1. Department of Earth and Planetary Sciences
Understanding the present and paleo record of the oxygen isotopes of sulfate
Becky Alexander
Postdoctoral Fellow
Department of Earth and Planetary Sciences
Harvard University
Carnegie Institution of Washington Geophysical Laboratory
October 20, 2003

2. Overview: Measurements Model
What can the D17O record of sulfate tell us about past and present variations in atmospheric chemistry?
Ice core record of D17O sulfate – what have we learned?
How can we interpret the data and improve our understanding of the long term record of atmospheric chemistry?
Measurements
Model
Constraint
Interpretation

3. ? Atmospheric Oxidation Capacity OH Climate change hn, H2O
Primary Species
Secondary Species H
S, SO
, CH
, CO, 2 2 4 CO
, H SO
, HNO , 2 2 4 3 CO
, NO, N O, 2 2
RCOOH, O3
particulates
Complex non-linear chemistry
Major challenge: “measure, understand and predict long-term trends in the oxidizing power of the atmosphere”
“understand the effects of climate change on air quality”
Industry
Volcanoes
Marine
Biomass
Continental
Biogenics
burning
Biogenics

4. Atmospheric Chemistry is controlled by atmospheric oxidants
“The Earth’s oxidizing capacity” O3
CH4 CO HC
NOx OH
H2O2
hu, O(1D)
O2, H2O
O3, NO
HO2
SOx
H2SO4
HNO3

5. Current knowledge of the past oxidative capacity of the atmosphere
Model results (vs Preindustrial 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
Thompson et al., 1993
Ice age: +12%
Indus: -0.15%
Indus: +80%
with NMHC
Conflicting results on OH, highly dependent on emission scenarios of NMHC, NOx which are not very well constrained

6. Current knowledge of the past oxidative capacity of the atmosphere
Measurement approach
Doubling of O3 between PIT/IT
Sigg & Neftel, 1991
Summit
Dye 3
50% increase of H2O2 between PIT/IT
Voltz & Kley, 1988
Calibration issue (O3), low stability in proxy records (H2O2).

7. 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-….)
d17O/d18O  0.5
D17O = d17O – 0.5*d18O = 0

8. Mass-Independent Fractionation
Thiemens and Heidenreich, 1983
O + O2  O3*
D17O
D17O = d17O – 0.5*d18O  0
Mass-dependent fractionation line: d17O/d18O  0.5
d17O/d18O  1

9. Explanation of Observations
(D17O  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*).
r(asymm) /r(symm) = 1.18
Gao and Marcus, 2001

10. 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

11. SO2 in isotopic equilibrium with H2O :
Source of D17O Sulfate
SO2 in isotopic equilibrium with H2O :
D17O of SO2 = 0 ‰
Aqueous
Gas
1) SO2 + O3 (D17O=30-35‰) D17O ~ 8-9 ‰
2) SO2-+ H2O2 (D17O=1-2‰)  D17O ~ ‰
3) SO2 + OH (D17O=0‰)  D17O = 0 ‰
D17O of SO42- a function relative amounts of OH, H2O2, and O3 oxidation
Savarino et al., 2000

12. Gas versus Aqueous-Phase Oxidation of Sulfate
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

13. pH dependency of O3 oxidation and its effect on D17O of SO42-
H2O2 O3
H2O2 O3
Lee et al., 2001

14. Sources of Sulfate in La Jolla, CA rainwater
pH = 5.1 (average of La Jolla rainwater)
Aqueous Gas
41% %
D17O (SO42-)aqueous = 1.82 ‰
D17O (SO42-)actual = 0.75 ‰
[Na+]
Sea salt
30%
Lee et al., 2001

15. Conservative Tracers in Ice cores
Na+
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?

16. Analytical Procedure

17. 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

18. Vostok, Antarctica
[SO42-] tracks [MSA-] suggesting a predominant DMS (oceanic biogenic) source

19. Climatic D17O (SO42-) fluctuations
Vostok Ice Core
Climatic D17O (SO42-) fluctuations
DTs data: Kuffey and Vimeux, 2001, Vimeux et al., 2002

20. Vostok sulfate three-isotope plot
slope1

21. Vostok sulfate three-isotope plot
100% O3 oxidation:
D17O (SO4) = ¼ * 32‰ = 8‰
100% OH oxidation:
D17O (SO4) = 0 ‰
100% H2O2 oxidation:
D17O(SO4) = ½*1.7‰ = 0.85 ‰
D17O range = 1.3 – 4.8 ‰

22. Results of calculations
OH (gas-phase) oxidation relatively greater in glacial period

23. Potential climate effects of SO42- over the ocean
Biological regulation of the climate?
(Charlson et al., Nature 1987) O3
CCN OH OH
DMS
SO2
H2SO4
New particle formation
NO3

24. Interpretation of Vostok D17O 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

25. GEOS-CHEM Global 3-D model of atmospheric chemistry
Global 3-D model of atmospheric chemistry
4ºx5º horizontal resolution, 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)

26. GEOS-CHEM D17O Sulfate Simulation
Use constant, global D17O value for oxidants
D17O ‰
method
reference O3 35
Photochemical model
Lyons 2001
27-32
Tropospheric measurements
Johnston and Thiemens 1997
H2O2
(1.7)
Rainwater measurements
Savarino and Thiemens 1999 OH
Experimental
Dubey et al., 1997
SO2 + OH (gas phase) D17O=0‰
S(IV) + H2O2 (in cloud, pH=4.5) D17O=0.85‰
S(IV) + O3 (in cloud, pH=4.5) D17O=8‰

27. GEOS-CHEM D17O Sulfate Simulation
D17O sulfate (January)
0.0
2.3
4.6
D17O > 1‰  O3 oxidation
D17O sulfate (July)
0.0
2.3
4.6
D17O
H2O2 (ppbv)
Winter: low H2O2
NH: High SO2
Preindustrial Antarctic ice core sulfate: D17O = ‰
(Alexander et al., 2001)
Missing O3 oxidation source?

28. 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)

29. GEOS-CHEM D17O Sulfate Simulation with Sea Salt Chemistry
January D17O sulfate
July D17O sulfate
INDOEX
0.0‰
3.5‰
7.0‰

30. INDOEX cruises – D17O sulfate
Pre-INDOEX Jan. 1997
INDOEX March 1998
Measurements: Charles C.W. Lee
Measurements: Joël Savarino

31. Pre-INDOEX cruise January 1997
ITCZ
ITCZ
ITCZ
Pre-INDOEX cruise January 1997
C.C.W. Lee, Ph.D. dissertation, 2000

32. INDOEX cruise March 1998 ITCZ ITCZ ITCZ
Measurements (unpublished) by J. Savarino

33. 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  SO42-
MBL SO2 concentrations decrease by 40% between 40º-70º S latitude
GCMs tend to over predict SO2 concentrations (while SO42- 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

34. Potential climate effects of SO42- in the MBL0%
50%
100%
Percent decrease in the rate of gaseous H2SO4 production (SO2+OH) after adding S(IV) oxidation on sea salt aerosols

35. Potential climate effects of SO42- over the ocean
Biological regulation of the climate?
(Charlson et al., Nature 1987) O3
CCN OH OH
DMS
SO2
H2SO4
New particle formation
NO3

36. 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 pH
Fog water sulfate:
Whiteface Mtn., NY, pH= Average D17O SO4: 0.3 ‰
Davis, CA, pH= Average D17O SO4: 4.3 ‰

37. Conclusions and Future Plans
D17O 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
D17O sulfate measurements provide an additional constraint for chemical transport models  improve our understanding of sulfur chemistry and the sulfur budget
S(IV) oxidation on dust and other aerosols (calculating pH?)  comparison with present day D17O measurements
Run NASA GISS model through one full climate cycle  use meteorology to drive GEOS-CHEM in the past
Quantitative interpretation of ice core D17O sulfate measurements

38. 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)