1. The Fate and Effects of Organics At Atmospheric Interfaces Joel Thornton Department of Atmospheric Sciences University of Washington, Seattle thornton@atmos.washington.edu Co-authors: V. Faye McNeill, Glenn Wolfe, Rob Wood

2. University of Washington, Seattle Department of Atmospheric Sciences Organics at Aerosol Interfaces Courtesy of PNNLO’Dowd, et al Nature Sea spray particles --likely contain organics that partition to surface --fatty acids, amino acids, proteinaceous material Pollution/Biogenic --phase separation possible even if “Water Soluble” Courtesy of Claudia Marcolli ETH, Zurich

3. University of Washington, Seattle Department of Atmospheric Sciences Kinetic limitation to water uptake Organics at Aerosol Interfaces ?? N2O5N2O5 OH NO 3 O 3 ?? SVOC H2OH2O

4. University of Washington, Seattle Department of Atmospheric Sciences Film Effects on Cloud Drop Number …an unconstrained problem FFC distributed by mass (open diamonds) FFC distributed by SA (circles) FFC only in 50% of the aerosol particles (+’s) See also: Feingold & Chuang, JAS 2006 Nenes, et al, GRL 2002

5. University of Washington, Seattle Department of Atmospheric Sciences Barrier to reactive uptake Organics at Aerosol Interfaces ?? N2O5N2O5 OH NO 3 O 3 ?? SVOC H2OH2O

6. University of Washington, Seattle Department of Atmospheric Sciences Barrier to Reactive Uptake (N 2 O 5 ) McNeill, et al JPC A 2007 McNeill, et al ACP 2006 Thornton, et al JPC A 2005 Antilla, et al, JPC A 2006 Folkers, et al, GRL 2003 Park, et al, JPC A 2007 (ASAP) Badger, et al, JPC A 2006 “Multilayer” “Monolayer”

7. University of Washington, Seattle Department of Atmospheric Sciences Aerosol Generation Atomizer solutions Dilute salt + trace soluble surfactant 60% RH: particles contain ~ ≤10 wt% surfactant R p ~ 150 nm Aerosol out Compressed N 2 ATOMIZER Vapor Deposition Salt particles coated with fatty acid by vapor deposition Coating thickness determined by CIMS calibration

8. University of Washington, Seattle Department of Atmospheric Sciences Surface Active Organics in Sea Salt Oleic Acid Hydrophilic Head + Hydrophobic Tail Palmitic Acid SDS Na+ - O 3 SO

9. University of Washington, Seattle Department of Atmospheric Sciences Suppression of N 2 O 5 Uptake See McNeill, et al ACP 2006 3.5wt% per particle 1.5x10 14 molec/cm 2 (0.75 of a monolayer) in constant ratio to NaCl D p ~ 300 nm Wall loss 10 wt% SDS NaCl only

10. University of Washington, Seattle Department of Atmospheric Sciences Suppression of N 2 O 5 Uptake What different results with different films tell us. C12, C16, C18 McNeill, et al Antilla, et al, JPC A 2006 Folkers, et al, GRL 2003 1.  N2O5 very sensitive to availability of surface water 2.Film organization important 3.Perhaps a chain length effect 4.Underlying composition?

11. University of Washington, Seattle Department of Atmospheric Sciences Oxidant attack Organics at Atmospheric Interfaces ?? N2O5N2O5 OH NO 3 O 3 ?? SVOC H2OH2O

12. University of Washington, Seattle Department of Atmospheric Sciences Surface Active Organics in Sea Salt Oleic Acid Hydrophilic Head + Hydrophobic Tail Palmitic Acid

13. University of Washington, Seattle Department of Atmospheric Sciences Oxidation – CI-AMS Aerosols O 3 or NO 3 /N 2 O 5 Turbo Drag and Mech. pumps Quadrupole Octopole Ion Guide CDC 210 Po Ion Source Mech. pump volatilization Turbo Drag and Mech. pumps Quadrupole Octopole Ion Guide CDC 210 Po Ion Source Mech. pump volatilization Aerosols and OH Precursors: HONO, H 2 O 2 or O 3 254 nm lamp Photo-reactor Flow Tube Chemical Ionization Mass Spectrometer Impactor

14. University of Washington, Seattle Department of Atmospheric Sciences Negative Ion Detection of Organics I - 127 amu I - H 2 O 145 amu I - oleic acid 409 amu 150 ºC Chemical ionization H2OH2O I -

15. University of Washington, Seattle Department of Atmospheric Sciences Negative Ion Detection of Organics 150 ºC Chemical ionization H2OH2O I - Impaction prior to volatilizationContinuous in-stream volatilization

16. University of Washington, Seattle Department of Atmospheric Sciences O 3 Oxidation of Oleate in Aq. Particles RH ~ 65% Decays consistent with 1 st order kinetics A range of O 3 concentrations and compositions examined Oleate + O 3  Products k I = 0.03 s -1 O 3 = 5 ppm O O O

17. University of Washington, Seattle Department of Atmospheric Sciences O 3 Oxidation of Oleate in Aq. Particles Mechanistic Information Decay constants are well described by Langmuir-Hinshelwood model K O3 ~ 4x10 -14 cm 3 molec -1 Compare to 10 -16 – 10 -13 on other surfaces

18. University of Washington, Seattle Department of Atmospheric Sciences Inefficient O 3 Oxidation of Oleate Apparent Reaction Probability 1x10 -5 <  < 4x10 -5  Factor of 25-100 lower than studies using pure oleic acid aerosols or coated walls. consistent w/single monolayer and reacto-diffusive length estimates O3O3 l~30 nm pure oleic  ~ 10 -3 2 nm thick oleic  ~ 2x10 -5

19. University of Washington, Seattle Department of Atmospheric Sciences OH Oxidation of Fatty Acids OH

20. University of Washington, Seattle Department of Atmospheric Sciences OH Oxidation of Fatty Acids Use SO 2 oxidation to constrain OH avg in reactor  OH determined w/chemical model using constrained OH avg  OH ~ 0.1 OH

21. University of Washington, Seattle Department of Atmospheric Sciences Oxidation Summary  O 3 oxidation of unsaturated fatty acids at the surface follow Langmuir-Hinshelwood mechanism.  oleic O3 ~ 1x10 -5  OH oxidation of saturated fatty acids at the surface of solid or aqueous particles is efficient:  OH ~ 0.1 Widely applicable mechanism: Poschl, et al JPC A 2001 Mmereki, et al JPC A 2003 Kwamena, et al JPC A 2004 McNeill, et al JPC A 2007 Rate may depend on underlying substrate Bertram, et al JPC 2000:  OH > 0.2 Molina, et al GRL:  OH > 0.2 Rate does not appear to depend on underlying substrate

22. University of Washington, Seattle Department of Atmospheric Sciences Implications: Film Lifetime  LH mechanism: alkenoic acid lifetime depends on K O3  Slower oxidation of oleate more consistent with atmospheric observations Robinson, et al JGR 2006  We infer  OH ~ 0.1 for palmitic and oleic acid on NaCl (s) and NaCl (aq) Range based on observed K O3 values saturated surfactant lifetime unsaturated surfactant lifetime oxidation lifetime (hours)

23. University of Washington, Seattle Department of Atmospheric Sciences Organics at Atmospheric Interfaces ?? N2O5N2O5 OH NO 3 O 3 ?? SVOC H2OH2O

24. University of Washington, Seattle Department of Atmospheric Sciences Oleate Ozonolysis Products I - oleic I - azelaic I - 9-oxononanoic I - nonanoic Signal (cps) Products consistent with those previously identified using pure oleic acid (+ some others) Dominant products are particle bound at room temperature

25. University of Washington, Seattle Department of Atmospheric Sciences OH Oxidation Products I - oleic

26. University of Washington, Seattle Department of Atmospheric Sciences Are Products Particle Bound?  Partitioning of ~ 90% of nonanoic acid to particles at room temperature not expected. Nonanoic acid an oligomeric unit?  OH oxidation products are numerous, some clearly small acids (formic, acetic) suggesting that volatilization by oxidation of surface organics is important source of OVOC and loss of aerosol HC.

27. University of Washington, Seattle Department of Atmospheric Sciences Take Home Points  Monolayer films significantly suppress the rates of gas- aerosol reactions, especially of hydrolysis reactions— likely a result of reduced water availability in the near- surface region.  OH oxidation of organics at the interface likely proceeds by true heterogeneous mechanism. Rapid,  OH ~ 0.1 (but not  OH ~ 1) for thin films.  Ozone oxidation of unsaturated surface organics appears to follow a very general Langmuir-Hinshelwood mechanism (often with similar rate constants).  Fate of oxidation products requires more investigation.

28. University of Washington, Seattle Department of Atmospheric Sciences Some of the Unresolved Issues  What extant of oxidation is required to remove barrier effect?  Is there a reservoir of surface active species in atmospheric particles that would replace those that get oxidized at the surface?  How does cloud processing change the surface composition of residual particles?  Need more field evidence for barrier effects and mixing state of organics in atmospheric particles

29. University of Washington, Seattle Department of Atmospheric Sciences Acknowledgements Anna Moon Glenn Wolfe Faye McNeill Reddy Yatavelli

30. University of Washington, Seattle Department of Atmospheric Sciences UW-CIMS Instrument < 150 kg < 1.5 kW < 0.75 m 3 >5 cps/ppt NO 3

31. University of Washington, Seattle Department of Atmospheric Sciences Palmitic Acid + OH on Salt Particles Palmitic Acid is efficiently oxidized by OH. By varying the OH source, we change the average OH in the reactor. Using  OH = 1 in a detailed model of the gas-particle interaction best reproduces the observed behavior.

32. University of Washington, Seattle Department of Atmospheric Sciences Aerosol Characterization SEM-EDAX: “Single Particle” Composition For every particle examined: internally mixed w/NaCl as dominant component

33. University of Washington, Seattle Department of Atmospheric Sciences Effects and Fate of Surface Active Organics Barrier to gas-aerosol mass transport? Enhance partitioning of semi-volatile organics? Surface bound organics bear the brunt of oxidant attack; source of volatile organics? N 2 O 5 ?? OxOx ? ? What are the rates and mechanisms for surface film oxidation and destruction?

34. University of Washington, Seattle Department of Atmospheric Sciences Working Questions--Salient Results Are the kinetics of alkenoic acid ozonolysis the same in mixed aqueous particles as for the pure substance? Does the oxidation of alkenoic acids occur at the gas- aerosol interface in mixed aqueous particles? Are ozonation products particle bound at room temperature? What dictates the lifetime of alkenoics in a surface film?

35. University of Washington, Seattle Department of Atmospheric Sciences Suppression of N 2 O 5 Uptake See McNeill, et al ACP 2006 3.5wt% per particle “0.05 M” at 65% RH 3.4x10 6 molec/particle 1.5x10 14 molec/cm 2 (0.75 of a monolayer) in constant ratio to NaCl D p ~ 300 nm

36. University of Washington, Seattle Department of Atmospheric Sciences Recent Work on Barrier Effect Clifford, et al, PCCP 2007 See also Daumer, et al, JAS 1992 Xiong, et al, ES&T v32, 1998

37. University of Washington, Seattle Department of Atmospheric Sciences Suppression of N 2 O 5 Uptake C6 Thornton & Abbatt Do different results with different reactants and films tell us anything? C12, C18 McNeill, et al C8 Clifford, et al C12, C16, C18 McNeill, et al

38. University of Washington, Seattle Department of Atmospheric Sciences Suppression of N 2 O 5 Uptake Do different results with different reactants and films tell us anything? C6 Thornton & Abbatt C12, C18 McNeill, et al C8 Clifford, et al C12, C16, C18 McNeill, et al Antilla, et al, JPC A 2006 Folkers, et al, GRL 2003