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

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

? 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

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

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

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).

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

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

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

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

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 ~ 0.5-1 ‰ 3) SO2 + OH (D17O=0‰)  D17O = 0 ‰ D17O of SO42- a function relative amounts of OH, H2O2, and O3 oxidation Savarino et al., 2000

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

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

Sources of Sulfate in La Jolla, CA rainwater pH = 5.1 (average of La Jolla rainwater) Aqueous Gas 41% 29% D17O (SO42-)aqueous = 1.82 ‰ D17O (SO42-)actual = 0.75 ‰ [Na+] Sea salt 30% Lee et al., 2001

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?

Analytical Procedure

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

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

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

Vostok sulfate three-isotope plot slope1

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 ‰

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) O3 CCN OH OH DMS SO2 H2SO4 New particle formation NO3

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

GEOS-CHEM Global 3-D model of atmospheric chemistry http://www-as.harvard.edu/chemistry/trop/geos/index.html 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)

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 1.3-2.2 (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‰

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 = 1.3-4.8‰ (Alexander et al., 2001) Missing O3 oxidation source?

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)

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

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

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

INDOEX cruise March 1998 ITCZ ITCZ ITCZ 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  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

Potential climate effects of SO42- in the MBL 0% 50% 100% 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- over the ocean Biological regulation of the climate? (Charlson et al., Nature 1987) O3 CCN OH OH DMS SO2 H2SO4 New particle formation NO3

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=2.9 Average D17O SO4: 0.3 ‰ Davis, CA, pH=6.2 Average D17O SO4: 4.3 ‰

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

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)