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The atmospheric cycle of mercury, focusing on its two stable oxidation states (hg(0) and hg(ii)), and discusses the biogeochemical cycle, including sources, sinks, and transport processes. It also covers the impact of anthropogenic activities on mercury emissions and atmospheric concentrations.
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Mass number = 80: 1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
6
4d
10
4f
14
5s
2
5p
6
5d
10
6s
2
Complete filling of subshells gives Hg(0) a low melting point, volatility
Two stable oxidation states: Hg(0) and Hg(II)
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Energetic arrangement of orbitalsis such that mercury (Z=80) hasall its subshells filled
Global mercury deposition has roughly tripled since preindustrial times
Dietz et al. [2009]
Mercury biomagnification factor
State fish consumption advisories
EPA reference dose (RfD) is 0.
μ
g kg
d
(about 2 fish meals per week)
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
LOCAL POLLUTION INFLUENCE FROM EMISSION OF Hg(II)
High-temperature combustion emits both Hg(0) and Hg(II)
photoreduction
Hg(II) concentrations in surface air:
circles = observed, background=model
Large variability of Hg(II) implies atmospheric lifetime of only days
against deposition
Selin et al. [2007]
Thus mercury is BOTH a global
and a local pollutant!
Hg(0) lifetime against oxidation must be ~ months
Observed variability of Hg(
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
x
sea saltsource
organohalogen
source
radicalcycling
non-radicalreservoirformation
heterogeneous recycling
3
2
2
From NASA aircraft campaigns over Pacific in April-June
Vertical profiles steeper for CHBr
3
(mean lifetime 21 days) than for CH
2
Br (91 days),
steeper in extratropics than in tropics
Parrella et al. [2012]
y
CHBr
3
2
Br
2
3
Br
Marine biosphere
Sea-salt debromination
(50% of 1-10 μm particles)
7-9 ppt Liang et al. [2010] stratospheric Bry model (upper boundary conditions)
Br
3.2 ppt
Volcanoes
Depositionlifetime 7 days
Sea salt is the dominant global source but is released in marine boundary layerwhere lifetime against deposition is short; CHBr
3
is major source in the free
troposphere
Parrella et al. [2012]
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y
Br
y
is 2-4 ppt, highest over
Southern Ocean (sea salt)
BrO increases with latitude(photochemical sink)
Br increases with altitude(BrO photolysis)
Parrella et al. [2012]
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[Theys et al., 2011]
model
model
TOMCAT has lower
=0.02 for
HOBr+HBr than GEOS-Chem,large polar spring source fromblowing snow
HOBr+HBr reaction critical forincreasing BrO with latitude,winter/spring NH max inGEOS-Chem
(9:30 am)
Parrella et al. [2012]
th
Standard models without bromine are too high, peak in winter-spring; brominechemistry corrects these biases
Model BrO is similar in pre-industrial and present atmosphere (canceling effects)
Parrella et al. [2012]
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
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]
