOL
photoreduction
40%
Hg(II)
Hg(II) concentrations in surface air:
circles = observed, background=model
NEAR-FIELD
DEPOSITION
MERCURY DEPOSITION
“HOT SPOT”
Large variability of Hg(II) implies
atmospheric lifetime of only days
against deposition
Observed Hg(II) ≡ reactive gaseous mercury (RGM)
+ particle-bound mercury (PBM)
Selin et al. [2007]
Atmospheric redox chemistry of mercury:
what laboratory studies and kinetic theory tell us
Older models
OH,
O3, Cl, Br
X X
Hg(0)
?
Hg(II)
HO
X2(aq)
• Oxidation of Hg(0) by OH or O3 is endothermic
• Oxidation by Cl and Br may be important:
Hg  Br  M
HgBr  M
HgBr  X  M  HgBrX  M
X  OH , Br , Cl
• 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
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
Working hypothesis: Br atoms could provide the dominant global Hg(0) oxidant
•
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
Atmospheric composition of Hg(II)?
precipitating cloud
aqueous aerosol/cloud
oxidation
Hg(0)
HgXY
Hg2+
XCl
Ygas-aerosol
partitioning
HgCl2,
others?
dry
deposition
wet
deposition
SURFACE
• 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?)
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
[PBM ]/PM 2.5
K
[RGM]
PM2.5 ≡ fine particulate matter
Rutter and Schauer [2007]
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.
Amos et al. [in prep]
Special case of Hg(II) uptake by sea salt
Observed RGM diurnal cycle suggests Br chemistry, deposition via sea salt uptake
Subtropical Pacific cruise data
Box model budget for marine boundary
layer (MBL)
Observed [Laurier et al., 2003]
Model Hg(0)+Br
Model Hg(0)+OH
Box model predicts that ~80% of Hg(II) in MBL should be in sea salt aerosol:
Hg(0)
Br
HgBr
Br, OH
sea-salt
aerosol
HgBrX
T
HgCl32-, HgCl42-
kinetics from Goodsite et al. [2004]
Holmes et al. [2009]
deposition
Bromine chemistry in the atmosphere
GOME-2 BrO columns
Inorganic bromine (Bry)
Halons
hv
O3
Br
BrO
hv, NO
OH
HBr
BrNO3
Thule
HOBr
Stratospheric BrO: 2-10 ppt
CH3Br
Stratosphere
Tropopause (8-18 km)
Troposphere
OH
Bry
deposition
Sea salt
industry
plankton
BrO column, 1013 cm-2
CHBr3
CH2Br2
Tropospheric BrO: 0.5-2 ppt
Satellite residual
[Theys et al., 2011]
TROPOSPHERIC BROMINE CHEMISTRY
simulated in GEOS-Chem global chemical transport model
GEOS-Chem
Observed
Vertical profiles of
short-lived
bromocarbons at
northern mid-latitudes
CH3Br
CH2 Br2
CHBr3
industry
Sea salt
plankton
Parrella et al. [in prep]
CHBr3
CH2Br2
440 Gg a-1
OH
1.1 years
OH
91 days
62 Gg a-1
Mean tropospheric concentrations (ppt)
0.09
Br
0.6
BrO
hv, OH
14 days
debromination
HBr
HOBr
1.4
0.9
deposition
0.3
BrNO3
including
HBr+HOBr
on aerosols
Model vs. observed tropospheric BrO columns
Theys et al. [2011] satellite
residuals
GEOS-Chem model
• 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
Parrella et al. [in prep]
GEOS-Chem global mercury model
• 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
anthropogenic
+ geogenic
primary
emissions
vegetation Hg(II)
Kinetics from Goodsite et al. [2004],
Donohoue et al. [2005]; Balabanov
et al. [2005]
Hg(0) + Br ↔ Hg(I) → Hg(II)
Hg(0)
ocean Hg(II)
mixed layer
Hg(0)
soil
natural + legacy boundary conditions
surface reservoirs
 ~ months
stable reservoirs
 ~ decades
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 Ocean
Holmes et al. [2010]
Mercury wet deposition fluxes over US, 2007-2009
Annual mean 2007-2009 MDN data (circles)
and GEOS-Chem model (background)
• 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.
Seasonal variation
Quantifying source-receptor relationships for mercury:
the grasshopper effect
Atmosphere
Hg
 = 6 months
 effective = 9 months
LAND
Hg(II)
legacy
Hg(0)
OCEAN
Hg(II)
Surface reservoirs
Hg(0)
legacy
 ~ months
Intermediate reservoirs
 ~ decades
GEOS-Chem influence functions for anthropogenic source regions
Extratropical NH
Tropical NH
SH
g m-2 Mg-1
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
New anthropogenic inputs to the world’s oceans
• 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
Corbitt et al., submitted
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)
Legacy source is highest in North Atlantic: past Hg(II) emissions from N. America?
Atmospheric Hg(0) data in March-May (circles)
compared to GEOS-Chem (background)
Soerensen et al. [2010], Corbitt et al., submitted
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
20-38% worldwide decrease
Slemr et al. [2011]
• 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
Effect of climate change on mercury in the Arctic Ocean
Atmospheric Hg depletion events
(AMDEs) associated w/ice leads
Hg(0)
Br
bromine
Sea salt
Hg(II)
deposition
light
SEA ICE
ICE LEAD
ARCTIC OCEAN
AMDEs
summer
rebound
Composite obs at Arctic sites
GEOS-Chem: standard
with Arctic rivers runoff
• 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 Ocean
Fisher et al., in prep.