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)