Understanding the present and paleo record of the oxygen

isotopes of sulfate

Carnegie Institution of Washington Geophysical Laboratory

October 20, 2003

Becky Alexander

Postdoctoral Fellow

Department of Earth and Planetary Sciences

Harvard University

Overview:

1. What can the 17O record of sulfate tell us about past and present variations in atmospheric chemistry?

2. Ice core record of 17O sulfate – what have we learned?

3. How can we interpret the data and improve our understanding of the long term record of atmospheric chemistry?

Measurements Model

Interpretation

Constraint

Atmospheric Oxidation Capacity

Industry Volcanoes Marine Biogenics

Biomass burning

Continental Biogenics

Primary Species H2S, SO2, CH4, CO,

CO2, NO, N2O, particulates

Secondary Species CO2, H2SO4, HNO3,

RCOOH, O3

OH

Climate change

hH2O

?

?

Atmospheric Chemistry is controlled by atmospheric oxidants

“The Earth’s oxidizing capacity”

O3

CH4

CO

HC

NOx

OH H2O2

h, O

(1 D)

O 2, H

2O

O3, NO

HO2

SO

xH

2SO

4

SO

x

H2 S

O4

SO

xH

2S

O4

NO

x

HN

O 3

NO

xH

NO

3

Current knowledge of the past oxidative capacity of the atmosphere

Model results (vs Preindustrial 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 atmosphere

Measurement approach

Doubling of O3 between PIT/IT

Voltz & Kley, 1988

Sigg & Neftel, 1991 Summit

Dye 3

50% increase of H2O2 between PIT/IT

Calibration issue (O3), low stability in proxy records (H2O2).

Stable Isotope Measurements:

Tracers of source strengths and/or chemical processing of atmospheric constituents

(‰) = [(Rsample/Rstandard) – 1] 1000

R = minorX/majorX

18O: R = 18O/16O

17O: R = 17O/16O

Standard = SMOW (Standard Mean Ocean Water)

(CO2, CO, H2O, O2, O3, SO42-….)

17O/18O 0.5

17O = 17O – 0.5*18O = 0

Mass-Independent Fractionation

17O/18O 1

-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

Thiemens and Heidenreich, 1983

17O

17O

17O = 17O – 0.5*18O 0

O + O2 O3*

Mass-dependent fractionation line: 17O/18O 0.5

Explanation of Observations

(17O 0)

•Rate coefficient advantage due to zero point energy differences (not mass-independent!):

k(16O+18O18O)/k(16O+16O16O) = 1.53

k(18O+16O16O)/k(16O+16O16O) = 0.93

Janssen et al., 1999; Mauersberger et al., 1999

•Density of quantum states of O3* coupled to exit channels is larger for asymmetric isotopomers (18O16O16O*) than for symmetric (16O18O16O*).

(asymm) /(symm) = 1.18

Gao and Marcus, 2001

25

10

5

50

75

100

10 20 50 100

SO4

CO

N2O

H2O2

NO3

CO2 strat.

O3

trop.

O3

strat.

18O

17O

17O measurements in the atmosphere

Source of 17O Sulfate

SO2 in isotopic equilibrium with H2O :

17O of SO2 = 0 ‰

1) SO2 + O3 (17O=30-35‰) 17O ~ 8-9 ‰

17O of SO42- a function relative amounts of OH, H2O2, and O3 oxidation

Savarino et al., 2000

3) SO2 + OH (17O=0‰) 17O = 0 ‰

2) SO2-+ H2O2 (17O=1-2‰) 17O ~ 0.5-1 ‰

Aqueous

Gas

Gas versus Aqueous-Phase Oxidation of Sulfate

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

pH dependency of O3 oxidation and its effect on 17O of SO4

2-

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

pH

Oxi

dat

ion

rat

e (M

/sec

)

H2O2

O3

1.0E-151.0E-141.0E-13

1.0E-121.0E-111.0E-101.0E-091.0E-08

1.0E-071.0E-061.0E-051.0E-041.0E-03

1.0E-021.0E-011.0E+00

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

pH

Oxi

dat

ion

rat

e (M

/sec

)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

17

O (

‰)

H2O2

O3

Lee et al., 2001

17O (SO42-)aqueous = 1.82 ‰

Sources of Sulfate in La Jolla, CA rainwater

pH = 5.1 (average of La Jolla rainwater)

17O (SO42-)actual = 0.75 ‰

Lee et al., 2001

[Na+]

Sea salt

30%

Aqueous Gas

41% 29%

Conservative Tracers in Ice cores

Na+

SO42-

Composition of gas bubbles

SO42- very stable

(34S) sources of sulfate

(33S) stratospheric influence

(17O) aqueous v. gas phase oxidation(17O) oxidant concentrations oxidation capacity of the atmosphere?

Analytical Procedure

Analytical Procedure

Old method:BaSO4 + C CO2 CO2 + BrF5 O2

(3 days of chemistry, 10 mol sulfate)

New method:Ag2SO4 O2 + SO2

(minutes of chemistry, 1-2 mol sulfate)

Faster, smaller sample sizes, O and S isotopes 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

Vostok Ice Core

Climatic 17O (SO42-) 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 plot

slope1

Vostok 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% H2O2 oxidation:

17O(SO4) = ½*1.7‰ = 0.85 ‰

17O range = 1.3 – 4.8 ‰

100% O3 oxidation:

17O (SO4) = ¼ * 32‰ = 8‰

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 calculations

OH (gas-phase) oxidation relatively greater in glacial period

Potential climate effects of SO42- over the

ocean

Biological regulation of the climate?

(Charlson et al., Nature 1987)

DMSOH

NO3 SO2 H2SO4

OH

O3

New particle formation

CCN

Interpretation of Vostok 17O data

Does more OH oxidation of S(IV) during the last glacial period mean:

• Greater atmospheric oxidation capacity (more OH)?

• Lower cloud processing efficiency?

• Changes in cloud/aerosol characteristics (i.e. pH, water content)?

Global 3-D model simulations of atmospheric sulfur chemistry

GEOS-CHEM

• Global 3-D model of atmospheric chemistry

•4ºx5º horizontal resolution, 26-30 layers in vertical

• Driven by assimilated meteorology (1985 –present). Eventually will be coupled to NASA-GISS meteorology for both past and future simulations.

• Includes aqueous and gas phase chemistry:

S(IV) + OH (gas-phase)

S(IV) + O3/H2O2 (in-cloud, pH=4.5)

• Off-line sulfur chemistry (uses monthly mean OH and O3 fields from a full chemistry, coupled aerosol simulation)

http://www-as.harvard.edu/chemistry/trop/geos/index.html

GEOS-CHEM 17O Sulfate Simulation

SO2 + OH (gas phase) 17O=0‰

S(IV) + H2O2 (in cloud, pH=4.5) 17O=0.85‰

S(IV) + O3 (in cloud, pH=4.5) 17O=8‰

Use constant, global 17O value for oxidants

17O ‰ method reference

O3 35 Photochemical model

Lyons 2001

27-32 Tropospheric measurements

Johnston and Thiemens 1997

H2O2 1.3-2.2 (1.7)

Rainwater measurements

Savarino and Thiemens 1999

OH 0 Experimental Dubey et al., 1997

GEOS-CHEM 17O Sulfate Simulation17O sulfate (January)

0.0 2.3 4.6

17O > 1‰ O3 oxidation

17O sulfate (July)

0.0 2.3 4.6

Preindustrial Antarctic ice core sulfate: 17O = 1.3-4.8‰

(Alexander et al., 2001)

Missing O3 oxidation source?

17O

H2O2 (ppbv)

Winter: low H2O2

NH: High SO2

17O

H2O2 (ppbv)

O3 oxidation on sea-salt aerosols

pH = 8

O3 oxidation dominant

Function of wind speed

GEOS-CHEM

S(IV) oxidation by O3 is a function of sea salt alkalinity flux to the atmosphere

Reaction can proceed until alkalinity is titrated (pH<6)

July 17O sulfate

0.0‰ 3.5‰ 7.0‰

January 17O sulfate

GEOS-CHEM 17O Sulfate Simulation with Sea Salt Chemistry

INDOEX

Pre-INDOEX Jan. 1997 INDOEX March 1998

INDOEX cruises – 17O sulfate

Measurements: Charles C.W. Lee Measurements: Joël Savarino

Pre-INDOEX cruise January 1997

0

0.5

1

1.5

2

2.5

3

3.5

4

-15.0 -5.0 5.0 15.0

Latitude (degrees)

SO

42-n

ss

17O

(‰

)ITCZ

0

0.5

1

1.5

2

2.5

3

3.5

4

-15.0 -5.0 5.0 15.0

Latitude (degrees)

SO

42-n

ss

17O

(‰

)ITCZ

0

0.5

1

1.5

2

2.5

3

3.5

4

-15.0 -5.0 5.0 15.0

Latitude (degrees)

SO

42-n

ss

17O

(‰

)ITCZ

C.C.W. Lee, Ph.D. dissertation, 2000

0.00

0.50

1.00

1.50

2.00

2.50

-15 -10 -5 0 5 10 15Latitude (degrees)

SO

42

- ns

s 1

7 O (

‰)

ITCZ

0.00

0.50

1.00

1.50

2.00

2.50

-15 -10 -5 0 5 10 15Latitude (degrees)

SO

42-

nss

17

O (

‰)

ITCZ

0.00

0.50

1.00

1.50

2.00

2.50

-15 -10 -5 0 5 10 15Latitude (degrees)

SO

42-n

ss

17O

(‰

)

ITCZ

INDOEX cruise March 1998

Measurements (unpublished) by J. Savarino

How does S(IV) oxidation by O3 on sea salt aerosols modify our understanding

of the sulfur budget in the MBL?Rapid oxidation of SO2 SO4

2-

MBL SO2 concentrations decrease by 40% between 40º-70º S latitude

GCMs tend to over predict SO2 concentrations (while SO4

2- predictions are more in line with observations)

Rapid deposition of SO42- formed on sea salt particles

MBL SO42- concentrations decrease by 14%

between 40º-70º S latitude

Rate of gas-phase H2SO4 production decreases

Percent decrease in the rate of gaseous H2SO4 production (SO2+OH) after adding S(IV) oxidation on sea salt aerosols

Potential climate effects of SO42- in the MBL

0% 50%

100%

Potential climate effects of SO42- over the

ocean

Biological regulation of the climate?

(Charlson et al., Nature 1987)

DMSOH

NO3 SO2 H2SO4

OH

O3

New particle formation

CCN

Other missing S(IV) oxidation pathways?Mineral dust:

O3 oxidation

Enhanced SO42- concentrations associated

with dust events have been observed (i.e. Jordan et al., 2003)

Other aerosols:

both H2O2 and O3 oxidation depending on pHFog water sulfate:

Whiteface Mtn., NY, pH=2.9 Average 17O SO4: 0.3 ‰

Davis, CA, pH=6.2 Average 17O SO4: 4.3 ‰

Conclusions and Future Plans

•S(IV) oxidation on dust and other aerosols (calculating pH?) comparison with present day 17O measurements

•Run NASA GISS model through one full climate cycle use meteorology to drive GEOS-CHEM in the past

•Quantitative interpretation of ice core 17O sulfate measurements

17O sulfate provides information on relative oxidation pathways (gas OH versus aqueous O3,H2O2) in the present and paleo atmosphere

Measurements from the Vostok ice core reveal that gas-phase OH oxidation of S(IV) was greater during the glacial period

17O sulfate measurements provide an additional constraint for chemical transport models improve our understanding of sulfur chemistry and the sulfur budget

Acknowledgements

Dr. Rokjin Park, Prof. Daniel J. Jacob, Bob Yantosca

Dr. Joël Savarino and Dr. Robert Delmas

Dr. Charles C.W. Lee and Prof. M.H. Thiemens

Laboratoire de Glaciologie et Geophysique de l’Environement

NOAA Climate and Global Change Postdoctoral Fellowship

Daly Postdoctoral Fellowship (Department of Earth and Planetary Sciences, Harvard University)