1. Paleo-oxidant variations and atmospheric aerosol formation: The ice-core record Becky Alexander Harvard University Department of Earth and Planetary Sciences USC May 3, 2004

2. Antarctic ice core record: Atmospheric composition and climate variations on the glacial/interglacial timescale  What controls the composition of the atmosphere? (importance of ocean, biosphere, and oxidizing power of the atmosphere)  How does this impact climate change? (greenhouse gases and aerosols)  What have we learned from the O-isotope record of sulfate? Greenland ice core record: Atmospheric composition variations in the Industrial Era  How have humans impacted atmospheric chemistry?  When did the anthropogenic era begin? Outline

3. The Vostok Ice Core Record: Greenhouse gases From Kotlyakov et al., 2001 050100150200250300350400 Age (kyr BP)  D (‰) CO 2 (ppbv) CH 4 (ppbv)

4. Eccentricity Tilt Periodicity ~41,000 years Periodicity ~22,000 years Precession Milankovitch Cycles Periodicity ~100,000 years What drives changes in CO 2 and CH 4 concentrations from glacial to interglacial periods?

5. Glacial/Interglacial CO 2 variations adapted from Ridgwell, 2002 Biological productivity: nutrients + DIC  POM POM Euphotic zone “Iron hypothesis” Martin, 1990 -0.80.80.0 Dust deposition and Chlorophyll Correlation coefficient from Erickson et al., 2000 Jouzel et al., 1993

6. Glacial/Interglacial CH 4 variations Kaplan, 2002 Wetland CH 4 emissions Present day LGM Wetland CH 4 emissions 24% less in LGM. Not enough to explain glacial/interglacial change in atmospheric CH 4 concentrations. Changes in atmospheric chemistry leading to CH 4 destruction?

7. Secondary Species CO 2, H 2 SO 4, O 3, … Oxidizing Power of the Atmosphere Volcanoes Marine Biogenics Biomass burning Continental Biogenics Primary Species H 2 S, SO 2, CH 4, CO, DMS, CO 2, NO, N 2 O, particulates ? Climate change OH h  H 2 O Primary Emissions DMS, SO 2, CH 4, … To what extent is the oxidizing power of the atmosphere controlled by the biosphere? Model simulations by Thompson et al. (1993), using the Vostok CH 4 constraint, calculate 32% greater OH concentrations during the LGM.

8. The Vostok Ice Core Record: Aerosols SO 4 2- (ppb)  D (‰)  D from Jouzel et al., 1987 [SO 4 2- ] from M. Legrand [SO 4 2- ] tracks [MSA - ] suggesting a predominant DMS (oceanic biogenic) source

9. SO 2 H 2 SO 4 OH New particle formation CCN Light scattering DMS OHNO 3 Phytoplankton O 3, H 2 O 2 SO 4 2- Aerosol Climate Effects Does the marine biosphere regulate the climate through the production of DMS?

10. Oxidants in the Sulfur Cycle 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

11. 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?

12. Current knowledge of the past oxidative capacity of the atmosphere Measurements Year AD 1870189019101930195019701990 0 10 20 30 40 50 60 O 3 (ppb) H 2 O 2 (  M) Sigg & Neftel, 1991 Summit 0 2 4 6 1750200018501950 18001900 H2O2H2O2 O3O3

13. O3O3 OH Model Estimates of Past OH and O 3 Ice Age Industrial Era Relative to preindustrial Holocene Karol et al., 1995 Thompson et al., 1993 Martinerie et al., 1995

14. Conservative Tracers in Ice cores Na + SO 4 2- Composition of gas bubbles SO 4 2- very stable (  17 O) oxidant concentrations  oxidation capacity of the atmosphere?

15. Stable Isotope Measurements: Tracers of source strengths and/or chemical processing of atmospheric constituents  (‰) = [(R sample /R standard ) – 1]  1000 R = minor X/ major X  18 O: R = 18 O/ 16 O  17 O: R = 17 O/ 16 O Standard = SMOW (Standard Mean Ocean Water) (CO 2, CO, H 2 O, O 2, O 3, SO 4 2- ….)  17 O /  18 O  0.5  17 O =  17 O – 0.5*  18 O = 0

16. Mass-Independent Fractionation  17 O /  18 O  1 Thiemens and Heidenreich, 1983  17 O  17 O  17 O =  17 O – 0.5 *  18 O  0 O + O 2  O 3 * Mass-dependent fractionation line:  17 O/  18 O  0.5

17. 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

18. Source of  17 O Sulfate SO 2 in isotopic equilibrium with H 2 O :  17 O of SO 2 = 0 ‰ 1) SO 3 2- + O 3 (  17 O=35‰)  SO 4 2-  1 7 O = 8.75 ‰  17 O of SO 4 2- a function relative amounts of OH, H 2 O 2, and O 3 oxidation Savarino et al., 2000 3) SO 2 + OH (  17 O=0‰)  SO 4 2-  17 O = 0 ‰ 2) HSO 3 - + H 2 O 2 (  17 O=1.7‰)  SO 4 2-  17 O = 0.85 ‰ Aqueous Gas

19. Analytical Procedure H 2 SO 4 Ag 2 SO 4 Decontamination ConcentrateIon ChromatographIonic separation

20. Ag 2 SO 4  O 2 + SO 2 Removable quartz tube 1050°C magnet To vacuum GC SO 2 trap He flow Sample loop 5A mol.sieve vent SO 2 port O 2 port Analytical Procedure Isotope Ratio Mass Spectrometer Faster, smaller sample sizes, O and S isotopes in same sample

21. Vostok Ice Core Climatic  17 O (SO 4 2- ) fluctuations  T s data: Kuffey and Vimeux, 2001, Vimeux et al., 2002 Alexander et al., 2002  17 O (‰) TsTs

22. Vostok 3-isotope plot slope  1

23. Vostok trendline Mass-Dependent line Vostok sulfate three-isotope plot Tropospheric O 3 Vostok trendline Mass-Dependent line Tropospheric O 3 H2O2H2O2 Vostok trendline Mass-Dependent line Tropospheric O 3 H2O2H2O2 H 2 O/OH Vostok trendline Mass-Dependent line 100% H 2 O 2 oxidation:  17 O(SO 4 ) = ½*1.7‰ = 0.85 ‰  17 O range = 1.3 – 4.8 ‰ Tropospheric O 3 100% O 3 100% OH Vostok trendline Mass-Dependent line 100% O 3 oxidation:  17 O (SO 4 ) = ¼ * 35‰ = 8.75‰ 100% OH oxidation:  17 O (SO 4 ) = 0 ‰ H 2 O/OH

24. Climate Variations in the Oxidation Pathways of Sulfate Formation OH (gas-phase) oxidation greater in glacial period compared to interglacial Age (kyr) % OH TsTs

25. Antarctica Ocean DMS OHNO 3 SO 2 OH H 2 SO 4 O3O3 SO 4 2- Transport Wet and dry deposition Vostok sulfate explanation CCN

26. Lessons from Vostok  17 O 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? OH

27. nssSO 4 2- (ppb) Site A, Greenland Ice Core GISP2 Site A GISP2 Site A Alexander et al., 2004 Mayewski et al., 1997

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

29.  17 O (‰) nssSO 4 2-  17 O (‰) NO 3 - Site A, Greenland  17 O (‰)

30. Preindustrial Biomass Burning Alexander et al., 2004 Savarino and Legrand, 1997

31. North America Greenland DMS SO 2 NO x S IV + O 3 S VI S IV + H 2 O 2 S VI S IV + O 3 S VI S IV + H 2 O 2 S VI N 2 O 5 + H 2 O 2HNO 3 NO 2 + O 3 NO 3 + O 2 NO 3 + HC HNO 3 NO 3 + NO 2 N 2 O 5 N 2 O 5 + H 2 O (aq) 2HNO 3 Ocean Ash, … NMHC NMHC O 3 NO 2 + OH HNO 3 SO 2 + OH + H 2 O H 2 SO 4 Wet/Dry Deposition NMHC O 3 NO+H/RO 2 NO 2 + H/RO NO+O 3 NO 2 + O 2 NMHC Wet/Dry Deposition (aq) Biomass burning emissions

32. Conclusions  17 O 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? Large biomass burning signal in  17 O of sulfate and nitrate  anthropogenic effects on atmospheric chemistry began prior to the Industrial Revolution

33. Future Directions Aerosol and snow pit samples from the Canadian Arctic  interactions between the Arctic climate, sea ice, marine productivity, and the formation of Arctic haze 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

34. Acknowledgements Mark H. Thiemens Charles Lee Greg Michalski Peter Zmolek Phoebe Glazer Karl Kreutz James Farquhar Mark Twickler Geoffrey Hargreaves Jeff Severinghaus Allison Shaw Joël Savarino Robert Delmas J.R. Petit

35. Glacial/Interglacial CO 2 variations “Sea-ice switch” From Gildor and Tziperman, 2001