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Challenges in Global Mercury Modeling
Ashu Dastoor
Meteorological Service of Canada
Environment Canada
Acknowledgements: Didier Davignon and Arturo Quintanar
Atmospheric Mercury Cycling
Gas-phase chemistryGEM↔RGM↔TPMO3, OH, H2O2, Halogens: tropospheric chemistry Q: reactions, rates and products?
Heterogeneous Chemistry ?
Aerosol dynamicsQ: TPM size distribution? Planetary Boundary Layer
Turbulent mixingmet model
Anthropogenic emissionsGEM, RGM, TPMQ: emission speciation andplume chemistry?
Flue gas chemistry?
Point sourcesPlume rise
Volcanic emissionsQ: inventory?
Bio-mass burningemissionsQ: inventory?
Surface natural and re-emission:soils, vegetation water bodies, snow, oceansQ: inventory/processes?
Wet depositionMet modelQ: precipitation scavenging?
EvaporationMet model
Gas/liquidexchange
TransportMet model
Transport
Dry depositionQ: deposition velocities?
Cloud propertiesMet models
Snow/Ice dynamicsIce model
Evaporation
Area emissions
Mercury transformation processesQ: residence time and revolatilization rate and species?
GEM↔RGM↔TPMO3, OH, HO2, (Cl), SO3,aerosols: tropospheric chemistryQ: reactions, rates and products?
Goals for a Global Mercury Model towards estimating Mercury in Lake Ontario
• What is the atmospheric flux of Hg that arrives to Lake Ontario from global anthropogenic sources?
• What is the atmospheric flux of Hg that arrives to lake Ontario from global natural and recycled sources?
• What are the sources of this distant mercury?
• What is the speciation of transported mercury?
• What is the time-space distribution of this flux?
• How are the fluxes changing with changing emissions?
• What is the contribution of distant mercury to the deposition in Lake Ontario basin? This question can be better answered by a regional model using the atmospheric background and boundary flux information from global models.
• Preliminary results from N. American model intercomparison study indicate significant impact of trans-boundary flows of mercury to the regional deposition. The study compares impact of boundary fluxes from three different global models.
Major Challenges faced by a global model in addressing the goals
• Emissions: Natural and re-emissions from land and ocean surfaces (very few flux measurements and no emissions inventory)
• Mercury Chemistry: Chemical reactions- products, rates and phase
• Mercury dry deposition process• What is the trend in natural and re-
emissions of mercury relative to the anthropogenic emissions?
THE MERCURY CYCLE: CURRENT
Wet & DryDeposition 3500
ATMOSPHERE5000
SURFACE SOILS1,000,000 OCEAN
288,000
Wet & DryDeposition3100
Oceanic Evasion
2600
Net burial200
Land emissions1600
Quantities in Mg/yearUncertainty ranges in parenthesesAdapted from Mason & Sheu, 2002
AnthropogenicEmissions 2400
Extraction from deep reservoirs2400
River200
(700-3500)
(1680-3120)
(700-3500)
Ocean flux distribution
0 100 200 300 kg
July ocean flux
Jan. ocean flux
latitude
Flux
(ng/
m2 )
Jan.July
• Higher flux in tropics due to high temperature and radiation
• High flux in regions of high deposition
• Seasonality, spatial variation due to temperature, npp, radiation, and mixed layer depth
• Diurnal variation: photochemistry
Hg0
1.7 ng/m3
GaseousPhase
AqueousPhase
Hg0
Henry’s Constant 0.11 M/atm
Particulate Phase
Oxidation
Hg2+
Products and phase
unclear
10-200 pg/m3
HgP
1-100 pg/m3
Hg2+
k=8.7(+/-2.8) x 10-14 cm3 s-1 (Sommar et al. 2001)k=9.3(+/-1.3) x 10-14 cm3 s-1(Pal & Ariya 2004)
Probably unimportant reaction (Goodsite et al. 2004)
k=3(+/-2) x 10-20 cm3 s-1 (Hall 1995) k=7.5(+/-0.9) x 10-19 cm3 s-1 (Pal and Ariya 2004)
Longer lifetime suggested (Calvert & Lindberg 05) Henry’s Constant 1.4x106 M/atm
OH
O3
Oxidation
HO2
??Reduction
SO3
k=1.1-1.7 x 104 M-1 s-1 (Pehkonen & Lin 1998)Shouldn’t occur (Gårdfeldt & Jonsson 2003)
k=0.0106 (+/- 0.0009) s-1
(vanLoon et al. 2000)Occurs only where high sulfur, low chlorine
Oxalate?
Is there Hope?
• Global model is a closed atmospheric system therefore observations can be used to constrain uncertainties.
• Observations available: atmospheric mercury concentrations, wet deposition, terrestrial and aquatic fluxes of mercury and measurements of long-range transport of mercury.
ATMOSPHERE
Hg0 Hg(II)
Via OH:10236Via OH:10236
Dry Deposition
Ocean Emissions
Land (Natural) Emissions
Anthropogenic Emissions
Land Re-emissions
Hg(P)
775775 204204
Via O3: 2377Via O3: 2377
1500150014461446
500500
20002000
Dry DepositionWet Deposition
Wet Deposition
10411041
53275327
191191
1111
MERCURY BUDGET IN GEOS-CHEM
Inventories in MgRates in Mg/yr
k=8.7 x 10-14 cm3 s-1
k=3 x 10-20 cm3 s-1
τ = 0.77 yr τ = 7 days τ = 3.5 daysNet ox: 5489
Reduction7124
How are we addressing the goals?
• Develop a well constrained global model with known chemistry and emissions.
• Use the constrained model to address the goals.• Perturb the system using emerging chemical mechanism for
mercury on the global balance of mercury and provide possible solutions.
• Assess the impact of emerging mercury chemistry on long range transport of mercury and trans-boundary fluxes of mercury for regional models.
• Evaluate the accuracy of trend in anthropogenic emissions inventory by modeling the global budgets and verify the changes against the observations.
• Develop detailed processes such as mercury evasion from snow, soil, vegetation etc and including aerosol dynamics.
90S 60S 30S Eq. 30N 60N 90N
0
0.5
1
1.5
2
2.5
TGM
ng/
m3
Latitude
Observed (left; Lamborg et al., 2002) and modeled (right)
Inter-hemispheric gradient of TGM Observed and from GRAHM simulation
Date (Julian Date)
0 100 200 300
Mer
cury
con
cent
ratio
n (n
g/m
3)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Mercury depletion
Mercury reduction
Mean mercury concentration in the Arctic
Elemental mercury vapor concentration at Alert for the whole year 1995
Rapid, near-complete depletion of mercury observed during spring in the Arctic,sub-Arctic and Antarctic is which is also correlated with ozone depletion.
Questions: Which halogen gases are responsible and what is the impact on the Arctic and global mercury deposition?
Mercury deposition without MDEs Mercury deposition with MDEs
Main conclusions:
• Br atoms (~.4 day) and BrO (~1 day) radicals are the most effective halogens driving mercury
oxidation to more hygroscopic species which are readily deposited and could be incorporated
in the biota.
• MDEs in the Arctic increase the net deposition into the Arctic by 100 tons/yr.
• Net accumulation in the Arctic 325 tons/yr
The Arctic: a sink for mercury
Slow oxidation of Hg0 by O3 in the troposphere – 1-2 years life timeFast oxidation of Hg0 by halogens in the polar regions and marine boundary layer – hours - 10 days life time
Hg0 HgII
Surface ocean
Evasion deposition
transport
reduction
Hg0 HgII
Marine Boundary Layer
Fast oxidation
Loss
• Reduction proportional to radiation and net primary productivity
transport
Sea-salt aerosolHalogen activation
Cl2, Br2, Cl, Br, BrO
Impact of mbl chemistry on transboundary flow from Asia
Surface air mercury concentrations(ng/m3)
MBL chemistry
No MBL chemistry
0.1
0.12
0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
J F M A M J J A S O N D
Canada
0.1
0.12
0.14
0.16
0.18
0.2
0.22
0.24
J F M A M J J A S O N D
0.1
0.12
0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
J F M A M J J A S O N D
0.1
0.6
1.1
1.6
2.1
2.6
3.1
3.6
J F M A M J J A S O N D
0.1
0.6
1.1
1.6
2.1
2.6
J F M A M J J A S O N D
0.1
1.1
2.1
3.1
4.1
5.1
6.1
7.1
J F M A M J J A S O N D
USA
Arctic
Surface air Hgo concentrations Total deposition in ug/m2/year
No MBL
MBL
Spring 2004 Experiment: Simultaneous Observations at Mt.Bachelor and Okinawa ( Jaffe et al. 2005)
MBO
Okinawa
Okinawa: Hg0,RGM, PHg, CO, O3, aerosols, etc.
MBO: Total Hg0 , CO, O3, aerosols, etc.
Mauna Loa
Hg transport to North America: April 25th, 2004
ΔHg/ΔCO=0.50 ng/m3/100 ppbv
Good tracer of Asian air masses
Suggests much larger Asian emissions? (Jaffe et al. 2005)
Possible causes for this discrepancy:• Underestimate of the industrial or domestic Hg emissions;• Natural emissions;• Re-emission of previously deposited Hg;• Too low a ratio of Hg0/total Hg in the inventory;• Conversion of RGM to Hg0 during transport;
Explanations/Hypotheses (Jaffe et al. 2005)
Using the observed Hg0/CO ratio, and the known CO emissions, Jaffe et al. calculate Hg0 emissions from Asia of 1460 mt/year (+/-30%);
This can be compared to 770 mt/year in the Pacyna et al., 2003 inventory.
Anthropogenic Air Emissions of Anthropogenic Air Emissions of Mercury: Distribution by Region in Mercury: Distribution by Region in
1990 and 20001990 and 2000
Total: 1,881 metric tons/yr Total: 2,269 metric tons/yr
Asia and Africa account for about 70% of global emissions and show steady, significant increases due to industrialization.
Based on Pacyna, J., Munthe J., Presentation at Workshop on Mercury: Brussels, March 29-30, 2004Slide courtesy Grace Howland, Air Pollution Prevention Directorate, Environment Canada
20001990Africa 9%
Asia38%
Australia 3%
Europe33%
North America14%
South America3%
Africa 18% Asia
52%
Australia 6%Europe
11%
North America9%
South America4%
JanuaryJanuary JulyJuly
GEM surface air concentrations ng/m**3 GEM surface air concentrations ng/m**3
GRAHM simulation with year 1990 global emissions
Impact of mercury emission reductions
• Impact of proposed Canada wide standards for coal-fired power plants-
Reduction of 1,224 kg/yr will result in ~ 580 kg/yr reduction in total mercury deposition over Canada
• Impact of proposed US mercury rule for coal-fired power plants- Reduction of 30,000 kg/yr will result in ~ 2,600 kg/yr reduction in
total mercury deposition over Canada
• Asia contributes ~ 24,700 (21,650 with mbl chemistry) kg/yr mercury deposition into Canada of which ~ 15,300 kg/yr comes from China.
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