MERCURY IN THE ATMOSPHERE, BIOSPHERE,
AND POLICY SPHERE:
Constraints from a global 3D land-ocean-atmosphere
model on mercury sources, cycling and deposition
Noelle Eckley Selin
Harvard University
Department of Earth and Planetary Sciences
Atmospheric Chemistry Modeling Group
Coauthors: D.J. Jacob, R.J. Park, R.M. Yantosca (Harvard)
S. Strode, L. Jaegle, D. Jaffe, P. Swartzendruber (U. Washington)
Princeton University
24 May 2007
MERCURY POLLUTION: A SCIENCE & POLICY PROBLEM
Mercury deposition has increased
by 300% since industrialization
Growing concern about human exposure
through methylmercury in fish
States with fish mercury advisories [EPA, 2004]
Particular concern in
Arctic ecosystems due
to bioaccumulation,
human exposure
Ice core record of deposition from
Wyoming, USA
[Schuster et al., ES&T 2002]
Mercury in polar bear fur
up 5-12X since 1890,
[Dietz et al., ES&T 2006]
POLITICAL ACTIONS AND UNCERTAINTIES
GLOBAL:
2002: Global Mercury Assessment: sufficient evidence to warrant international action
2003, 2005, 2007: UNEP Governing Council meetings reject proposed global mercury
agreement. Mercury Programme and voluntary partnerships established.
REGIONAL:
-U.S./Mexico/Canada regional action plan under Commission for
Environmental Cooperation (1997,2000)
-U.S./Canada/Europe/former Soviet Union countries agreement
on heavy metals under Convention on Long Range
Transboundary Air Pollution (1998)
U.S. :
2005: CLEAN AIR MERCURY RULE establishes a “cap and
trade” approach to regulating mercury from coal-fired power
plants
• What are the relative contributions of global, regional and domestic
sources to deposition?
• What is the impact of anthropogenic emissions (past & present) on
the global mercury cycle?
[Selin, Environment, 2005; Selin and Selin, RECIEL, 2006]
SCEINTIFIC UNCERTAINTIES: SOURCES AND SINKS
ATMOSPHERE
5000
(3x pre-industrial)
Anthropogenic
Emissions
2400
(1680-3120)
Wet & Dry
Deposition
2600
(1800-3600)
Land
emissions
1600
(700-3500)
Oceanic
Evasion
1500
(700-3500)
SURFACE SOILS
1,000,000
Extraction from deep
reservoirs
2400 (1680-3120)
Rivers
200
Quantities in Mg/year (106 g, or metric tonnes)
Uncertainty ranges in parentheses
Adapted from Mason & Sheu, 2002
Wet & Dry
Deposition
1900
(1300-2600)
OCEAN
289,000
Net burial
200
SCIENTIFIC UNCERTAINTIES: ATMOSPHERIC CHEMISTRY
REACTIVE GASEOUS
MERCURY (RGM)
TOTAL GASEOUS MERCURY (TGM)
GAS PHASE
Hg(0)
Oxidation
OH, O3, Br(?)
SOLID PHASE
VERY
SOLUBLE
RELATIVELY
INSOLUBLE
ATMOSPHERIC
LIFETIME:
ABOUT 1 YEAR
AQUEOUS PHASE
Hg(II)
Reduction
Photochemical
aqueous (?)
TYPICAL
LEVELS:
1.7 ng m-3
EMITTED BY
ANTHROPOGENIC
SOURCES
Hg(II)
LIFETIME:
DAYS TO
WEEKS
TYPICAL
LEVELS:
1-100 pg m-3
Hg(P)
DRY AND WET DEPOSITION
ECOSYSTEM INPUTS
CONSTRAINING POLICY-RELEVANT UNCERTAINTIES
WITH A GLOBAL ATMOSPHERIC MODEL
Global, 3D tropospheric
chemistry model (GEOSChem) simulation, 4x5
degree resolution
Mercury budget in
GEOS-Chem
Reproduces annual average
concentration at 22 land-based sites,
interhemispheric gradient
Measured: 1.58 ± 0.19 ng/m3
Simulated: 1.63 ± 0.10 ng/m3
High Atlantic cruise data (enrichment from
past decades emissions in North
Atlantic?)
[Selin et al. JGR 2007 (atmosphere);
Strode et al. GBC 2007 (ocean)]
OXIDATION AND REDUCTION PROCESSES
• Seasonal variation of TGM is consistent • Diurnal variation of RGM (at Okinawa,
with a photochemical oxidation of Hg(0)
partially balanced by reduction of Hg(II)
Observations
GEOS-Chem
No reduction (oxidation by OH)
Japan, measured by Jaffe et al. 2005)
supports a photochemical source
Observations
GEOS-Chem
• In most models (including GEOS-Chem)
OH is the dominant Hg(0) oxidant.
• But the Hg+OH reaction may not occur
[Calvert & Lindberg 2005]
•Could the dominant oxidant be Br?
[Holmes et al. 2006]
[Selin et al. JGR 2007]
HIGH LEVELS OF RGM IN THE FREE TROPOSPHERE
AND STRATOSPHERE
Vertical profile of GEOS-Chem vs.
800 mb Hg(II) fields show the influence
measurements at Mt. Bachelor, Oregon (2.7
of large-scale subsidence (contributes
km) show elevated levels relative to surface
to high levels of Hg(II) deposition in the
[Swartzendruber et al. JGR 2006]
subtropics) [Selin et al. in prep for GBC]
=daytime
▲
● (blue)=all
◊ = nighttime
DEPOSITION: LOCAL VS. GLOBAL SOURCES
Two patterns of mercury wet
deposition over the U.S.
(background=model, dots=measured)
1) Latitudinal gradient (higher in the
subtropics). From oxidation of
global pool of Hg(0) and
subsequent rainout; influence of
subsidence.
2) Near-source wet deposition of
locally-emitted Hg(II) and Hg(P)
(underestimated in GEOS-Chem)
% contribution of North American
sources to total (wet + dry) deposition
GEOS-Chem model
U.S. mean: 20%
Reflects influence of locally-deposited
Hg(II) and Hg(P) in source regions
Measurements [Mercury Deposition Network, 2006]; GEOS-Chem [Selin et al., JGR, 2007]
CONSTRAINING NATURAL AND RECYCLED SOURCES
THROUGH A PRE-INDUSTRIAL MODEL
Deposition
Steady state assumption:
-Soil Hg comes from the atmosphere
(for about 90% of land area)
-What goes down, must come up…
=
Evasion
GEOS-Chem
(4x5) grid box
Runoff:
negligible
Soil volatilization:
F(T, [Hg], solar radiation)
Evapotranspiration:
F([Hg], transp. rate)
Prompt recycling:
“New” Hg can be more
easily reduced/emitted
than resident Hg
[Hintelmann et al. 2002]
g m-2 y-1
[Selin et al. in prep for GBC]
EVALUATING MERCURY CYCLE AND LIFETIMES
GEOS-Chem Pre-industrial Hg Cycle
Hg is very long-lived in
the soil (1000 y);
however, the surface
ocean recycles Hg
efficiently (1 y)
Recycling in the surface
ocean more than
doubles the effective
atmospheric lifetime of
emitted Hg
Future work: coupling
with intermediate/deep
ocean reservoirs
Quantities in Mg, Fluxes in Mg/y
[Selin et al. in prep for GBC]
ESTIMATING THE ANTHROPOGENIC, RECYCLED AND
NATURAL CONTRIBUTIONS TO DEPOSITION
Anthropogenic Enrichment Factor (Present/Preindustrial Deposition)
Deposition to the U.S.:
20% from North American
anthropogenic emissions
22% from outside North
America anthropogenic
26% from recycled
anthropogenic emissions
32% natural
Table 1: Total deposition to t he U.S. from natural, anthropogenic, and recycled emi ssions (Mg)
Source
Natural
Primary Anthropogenic
(North America)
Primary Anthropogenic
(Outside North America)
Recycled Anthropogenic
Total
Dry Hg(0)
39
8
Wet Hg(II)+(P)
15
14
Dry Hg(II)+(P)
29
30
Total
83 (32%)
52 (20%)
24
11
22
57 (22%)
29
100
14
54
25
106
68 (26%)
260 (100%)
[Selin et al. in prep for GBC]
TAKE-HOME MESSAGES FOR POLICY
• Domestic, regional, and global regulation are all
important in addressing the mercury problem
vs.
In the US, Florida and Ohio both see
high deposition -- but the source
patterns are very different
• Hg(0), Hg(II) and Hg(P) emissions have different
deposition patterns, and may need different
regulatory strategies
• Need for better understanding of redox chemistry,
and cycling in land & ocean reservoirs (will
climate change have an effect?)
• Need for improved cross-scale governance