Sulfur and oxygen isotopic tracers of past and present atmospheric chemistry

1
Sulfur and oxygen isotopic tracers of past and
present atmospheric chemistry
Becky Alexander Harvard University April 14, 2003
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Overview

• What controls atmospheric chemistry and why do we
  care?
• Stable isotope measurements limitations and
  advantages
• Mass-independent fractionation in O and S
  isotopes (NO3- and SO42-)
• Ice core sulfate and nitrate past variations in
  atmospheric chemistry
• Preliminary modeling insights
• Summary and conclusions

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The Atmospheric Reactor
Climate
Pollution
Industry
Volcanoes
Marine
Biomass
Continental
Biogenics
burning
Biogenics
4
Atmospheric Chemistry is controlled by
atmospheric oxidants
The Earths oxidizing capacity
CH4 CO HC NOx
O3
H2O2
OH
5
Models Coupled chemistry/climate global models
Measurements Field studies Laboratory studies
Global picture
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Stable Isotope Measurements Tracers of source
strengths and chemical processing of atmospheric
constituents
?() (Rsample/Rstandard) 1 ? 1000 R
minorX/majorX ?18O R 18O/16O (CO2, CO, H2O,
O2, O3, SO42-.) ?34S R 34S/32S (SO2, SO42-,
H2S)
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Overlapping Source Signatures

()

d
3
4
S
-
10


10

20

30

-
2
0

Volcanic/Mineral


CONTINENT
Biogenic


OCEAN
Marine Biogenic

Sea water


COMBUSTION
Coal
Oil

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Chemical isotopic fractionation

• d34SSO4/d34SSO2
• SO2 OH ? SO4 a gt 1.07
• (Luong et al., 2001)
• SO2 O3/H2O2? SO4 a 1.0165
• (Eriksen, 1972)

Oxidation of the heavier isotope is favored
resulting in an increasing degree of 34S
depletion at progressively later times
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Mass-Dependent Fractionation
d17O ? 0.5d18O D17O d17O 0.5d18O 0 d33S
? 0.5d34S D33S d33S 0.5d34S 0
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O3 formation in the laboratory
D17O
Thiemens and Heidenreich, 1983
d17O/d18O ? 1 D17O d17O 0.5d18O ? 0
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Mass-independent isotope effects symmetry
explanation
Symmetry C2v
Symmetry Cs
O2 O(3P) O3
Vibrational States
Rotational States
Rotational States
Vibrational States
E
vi1
vi1
v i
v i
De
De
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All D17O measurements in the atmosphere
O3 strat.
d17O
100
O3 trop.
75
CO2 strat.
50
NO3
25
N2O
10
H2O2
CO
5
d18O
SO4
10
20
50
100
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Tropospheric oxidation
D17O of HNO3 a function of RO2/O3 and the
terminal reaction
D17O of NOx is a function of RO2/O3 oxidation
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Tropospheric oxidation
D17O of SO4 a function relative amounts of OH,
H2O2, and O3 oxidation
SO2 in isotopic equilibrium with H2O No source
effect D17O of SO2 0
HSO3- O3 ? D17O 8.0 , pH gt 5.6 HSO3- H2O2
? D17O 0.5 , pH lt 5.6 SO2 OH ? D17O 0
Aqueous
Gas
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Gas versus Aqueous-Phase Oxidation
Aqueous-phase SO2 O3/H2O2 ? growth of existing
aerosol particle
Gas-phase SO2 OH ? new aerosol particle ?
increased aerosol number concentrations
Microphysical/optical properties of clouds
Cloud albedo and climate
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O3/H2O2 oxidation depends on pH of aqueous phase
D17O
Lee et al., 2001
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Estimated sulfate contribution from different
sources in La Jolla, CA rainwater
pH 5.1 (average of La Jolla rainwater)
D17O (SO4)aqueous 1.82
D17O (SO4)actual 0.75
Seasalt Aqueous Gas 30 41 29
Lee et al., 2001
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Oxygen (D17O) ? relative oxidation pathways
(oxidant chemistry)Gas/Aqueous phase chemistry
? climateRelative oxidation concentrations ?
oxidation efficiency

• Sulfur (D33S) ?

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SO2 photolysis
Volcanic sulfate in South Pole ice
Continuum gt 220nm

• 1991 Pinatubo
• D33S 0.7 0.1
• 1259 Unknown
• D33S -0.5 0.1
• 1991 Cerro Hudson
• D33S -0.1 0.1

Sulfate
Mass-fractionation line
Residual SO2
Savarino et al., 2002
Farquhar et al., 2001
Non-zero D33S ? stratospheric influence
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Conservative Tracers in Ice cores
Na NO3- SO42- Composition of gas bubbles
SO42- very stable (d34S) sources of
sulfate (D33S) stratospheric influence (D17O)
aqueous v. gas phase oxidation
(D17O) oxidant concentrations ? oxidation
capacity of the atmosphere
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Current knowledge of the past oxidative capacity
of the atmosphere
Model results (vs Pre Indus. Holocene)
Model author OH O3 Remarks
Martinerie et al., 1995 Ice age 17 Indus 6 Ice age -15 Indus 150 2 D model, No NMHC
Karol et al., 1995 Ice age -35 Indus 9 Ice age -20 Indus 70 1D model No NMHC
Thompson et al., 1993 Ice age 12 Indus -0.15 Ice age -20 Indus 80 1D model with NMHC
Conflicting results on OH, highly dependent on
emission scenarios of NMHC, NOx which are not
very well constrained
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Current knowledge of the past oxidative capacity
of the atmosphere
Measurement approach
Doubling of O3 between PIT/IT
50 increase of H2O2 between PIT/IT
Sigg Neftel, 1991
Voltz Kley, 1988
Summit
Dye 3
But calibration issue, not representative of
global conditions, or stability in proxy records.
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Antarctica
Greenland
Sulfate concentration reflects anthropogenic
emissions
Sulfate concentration varies with climate
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Analytical Procedure

• Old method
• BaSO4 C ? CO2
• CO2 BrF5 ? O2
• (3 days of chemistry, 10 mmol sulfate)
• New method
• Ag2SO4 ? O2 SO2
• (minutes of chemistry, 1-2 mmol sulfate)
• Faster, smaller sample sizes, O and S isotopes in
  same sample

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Vostok, Antarctica Ice Core
SO42- tracks MSA- suggesting a predominant
DMS (oceanic biogenic) source
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Vostok Ice Core Climatic D17O (SO4) fluctuations
DTs data Kuffey and Vimeux, 2001, Vimeux et al.,
2002
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Vostok sulfate three-isotope plot
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Extended 3-isotope plot
100 O3 oxidation D17O (SO4) ¼ 32
7.5 100 OH oxidation D17O (SO4) 0
100 H2O2 oxidation D17O(SO4) ½1 0.5
D17O range 1.3 4.8
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Results of calculations
OH (gas-phase) oxidation relatively greater in
glacial period
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GCM sensitivity studies
What can cause this climate variation?

• Stratospheric influence? ? NO

D33S 0 for all Vostok samples

• Changes in oxidant concentrations in the
  atmosphere?
• Oxidation capacity of the atmosphere

• Changes in cloud processing/liquid water content?
• Cloud/water content of the atmosphere

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Sulfur oxidation pathways have a natural
variation on the glacial/interglacial timescale.
Do we see a variation as a result of
anthropogenic activities?
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Sulfate and nitrate in Greenland ice cores
Fossil fuel burning trends
from Graedel and Crutzen, Atmospheric Change.
Mayewski et al., 1990
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Site A NO3-
Site A SO42-
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Pre-Industrial Biomass Burning
Fire index data Savarino and Legrand, 1998
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• Biomass burning can affect D17O of sulfate and
  nitrate by
• Altering oxidant (O3) concentrations
• Increase aerosol loading affecting heterogeneous
  oxidation pathways

Are D17O measurements of sulfate/nitrate proxies
of Oxidation capacity? Aerosol concentrations?
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Resolving D17O sulfate in GEOS-CHEM
Resolve sulfate sources SO2 OH ? SO4A HSO3-
H2O2 ? SO4B SO32- O3 ? SO4C primary sulfate
SO4D (currently direct anthropogenic emissions)
D17O (10.5SO4B 320.25SO4C)/ (SO4A SO4B
SO4C SO4D)
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Oxidation by O3 only important during winter in
high northern latitudes
D17O gt 1 ? O3 oxidation
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D17O sulfate versus cloud processing
D17O
Cloud liquid water content
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D17O sulfate versus O3 concentration
D17O
O3 ppbv
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D17O sulfate versus H2O2 concentration
D17O sulfate versus OH concentration
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D17O versus H2O2 January
D17O
H2O2 ppbv
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D17O of sulfate is strongly affected by (oxidant)
H2O2 concentrations Less so by cloud content
Importance of oxidation by O3 is not
represented Aqueous-phase oxidation occurs in
clouds only (pH 4.5)
Aqueous oxidation occurs on deliquescent sea-salt
aerosols (initial pH8, large buffering capacity)
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Oxidation on sea-salt aerosols
Sea salt flux to atmosphere 1.01 x 104 Tg/year
? 11.1 Tg(S)/year (Gong et al., 2002) Global DMS
emissions 15-25 Tg(S)/year (Seinfeld and Pandis,
1998)
44 -74 of SO2 (from DMS) oxidized to sulfate by
O3 on sea-salt aerosols
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Conclusions and Future Directions

• D17O measurements of both sulfate and nitrate
  reflect variations in
• Changes in the oxidation capacity ? Potential
  buildup of pollutants
• Changes in aerosol/cloud properties ? Climate
  change

Model sensitivity studies can determine the
importance of each on D17O
Simulation of heterogeneous chemistry must be
improved in GCMs ? current D17O measurements
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Acknowledgements
Prof. Mark Thiemens UCSD Dr. Joël Savarino
CNRS/LGGE Laboratoire de Glaciologie et
Géophysique de l'Environnement (LGGE) The
National Ice Core Laboratory (USGS) Prof. Daniel
Jacob Harvard Dr. Rokjin Park Harvard Bob
Yantosca - Harvard