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Sulfur and oxygen isotopic tracers of past and present atmospheric chemistry

Sulfur and oxygen isotopic tracers of past and present atmospheric chemistry. Becky Alexander Harvard University April 14, 2003. Overview. What controls atmospheric chemistry and why do we care? Stable isotope measurements: limitations and advantages

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Sulfur and oxygen isotopic tracers of past and present atmospheric chemistry

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  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 (NO3- and SO42-) • Ice core sulfate and nitrate – past variations in atmospheric chemistry • Preliminary modeling “insights” • Summary and conclusions

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

  4. NOx NOx HNO3 HNO3 O3, NO hu, O(1D) O2, H2O HO2 H2SO4 H2SO4 H2SO4 SOx SOx SOx Atmospheric Chemistry is controlled by atmospheric oxidants “The Earth’s oxidizing capacity” CH4 CO HC NOx O3 H2O2 OH

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

  6. Stable Isotope Measurements: Tracers of source strengths and chemical processing of atmospheric constituents (‰) = [(Rsample/Rstandard) – 1]  1000 R = minorX/majorX 18O: R = 18O/16O (CO2, CO, H2O, O2, O3, SO42-….) 34S: R = 34S/32S (SO2, SO42-, H2S)

  7. Overlapping Source Signatures (‰) d 3 4 S - 2 0 - 10 + 10 +20 +30 Volcanic/Mineral CONTINENT Biogenic OCEAN Marine Biogenic Sea water COMBUSTION Coal Oil

  8. Chemical isotopic fractionation • = d34SSO4/d34SSO2 • SO2 + OH  SO4: a > 1.07 • (Luong et al., 2001) • SO2 + O3/H2O2 SO4: a = 1.0165 • (Eriksen, 1972) Oxidation of the heavier isotope is favored resulting in an increasing degree of 34S depletion at progressively later times

  9. Mass-Dependent Fractionation d17O  0.5*d18O :D17O = d17O – 0.5*d18O = 0 d33S  0.5*d34S :D33S = d33S – 0.5*d34S = 0

  10. O3 formation in the laboratory D17O Thiemens and Heidenreich, 1983 d17O/d18O  1 D17O = d17O – 0.5*d18O  0

  11. 16 16 16 17 or 18 16 16 Mass-independent isotope effects – symmetry explanation Symmetry C2v Symmetry Cs O2 + O(3P) O3* Vibrational States Rotational States Rotational States Vibrational States E v=i+1 v=i+1 v = i v = i De De

  12. All D17O measurements in the atmosphere O3 strat. d17O 100 O3 trop. 75 CO2strat. 50 NO3 25 N2O 10 H2O2 CO 5 d18O SO4 10 20 50 100

  13. Tropospheric oxidation The D17O of HNO3 depends also on the dilution factor due to the terminal reaction NO2 + OH  HNO3 NO3 + RH  HNO3 N2O5 + H2O(aq) 2HNO3 D17O of HNO3 a function of RO2/O3 and the terminal reaction D17O of NOx is a function of RO2/O3 oxidation

  14. Tropospheric oxidation D17O of SO4 a function relative amounts of OH, H2O2, and O3 oxidation SO2 in isotopic equilibrium with H2O : No source effect: D17O of SO2 = 0 ‰ HSO3- + O3  D17O ~ 8.0 ‰, pH > 5.6 HSO3- + H2O2  D17O ~ 0.5 ‰, pH < 5.6 SO2 + OH  D17O = 0 ‰ Aqueous Gas

  15. Gas versus Aqueous-Phase Oxidation Aqueous-phase: 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

  16. O3/H2O2 oxidation depends on pH of aqueous phase D17O Lee et al., 2001

  17. [Na+] Estimated sulfate contribution from different sources in La Jolla, CA rainwater pH = 5.1 (average of La Jolla rainwater) D17O (SO4)aqueous = 1.82 ‰ D17O (SO4)actual = 0.75 ‰ Seasalt Aqueous Gas 30% 41% 29% Lee et al., 2001

  18. Oxygen (D17O)  relative oxidation pathways (oxidant chemistry)Gas/Aqueous phase chemistry  climateRelative oxidation concentrations  oxidation efficiency Sulfur (D33S) ?

  19. SO2photolysis Volcanic sulfate in South Pole ice Continuum > 220nm • 1991 Pinatubo : • D33S = 0.7 ± 0.1 ‰ • 1259 Unknown: • D33S = -0.5 ± 0.1 ‰ • 1991 Cerro Hudson : • D33S = -0.1 ± 0.1 ‰ Sulfate Mass-fractionation line Residual SO2 Savarino et al., 2002 Farquhar et al., 2001 Non-zero D33S  stratospheric influence

  20. Conservative Tracers in Ice cores: Na+ NO3- 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

  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, NOx which are not very well constrained

  22. Current knowledge of the past oxidative capacity of the atmosphere Measurement approach Doubling of O3 between PIT/IT 50 % increase of H2O2 between PIT/IT Sigg & Neftel, 1991 Voltz & Kley, 1988 Summit Dye 3 But calibration issue, not representative of global conditions, or stability in proxy records.

  23. Antarctica Greenland Sulfate concentration reflects anthropogenic emissions Sulfate concentration varies with climate

  24. 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

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

  26. Vostok Ice Core – Climatic D17O (SO4) fluctuations DTs data: Kuffey and Vimeux, 2001, Vimeux et al., 2002

  27. Vostok sulfate three-isotope plot

  28. Extended 3-isotope plot 100% O3 oxidation: D17O (SO4) = ¼ * 32‰ = 7.5‰ 100% OH oxidation: D17O (SO4) = 0 ‰ 100% H2O2 oxidation: D17O(SO4) = ½*1‰ = 0.5 ‰ D17O range = 1.3 – 4.8 ‰

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

  30. GCM sensitivity studies What can cause this climate variation? • Stratospheric influence?  NO D33S = 0 for all Vostok samples • 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

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

  33. Site A NO3- Site A SO42-

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

  35. Biomass burning can affect D17O of sulfate and nitrate by: • Altering oxidant (O3) concentrations • Increase aerosol loading affecting heterogeneous oxidation pathways Are D17O measurements of sulfate/nitrate proxies of: Oxidation capacity? Aerosol concentrations?

  36. Resolving D17O sulfate in GEOS-CHEM Resolve sulfate sources: SO2 + OH  SO4A HSO3- + H2O2  SO4B SO32- + O3  SO4C primary sulfate = SO4D (currently direct anthropogenic emissions) D17O = (1*0.5*SO4B + 32*0.25*SO4C)/ (SO4A + SO4B + SO4C + SO4D)

  37. Oxidation by O3 only important during winter in high northern latitudes D17O > 1  O3 oxidation

  38. D17O sulfate versus cloud processing D17O Cloud liquid water content

  39. D17O sulfate versus O3 concentration D17O O3 ppbv

  40. D17O sulfate versus H2O2 concentration D17O sulfate versus OH concentration

  41. D17O versus H2O2 : January D17O H2O2 ppbv

  42. D17O of sulfate is strongly affected by (oxidant) H2O2 concentrations Less so by cloud content Importance of oxidation by O3 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 104 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 SO2 (from DMS) oxidized to sulfate by O3 on sea-salt aerosols

  44. Conclusions and Future Directions • D17O 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 D17O Simulation of heterogeneous chemistry must be improved in GCMs  “current” D17O 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

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