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Sulfur and oxygen isotopic Sulfur and oxygen isotopic tracers of past and present tracers of past and present
atmospheric chemistryatmospheric 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 ReactorThe Atmospheric Reactor
Industry Volcanoes Marine Biogenics
Biomass burning
Continental Biogenics
Deposition,
Biosphere interaction
Primary Species H2S, SO2, CH4, CO,
CO2, NO, N2O, particulates
Secondary Species CO2, H2SO4, HNO3,
RCOOH
Oxidation Capacity
Photochemistry
ClimatePollution
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Atmospheric Chemistry is controlled by Atmospheric Chemistry is controlled by atmospheric oxidantsatmospheric oxidants
““The Earth’s oxidizing capacity”The Earth’s oxidizing capacity”
O3
CH4
CO
HC
NOx
OH H2O2
h, O
(1 D)
O 2, H 2
O
O3, NO
HO2
SO
xH
2SO
4
SOx
H2 SO
4
SOx
H2S
O4
NO x
HN
O 3
NO
xH
NO
3
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MeasurementsMeasurementsField studies
Laboratory studies
ModelsModelsCoupled
chemistry/climate global models
Global pictureGlobal picture
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Stable Isotope Measurements: Stable Isotope Measurements:
Tracers of source strengths and Tracers of source strengths and chemical processing of atmospheric chemical processing of atmospheric
constituentsconstituents
(‰) = [(Rsample/Rstandard) – 1] 1000
R = minorX/majorX
18O: R = 18O/16O
(CO2, CO, H2O, O2, O3, SO42-….)
3434S: R = S: R = 3434S/S/3232SS
(SO2, SOSO442-2-,, H2S)
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Sea water
34S (‰) -10 +10 +20 +30 -20
Volcanic/Mineral
Biogenic
Marine Biogenic
Coal
Oil
CONTINENTCONTINENT
OCEANOCEAN
COMBUSTIONCOMBUSTION
Overlapping Source SignaturesOverlapping Source Signatures
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= = 3434SSSO4SO4//3434SSSO2SO2
SO2 + OH SO4: > 1.07 > 1.07
(Luong et al., 2001)
SO2 + O3/H2O2 SO4: = 1.0165 = 1.0165
(Eriksen, 1972)
Oxidation of the heavier isotope is favored resulting in an increasing degree of 34S depletion at
progressively later times
Chemical isotopic fractionationChemical isotopic fractionation
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Mass-Dependent FractionationMass-Dependent Fractionation
-20
-15
-10
-5
0
5
10
15
20
-40 -30 -20 -10 0 10 20 30 40
O‰
O‰
Air O2
Slope= 0.5
Basaltic and Sedementary Rocks
Rain and Cloud Water
SMOW
17O 0.5*18O : 17O = 17O – 0.5*18O = 0
33S 0.5*34S : 33S = 33S – 0.5*34S = 0
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-80
-60
-40
-20
0
20
40
60
-100 -80 -60 -40 -20 0 20 40 60 8018O
17O
Product Ozone
Residual Oxygen
Starting Oxygen
OO33 formation in the laboratory formation in the laboratory
Thiemens and Heidenreich, 1983
17O/18O 1
17O = 17O – 0.5*18O 0
17O
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Mass-independent isotope effects – symmetry Mass-independent isotope effects – symmetry explanationexplanation
Symmetry C2v Symmetry Cs
16
16 16
16
16 17 or 18
O2 + O(3P) O3*
EVibrational
StatesRotational
States
De
v = i
v=i+1
RotationalStates
VibrationalStates
De
v = i
v=i+1
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25
10
5
50
75
100
10 20 50 100
SO4
CO
N2O
H2O2
NO3
CO2 strat.
O3
trop.
O3
strat.
18O
17O
All All 1717O measurements in the O measurements in the atmosphereatmosphere
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Tropospheric oxidationTropospheric oxidation
17O of HNO3 a function of RO2/O3 and the terminal reaction
17O of NOx is a function of RO2/O3 oxidation
The 17O of HNO3 depends also on the dilution
factor due to the terminal reaction
NO2 + OH HNO3
NO3 + RH HNO3
N2O5 + H2O(aq) 2HNO3
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Tropospheric oxidationTropospheric oxidation
SO2 in isotopic equilibrium with H2O : No source effect:
17O of SO2 = 0 ‰
HSO3- + O3 17O ~ 8.0 ‰, pH > 5.6
HSO3- + H2O2 17O ~ 0.5 ‰, pH < 5.6
SO2 + OH 17O = 0 ‰
17O of SO4 a function relative amounts of OH, H2O2, and O3 oxidation
Aqueous
Gas
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Gas versus Aqueous-Phase OxidationGas versus Aqueous-Phase Oxidation
Gas-phase:
SO2 + OH new aerosol particle increased aerosol number
concentrations
Aqueous-phase:
SO2 + O3/H2O2 growth of existing aerosol particle
Cloud albedo and climate
Microphysical/optical
properties of clouds
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1.0E-15
1.0E-13
1.0E-11
1.0E-09
1.0E-07
1.0E-05
1.0E-03
1.0E-01
1.0E+01
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
pH
S(IV
) oxi
datio
n ra
te (M
/sec
)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
17
O(S
(VI))
[SM
OW
‰]
H2O2
O3
Lee et al., 2001
OO33/H/H22OO22 oxidation depends on pH of oxidation depends on pH of aqueous phaseaqueous phase
17O
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17O (SO4)aqueous = 1.82 ‰
Estimated sulfate contribution from Estimated sulfate contribution from different sources in La Jolla, CA rainwaterdifferent sources in La Jolla, CA rainwater
pH = 5.1 (average of La Jolla rainwater)
17O (SO4)actual = 0.75 ‰
Seasalt Aqueous Gas
30% 41% 29%
Lee et al., 2001
[Na+]
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Oxygen (Oxygen (1717O) O) relative oxidation relative oxidation pathways (oxidant chemistry)pathways (oxidant chemistry)
Gas/Aqueous phase chemistry climateRelative oxidation concentrations
oxidation efficiency
Sulfur (Sulfur (3333S) ?S) ?
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-30
-20
-10
0
10
20
30
-40 -30 -20 -10 0 10 20 30 40 50
34S
33S
Continuum > 220nm
Mass-fractionation line
SOSO22 photolysisphotolysis
Farquhar et al., 2001
Volcanic sulfate in Volcanic sulfate in South Pole iceSouth Pole ice
Sulfate
Residual SO 2
•1991 Pinatubo :
33S = 0.7 ± 0.1 ‰
•1259 Unknown :
33S = -0.5 ± 0.1 ‰
•1991 Cerro Hudson :
33S = -0.1 ± 0.1 ‰
Savarino et al., 2002
Non-zero Non-zero 3333S S stratospheric influence stratospheric influence
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Conservative Tracers in Ice cores:Conservative Tracers in Ice cores:Na+
NO3-
SOSO442-2-
Composition of gas bubbles
SO42- very stable
(34S) sources of sulfate
(33S) stratospheric influence
((1717O) aqueous v. gas phase oxidationO) aqueous v. gas phase oxidation(17O) oxidant concentrations oxidation capacity of the atmosphere
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Current knowledge of the past oxidative capacity of the Current knowledge of the past oxidative capacity of the atmosphereatmosphere
Model results (vs Pre Indus. Model results (vs Pre Indus. Holocene)Holocene)
Conflicting results on OH, highly dependent on emission scenarios of NMHC, NOx which are not
very well constrained
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
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Current knowledge of the past oxidative capacity of the Current knowledge of the past oxidative capacity of the atmosphereatmosphere
Doubling of O3 between PIT/IT
Measurement approach
Voltz & Kley, 1988 Sigg & Neftel, 1991 Summit
Dye 3
50 % increase of H2O2 between PIT/IT
But calibration issue, not representative of global conditions, or stability in proxy records.
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AntarcticaAntarctica GreenlandGreenland
Sulfate concentration
varies with climate
Sulfate concentration
reflects anthropogenic
emissions
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Analytical ProcedureAnalytical Procedure
Old methodBaSO4 + C CO2 CO2 + BrF5 O2
(3 days of chemistry, 10 mol sulfate)
New methodAg2SO4 O2 + SO2
(minutes of chemistry, 1-2 mol sulfate)
Faster, smaller sample sizes, O and S Faster, smaller sample sizes, O and S isotopes in same sampleisotopes in same sample
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[SO42-] tracks [MSA-] suggesting a predominant
DMS (oceanic biogenic) source
0
50
100
150
200
250
300
350
0 20 40 60 80 100 120
Age (kyr)
SO4 (ppb)
-500-490-480-470-460-450-440-430-420-410
D (‰)
Vostok, Antarctica Ice CoreVostok, Antarctica Ice Core
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Vostok Ice Core – Climatic Vostok Ice Core – Climatic 1717O (SOO (SO44) fluctuations) fluctuations
0
1
2
3
4
5
6
0 20 40 60 80 100 120 140
Age (kyr)
17O
-6
-5
-4
-3
-2
-1
0
1
2
3
Ts
Ts data: Kuffey and Vimeux, 2001, Vimeux et al., 2002
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Vostok trendlineR2 = 0.94
-10
-8
-6
-4
-2
0
2
4
6
8
10
-15 -13 -11 -9 -7 -5 -3 -1 1 3 5
O‰
O‰
Terrestrial Mass Fractionation LineSlope= 0.52
Vostok sulfate three-isotope plotVostok sulfate three-isotope plot
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Vostok trendline
-60
-40
-20
0
20
40
60
80
100
-70 -20 30 80
18O
17O
Mass-dependent line
100% O3
oxidation100% OH oxidation
Tropospheric O3
OH and H2O
H2O2
100% H100% H22OO22 oxidation: oxidation:
17O(SO4) = ½*1‰ = 0.5 ‰
17O range = 1.3 – 4.8 ‰
Extended 3-isotope plotExtended 3-isotope plot
100% O100% O33 oxidation: oxidation:
17O (SO4) = ¼ * 32‰ = 7.5‰
100% OH oxidation:100% OH oxidation:
17O (SO4) = 0 ‰
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71294.8130.2Eemian
70304.7121.9Eemian
42582.8109.9Glacial
20801.360.2Glacial
28721.914.3Glacial
34662.311.2Holocene
50503.48.7Holocene
46543.15.7Holocene
%O3% OH17OAge (kya)
Period
71294.8130.2Eemian
70304.7121.9Eemian
42582.8109.9Glacial
20801.360.2Glacial
28721.914.3Glacial
34662.311.2Holocene
50503.48.7Holocene
46543.15.7Holocene
%O3% OH17OAge (kya)
Period
Results of calculationsResults of calculations
OH (gas-phase) oxidation relatively greater in glacial period
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33S = 0 for all Vostok samples
What can cause this climate What can cause this climate variation?variation?
•Stratospheric influence? NO
•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
GCM sensitivity studiesGCM sensitivity studies
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Sulfur oxidation Sulfur oxidation pathways have a pathways have a
natural variation on natural variation on the glacial/interglacial the glacial/interglacial
timescale.timescale.
Do we see a variation as a result
of anthropogenic activities?
M i l c e n t S i t e A
G I S P 2
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Mayewski et al., 1990
Sulfate and nitrate in Sulfate and nitrate in Greenland ice coresGreenland ice cores
Fossil fuel burning Fossil fuel burning trendstrends
from Graedel and Crutzen, “Atmospheric Change”.
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Site A Site A
NONO33--
Site ASite A
SOSO442-2-
26
26.5
27
27.5
28
28.5
29
29.5
30
1680 1730 1780 1830 1880 1930 1980
[NO3]g/L30
40
50
60
70
80
90
100
110
120
13017O
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
1680 1730 1780 1830 1880 1930 1980
Year (AD)
17O
0
20
40
60
80
100
120
140
160
[SO4] g/L
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1600 1700 1800 1900 2000Year (AD)
fire index
0
0.5
1
1.5
2
2.5
3
3.5
Sulfate 17O (‰)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1600 1700 1800 1900 2000Year (AD)
fire index
26.5
27
27.5
28
28.5
29
29.5
30Nitrate
17O (‰)
Fire index data: Savarino and Legrand, 1998
Pre-Industrial Biomass BurningPre-Industrial Biomass Burning
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Biomass burning can affect Biomass burning can affect 1717O of O of sulfate and nitrate by:sulfate and nitrate by:
1) Altering oxidant (O3) concentrations
2) Increase aerosol loading affecting heterogeneous oxidation pathways
Are 17O measurements of sulfate/nitrate proxies of:
Oxidation capacity?
Aerosol concentrations?
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Resolving Resolving 1717O sulfate in GEOS-CHEMO sulfate in GEOS-CHEM
Resolve sulfate sources:
SO2 + OH SO4A
HSO3- + H2O2 SO4B
SO32- + O3 SO4C
primary sulfate = SO4D
(currently direct anthropogenic emissions)
17O = (1*0.5*SO4B + 32*0.25*SO4C)/
(SO4A + SO4B + SO4C + SO4D)
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Oxidation by OOxidation by O33 only important during only important during winter in high northern latitudeswinter in high northern latitudes
17O > 1 O3 oxidation
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1717O sulfate versus cloud processingO sulfate versus cloud processing1
717 OO
Cloud liquid water contentCloud liquid water content
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1717O sulfate versus OO sulfate versus O33 concentration concentration1
717 OO
OO33 ppbv ppbv
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1717O sulfate versus O sulfate versus HH22OO22 concentration concentration
1717O sulfate versus OH O sulfate versus OH concentrationconcentration
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1717O versus HO versus H22OO22 : January : January1
717 OO
HH22OO22 ppbv ppbv
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1717O of sulfate is strongly affected O of sulfate is strongly affected by (oxidant) Hby (oxidant) H22OO22 concentrations concentrations
Less so by cloud contentLess 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 pH=8, large
buffering capacity)
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Oxidation on sea-salt aerosolsOxidation 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 DirectionsConclusions and Future Directions
17O 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 17O
Simulation of heterogeneous chemistry must be improved in GCMs “current”
17O measurements
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AcknowledgementsAcknowledgements
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