What the Clean Air Mercury Rule and Clean Air Interstate Rules Will Mean to You
MERCURY IN THE AIR
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Transcript of MERCURY IN THE AIR
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.