1. Oxygen isotope tracers of atmospheric sulfur/oxidant chemistry Becky Alexander Harvard University NOAA Postdoctoral Fellow ACCESS, September 2003

2. 1)Oxygen isotope measurements: tracers of sulfate formation processes (climatic and human health implications) 2) Oxygen isotope modeling (GEOS-CHEM): resolve sulfur chemistry and the sulfur budget Overview:

3. 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- ….)

4. Mass-Dependent Fractionation  17 O/  18 O  0.5  17 O =  17 O – 0.5*  18 O = 0

5. Mass-Independent Fractionation  17 O/  18 O  1 Thiemens and Heidenreich, 1983  17 O  17 O =  17 O – 0.5*  18 O  0

6. Source of  17 O Sulfate SO 2 in isotopic equilibrium with H 2 O : No source effect:  17 O of SO 2 = 0 ‰ 1) SO 3 2 - + O 3   17 O ~ 8 ‰ 2) HSO 3 - + H 2 O 2   17 O ~ 1 ‰ 3) SO 2 + OH   17 O = 0 ‰  17 O of SO 4 2- a function relative amounts of OH, H 2 O 2, and O 3 oxidation Aqueous Gas

7. 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

8. pH dependency of O 3 oxidation and its effect on  17 O of SO 4 2- H2O2H2O2 O3O3 H2O2H2O2 O3O3 Lee et al., 2001

9.  17 O (SO 4 2- ) aqueous = 1.82 ‰ Sources of Sulfate in La Jolla, CA rainwater pH = 5.1 (average of La Jolla rainwater)  17 O (SO 4 2- ) actual = 0.75 ‰ Lee et al., 2001 Aqueous Gas 41% 29% [Na + ] Sea salt 30%

10. GEOS-CHEM  17 O Simulation SO 2 + OH (gas phase)  17 O=0‰ S(IV) + H 2 O 2 (in cloud, pH=4.5)  17 O=1‰ S(IV) + O 3 (in cloud, pH=4.5)  17 O=8‰  17 O sulfate (July) 0.0 2.3 4.6  17 O sulfate (January) 0.0 2.3 4.6  17 O > 1‰  O 3 oxidation  17 O H 2 O 2 (ppbv) Winter: low H 2 O 2 NH: High SO 2 Missing O 3 oxidation source? Preindustrial Antarctic ice core sulfate:  17 O = 1.3-4.8‰ (Alexander et al., 2001)

11. O 3 oxidation on sea-salt aerosols Sea-salt pH = 8  O 3 oxidation dominant S(IV) + O 3 + ALK  SO 4 2- Reaction can proceed until alkalinity (ALK) is titrated (pH<6).

12.  17 O sulfate measurements from Lee 2000 (Ph.D. dissertation) ITCZ INDOEX cruise January 1997

13. Percent decrease in the rate of gaseous H 2 SO 4 production (SO 2 +OH) after adding S(IV) oxidation on sea salt aerosols Implications for the Sulfur budget 0% 50% 100%

14. Conclusions Measurements Model Interpretation Constraint  17 O SO 4 2- provides a means to measure the chemical formation pathways of atmospheric sulfate Improved understanding of the sulfur budget and the radiative effects of sulfate aerosols

15. Acknowledgements Dr. Rokjin Park – Harvard Prof. Daniel Jacob – Harvard Dr. Charles Lee – URS Corp. NOAA CGC postdoctoral fellowship