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

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

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

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

MeasurementsMeasurementsField studies

Laboratory studies

ModelsModelsCoupled

chemistry/climate global models

Global pictureGlobal picture

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)

Sea water

34S (‰) -10 +10 +20 +30 -20

Volcanic/Mineral

Biogenic

Marine Biogenic

Coal

Oil

CONTINENTCONTINENT

OCEANOCEAN

COMBUSTIONCOMBUSTION

Overlapping Source SignaturesOverlapping Source Signatures

= = 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

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

-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

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

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

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

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

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

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

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+]

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) ?

-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

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

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

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.

AntarcticaAntarctica GreenlandGreenland

Sulfate concentration

varies with climate

Sulfate concentration

reflects anthropogenic

emissions

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

[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

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

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

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 ‰

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

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

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

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”.

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

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

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?

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)

Oxidation by OOxidation by O33 only important during only important during winter in high northern latitudeswinter in high northern latitudes

17O > 1 O3 oxidation

1717O sulfate versus cloud processingO sulfate versus cloud processing1

717 OO

Cloud liquid water contentCloud liquid water content

1717O sulfate versus OO sulfate versus O33 concentration concentration1

717 OO

OO33 ppbv ppbv

1717O sulfate versus O sulfate versus HH22OO22 concentration concentration

1717O sulfate versus OH O sulfate versus OH concentrationconcentration

1717O versus HO versus H22OO22 : January : January1

717 OO

HH22OO22 ppbv ppbv

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)

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

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

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