1. Sulfur and oxygen isotopic tracers of past and present atmospheric chemistry Becky Alexander Harvard University April 14, 2003

2. Overview What controls atmospheric chemistry and why do we care? Stable isotope measurements: limitations and advantages Mass-independent fractionation in O and S isotopes (NO 3 - and SO 4 2- ) Ice core sulfate and nitrate – past variations in atmospheric chemistry Preliminary modeling “insights” Summary and conclusions

3. The Atmospheric Reactor IndustryVolcanoes Marine Biogenics Biomass burning Continental Biogenics Deposition, Biosphere interaction Primary Species H 2 S, SO 2, CH 4, CO, CO 2, NO, N 2 O, particulates Secondary Species CO 2, H 2 SO 4, HNO 3, RCOOH Oxidation Capacity Photochemistry ClimatePollution

4. Atmospheric Chemistry is controlled by atmospheric oxidants “The Earth’s oxidizing capacity” O3O3 CH 4 CO HC NO x OH H2O2H2O2 h , O( 1 D) O 2, H 2 O O 3, NO HO 2 SO x H 2 SO 4 SO x H 2 SO 4 SO x H 2 SO 4 NO x HNO 3 NO x HNO 3

5. Measurements Field studies Laboratory studies Models Coupled chemistry/climate global models Global picture

6. Stable Isotope Measurements: Tracers of source strengths and chemical processing of atmospheric constituents  (‰) = [(R sample /R standard ) – 1]  1000 R = minor X/ major X  18 O: R = 18 O/ 16 O (CO 2, CO, H 2 O, O 2, O 3, SO 4 2- ….)  34 S: R = 34 S/ 32 S SO 4 2-, (SO 2, SO 4 2-, H 2 S)

7. Sea water  34 S ( ‰ ) -10 + +20 +30 -20 Volcanic/Mineral Biogenic Marine Biogenic Coal Oil CONTINENT OCEAN COMBUSTION Overlapping Source Signatures

8.  =  34 S SO4 /  34 S SO2  > 1.07 SO 2 + OH  SO 4 :  > 1.07 (Luong et al., 2001)  = 1.0165 SO 2 + O 3 /H 2 O 2  SO 4 :  = 1.0165 (Eriksen, 1972) Oxidation of the heavier isotope is favored resulting in an increasing degree of 34 S depletion at progressively later times Chemical isotopic fractionation

9. Mass-Dependent Fractionation  17 O  0.5*  18 O :  17 O =  17 O – 0.5*  18 O = 0  33 S  0.5*  34 S :  33 S =  33 S – 0.5*  34 S = 0

10. O 3 formation in the laboratory Thiemens and Heidenreich, 1983  17 O/  18 O  1  17 O =  17 O – 0.5*  18 O  0  17 O

11. Mass-independent isotope effects – symmetry explanation Symmetry C 2v Symmetry C s 16 17 or 18 O 2 + O( 3 P) O 3 * E Vibrational States Rotational States DeDe v = i v=i+1 Rotational States Vibrational States DeDe v = i v=i+1

12. 25 10 5 50 75 100 102050100 SO 4 CO N2ON2O H2O2H2O2 NO 3 CO 2 strat. O 3 trop. O 3 strat.  18 O  17 O All  17 O measurements in the atmosphere

13. Tropospheric oxidation  17 O of HNO 3 a function of RO 2 /O 3 and the terminal reaction  17 O of NO x is a function of RO 2 /O 3 oxidation The  17 O of HNO 3 depends also on the dilution factor due to the terminal reaction NO 2 + OH  HNO 3 NO 3 + RH  HNO 3 N 2 O 5 + H 2 O (aq)  2HNO 3

14. Tropospheric oxidation SO 2 in isotopic equilibrium with H 2 O : No source effect:  17 O of SO 2 = 0 ‰ HSO 3 - + O 3   17 O ~ 8.0 ‰, pH > 5.6 HSO 3 - + H 2 O 2   17 O ~ 0.5 ‰, pH < 5.6 SO 2 + OH   17 O = 0 ‰  17 O of SO 4 a function relative amounts of OH, H 2 O 2, and O 3 oxidation Aqueous Gas

15. Gas versus Aqueous-Phase Oxidation Gas-phase: SO 2 + OH  new aerosol particle  increased aerosol number concentrations Aqueous-phase: SO 2 + O 3 /H 2 O 2  growth of existing aerosol particle Cloud albedo and climate Microphysical/optical properties of clouds

16. Lee et al., 2001 O 3 /H 2 O 2 oxidation depends on pH of aqueous phase  17 O

17.  17 O (SO 4 ) aqueous = 1.82 ‰ Estimated sulfate contribution from different sources in La Jolla, CA rainwater pH = 5.1 (average of La Jolla rainwater)  17 O (SO 4 ) actual = 0.75 ‰ SeasaltAqueousGas 30% 41% 29% Lee et al., 2001 [Na + ]

18. Oxygen (  17 O)  relative oxidation pathways (oxidant chemistry) Oxygen (  17 O)  relative oxidation pathways (oxidant chemistry) Gas/Aqueous phase chemistry  climate Relative oxidation concentrations  oxidation efficiency Sulfur (  33 S) ?

19. Continuum > 220nm Mass-fractionation line SO 2 photolysis Farquhar et al., 2001 Volcanic sulfate in South Pole ice Sulfate Residual SO 2 1991 Pinatubo :  33 S = 0.7 ± 0.1 ‰ 1259 Unknown :  33 S = -0.5 ± 0.1 ‰ 1991 Cerro Hudson :  33 S = -0.1 ± 0.1 ‰ Savarino et al., 2002 Non-zero  33 S  stratospheric influence

20. Conservative Tracers in Ice cores: Na + NO 3 - SO 4 2- Composition of gas bubbles SO 4 2- very stable (  34 S) sources of sulfate (  33 S) stratospheric influence (  17 O) aqueous v. gas phase oxidation (  17 O) oxidant concentrations  oxidation capacity of the atmosphere

21. Current knowledge of the past oxidative capacity of the atmosphere Model results (vs Pre Indus. Holocene) Conflicting results on OH, highly dependent on emission scenarios of NMHC, NO x which are not very well constrained Model author OHO3O3 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

22. Current knowledge of the past oxidative capacity of the atmosphere Doubling of O 3 between PIT/IT Measurement approach Voltz & Kley, 1988 Sigg & Neftel, 1991 Summit Dye 3 50 % increase of H 2 O 2 between PIT/IT But calibration issue, not representative of global conditions, or stability in proxy records.

23. AntarcticaGreenland Sulfate concentration varies with climate Sulfate concentration reflects anthropogenic emissions

24. Analytical Procedure Old method BaSO 4 + C  CO 2 CO 2 + BrF 5  O 2 (3 days of chemistry, 10  mol sulfate) New method Ag 2 SO 4  O 2 + SO 2 (minutes of chemistry, 1-2  mol sulfate) Faster, smaller sample sizes, O and S isotopes in same sample

25. [SO 4 2- ] tracks [MSA - ] suggesting a predominant DMS (oceanic biogenic) source Vostok, Antarctica Ice Core

26. Vostok Ice Core – Climatic  17 O (SO 4 ) fluctuations  T s data: Kuffey and Vimeux, 2001, Vimeux et al., 2002

27. Vostok sulfate three-isotope plot

28. 100% H 2 O 2 oxidation:  17 O(SO 4 ) = ½*1‰ = 0.5 ‰  17 O range = 1.3 – 4.8 ‰ Extended 3-isotope plot 100% O 3 oxidation:  17 O (SO 4 ) = ¼ * 32‰ = 7.5‰ 100% OH oxidation:  17 O (SO 4 ) = 0 ‰

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

30.  33 S = 0 for all Vostok samples What can cause this climate variation? Stratospheric influence?  NO Changes in oxidant concentrations in the atmosphere? Oxidation capacity of the atmosphere Changes in cloud processing/liquid water content? Cloud/water content of the atmosphere GCM sensitivity studies

31. Sulfur oxidation pathways have a natural variation on the glacial/interglacial timescale. Do we see a variation as a result of anthropogenic activities?

32. Mayewski et al., 1990 Sulfate and nitrate in Greenland ice cores Fossil fuel burning trends from Graedel and Crutzen, “Atmospheric Change”.

33. Site A NO 3 - Site A SO 4 2-

34. Fire index data: Savarino and Legrand, 1998 Pre-Industrial Biomass Burning

35. Biomass burning can affect  17 O of sulfate and nitrate by: 1)Altering oxidant (O 3 ) concentrations 2)Increase aerosol loading affecting heterogeneous oxidation pathways Are  17 O measurements of sulfate/nitrate proxies of: Oxidation capacity? Aerosol concentrations?

36. Resolving  17 O sulfate in GEOS-CHEM Resolve sulfate sources: SO 2 + OH  SO 4 A HSO 3 - + H 2 O 2  SO 4 B SO 3 2- + O 3  SO 4 C primary sulfate = SO 4 D (currently direct anthropogenic emissions)  17 O = (1*0.5*SO 4 B + 32*0.25*SO 4 C)/ (SO 4 A + SO 4 B + SO 4 C + SO 4 D)

37. Oxidation by O 3 only important during winter in high northern latitudes  17 O > 1  O 3 oxidation

38.  17 O sulfate versus cloud processing  17 O Cloud liquid water content

39.  17 O sulfate versus O 3 concentration  17 O O 3 ppbv

40.  17 O sulfate versus H 2 O 2 concentration  17 O sulfate versus OH concentration

41.  17 O versus H 2 O 2 : January  17 O H 2 O 2 ppbv

42.  17 O of sulfate is strongly affected by (oxidant) H 2 O 2 concentrations Less so by cloud content Importance of oxidation by O 3 is not represented Aqueous-phase oxidation occurs in clouds only (pH = 4.5) Aqueous oxidation occurs on deliquescent sea-salt aerosols (initial pH=8, large buffering capacity)

43. Oxidation on sea-salt aerosols Sea salt flux to atmosphere: 1.01 x 10 4 Tg/year  11.1 Tg(S)/year (Gong et al., 2002) Global DMS emissions: 15-25 Tg(S)/year (Seinfeld and Pandis, 1998) 44 -74% of SO 2 (from DMS) oxidized to sulfate by O 3 on sea-salt aerosols

44. Conclusions and Future Directions  17 O measurements of both sulfate and nitrate reflect variations in : Changes in the oxidation capacity  Potential buildup of pollutants Changes in aerosol/cloud properties  Climate change Model sensitivity studies can determine the importance of each on  17 O Simulation of heterogeneous chemistry must be improved in GCMs  “current”  17 O measurements

45. Acknowledgements Prof. Mark Thiemens – UCSD Dr. Joël Savarino – CNRS/LGGE Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE) The National Ice Core Laboratory (USGS) Prof. Daniel Jacob – Harvard Dr. Rokjin Park – Harvard Bob Yantosca - Harvard