Paleo-oxidant variations and atmospheric sulfate aerosol formation

1
Paleo-oxidant variations and atmospheric sulfate
aerosol formation
Becky Alexander Harvard University Department of
Earth and Planetary Sciences
UW PCC April 29, 2004
2
Overview

• Vostok Climate Record Greenhouse gases,
  aerosols, and the oxidizing power of the
  atmosphere
• Vostok D17O sulfate Aerosol oxidation pathways
• GEOS-CHEM Global 3D model
• INDOEX INDian Ocean EXperiment
• Sulfate formation in the marine boundary layer
  (MBL)
• Future Plans

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The Vostok Ice Core Record Greenhouse gases
What drives changes in CO2 and CH4 concentrations
from glacial to interglacial periods?
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The Vostok Ice Core Record Aerosols
Does the marine biosphere regulate the climate
through the production of DMS?
SO42- (ppb)
dD ()
dD from Jouzel et al., 1987 SO42- from M.
Legrand
SO42- tracks MSA- suggesting a predominant
DMS (oceanic biogenic) source (Legrand et al.,
1991)
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Effects of Aerosols on Climate
Direct Effect
Indirect Effect
Cloud droplet number density (cm-3)
Aerosol number density (cm-3)
Ramanathan et al., 2001
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Gas versus aqueous-phase production
SO42-
O3, H2O2
CCN
SO2
OH
New particle formation
H2SO4
Light scattering
OH
NO3
DMS
Phytoplankton
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Oxidizing Power of the Atmosphere
Primary Species
H
S, SO
, CH
, CO,
2
2
4
DMS, CO
, NO, N
O,
2
2
particulates
Marine
Biomass
Continental
Volcanoes
Biogenics
burning
Biogenics
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Key Questions
How have anthropogenic emissions affected the
oxidation capacity of the atmosphere?
How has the oxidation capacity of the atmosphere
varied in the past (glacial/interglacial cycles)?
What can we expect in the future?
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Current knowledge of the past oxidative capacity
of the atmosphere
Measurements
H2O2
O3
Year AD
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Model Estimates of Past OH and O3
Ice Age
O3
Relative to preindustrial Holocene
OH
Industrial Era
Relative to preindustrial Holocene
Martinerie et al., 1995
Karol et al., 1995
Thompson et al., 1993
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Conservative Tracers in Ice cores
Na SO42- Composition of gas bubbles
SO42- very stable (D17O) sulfate formation
pathways ? oxidation capacity of the atmosphere?
(Alexander et al., 2003, 2004)
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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 0.5d18O 0
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Mass-Independent Fractionation
O O2 ? O3
Mass-dependent fractionation line d17O/d18O ? 0.5
Thiemens and Heidenreich, 1983
d17O/d18O ? 1
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Source of D17O Sulfate
SO2 in isotopic equilibrium with H2O D17O of
SO2 0
1) SO32- O3 (D17O35)? SO42- D17O 8.75
2) HSO3- H2O2 (D17O1.7) ? SO42- D17O 0.85
3) SO2 OH (D17O0) ? SO42- D17O 0
D17O of SO42- a function relative amounts of OH,
H2O2, and O3 oxidation
Savarino et al., 2000
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Analytical Procedure
Decontamination
Concentrate
Ion Chromatograph
Ionic separation
SO42-
Ag2SO4
Savarino, Alexander, and Thiemens, 2001
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Analytical Procedure
Isotope Ratio Mass Spectrometer
Faster, smaller sample sizes, O and S isotopes in
same sample
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Vostok Ice Core Climatic D17O (SO42-)
fluctuations
D17O ()
DTs
DTs data Kuffey and Vimeux, 2001, Vimeux et al.,
2002 Alexander et al., 2002
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Vostok sulfate three-isotope plot
slope?1
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Vostok sulfate three-isotope plot
100 H2O2 oxidation D17O(SO4) ½1.7 0.85
D17O range 1.3 4.8
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Climate Variations in the Oxidation Pathways of
Sulfate Formation
DTs
OH
Age (kyr)
OH (gas-phase) oxidation greater in glacial
period compared to interglacial
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Vostok sulfate explanation
CCN
H2SO4
SO42-
OH
O3
Transport
SO2
Wet and dry deposition
OH
NO3
DMS
Antarctica
Ocean
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Glacial/Interglacial CH4 variations
Wetland CH4 emissions
Present day
Wetland CH4 emissions 24 less in LGM. Not
enough to explain glacial/interglacial change in
atmospheric CH4 concentrations.
Larger OH sink during the glacial?
LGM
Kaplan, 2002
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Lessons from Vostok
D17O of sulfate varies with climate, reflects
variations in oxidant concentrations and/or cloud
processing efficiency
Increased (30-80 range) gas-phase formed sulfate
during the glacial period ? positive climate
feedback?
This is just the beginning! Higher resolution
data over various timescales ? WAISCORES
Global model simulations using oxygen isotope
tracers ? interpret and quantify existing data
sets
? direct future measurement sites
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GEOS-CHEM
http//www-as.harvard.edu/chemistry/trop/geos/inde
x.html

• Global 3-D model of atmospheric chemistry
• Driven by assimilated meteorology (1987
  present).
• 4ºx5º horizontal resolution
• Includes aqueous and gas phase chemistry
• S(IV) OH (gas-phase)
• S(IV) O3/H2O2 (in-cloud, pH4.5)
• Off-line sulfur chemistry (uses monthly mean OH
  and O3 fields from a full chemistry, coupled
  aerosol simulation)

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D17O sulfate GEOS-CHEM and measurements
January 2001
July 2001
Missing O3 oxidation source
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pH dependency of O3 oxidation and its effect on
D17O of SO42-
Lee et al., 2001
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O3 oxidation on sea-salt aerosols
Sea-salt emissions are a function of wind
speed pH 8 O3 oxidation dominant
How important is this reaction in the marine
boundary layer? What does this mean for the
marine biological control of climate?
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INDOEX cruises D17O sulfate
Pre-INDOEX Jan. 1997
INDOEX March 1998
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Pre-INDOEX Cruise January 1997
ITCZ
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INDOEX Cruise March 1998
ITCZ
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Effect of sea-salt chemistry on gas-phase sulfate
production rates
Percent () decrease (seasonal average)
Mar/Apr/May
Jun/Jul/Aug
Sep/Oct/Nov
Dec/Jan/Feb
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Effects of Sea-Salt on Sulfate Production in the
MBL
H2O2
Aqueous-phase
CCN
Gas-phase
OH
DMS
SO2
H2SO4
New particle formation
NO3
OH
Light scattering
Phytoplankton
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D17O sulfate GEOS-CHEM and measurements
Davis, CA fogwater 4.3 pH6.2
Whiteface Mtn, NY fogwater 0.3 pH2.9
Site A, Greenland ice core 2
January 1997
July 1997
Vostok Dome C ice cores 1.3-4.8
La Jolla rainwater 1.1 pH5.1
La Jolla aerosol 0.2-1.4
White Mtn, CA aerosol 1-1.7
South Pole aerosol 0.8-2
INDOEX aerosol 0.5-3
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Future Plans Dust
from D. Jaffe, UW, Bothell
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Future Plans Organic Aerosols
Current understanding is very limited! d13C is
the only isotope system measured.
OH, O3 NO3
Formic acid Acetic acid
a-pinene
b-pinene
100 ppt in remote MBL!
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Acknowledgements
Mark H. Thiemens Charles Lee Greg
Michalski Jeff Severinghaus V. Ramanathan
Allison Shaw Joël Savarino Robert
Delmas Michele Legrand
Daniel Jacob Dan Schrag Ann Pearson Rokjin
Park Qinbin Li Bob Yantosca
James Farquhar
Karl Kreutz