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Coastal New Particle Formation

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Coastal New Particle Formation

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  1. Aerosol Formation from Photooxidation of Diiodomethane (CH2I2)J. L. Jimenez, D. R. Cocker III, R. Bahreini, H. Zhuang, V. Varutbangkul, R. C. Flagan, and J. H. SeinfeldCalifornia Institute of TechnologyColin O'DowdUniversity of Helsinki & National University of Ireland Thorsten HoffmannInst. of Spectrochemistry and Applied Spectroscopy, Dortmund, GermanyAAAR Annual MeetingNov. 16th, 2001

  2. Coastal New Particle Formation • Frequent observation of coastal nucleation • John Aitken 1890s • Scotland, Ireland, England, France, Antarctica,Tasmania • Mace Head (Ireland): PARFORCE 1998-99 • Possibly the most intense natural new particle source • Coastal: 1-7 x 105 cm-3 s-1 / Other locations: ~ 1 cm-3 s-1 (Weber et al., 1999) • Regional impact on scattering and CCN • May be globally significant MBL particle source • CH2I2 • Strong correlation with tide and sunlight • Iodine always in nucleation-mode particles • No other species can explain observed growth • Kulmala et al.: Iodine condenses on H2SO4-NH3-H2O clusters (JGR, in press) • Hoffman et al. (2001): CH2I2 + hv + O3Þ Particles

  3. Can CH2I2 + hv + O3 explain coastal nucleation? Caltech Chamber Study • Caltech Environmental Chamber • 28 m3 • UV Light intensity ~ Ground Level w/ 0o Zenith • Instrumentation • Number concentration • Number distribution • Mass Distribution and Chemical Composition (AMS) • Hygroscopicity • No particles without either hv or O3

  4. Chamber Experiment Overview Nucleation Condensation Coagulation Wall loss

  5. Radiation has same effect as CH2I2 • Humidity has no effect Effect of Initial CH2I2 (II)

  6. Iodine oxide(s) • HIO3 Aerosol Mass Spectrum

  7. PARFORCE Vakeva et al., JGR, 2002, in press RH and Particle Hygroscopicity Caltech

  8. Aerosol a = 0.02 HI a = 0.02 HOI OH HO2 hv (94s) a = 0.036 a = 0.036 OH HO2 hv (70 s) O3 IO I IO OIO hv (2.8s) IO hv (85 s) IO I2O2 Kinetic Simulations CH2I2 Photolysis: Reaction: Aerosol Uptake:

  9. Qualitative agreement with effect of CH2I2, RH, radiation, O3 • Also for O3 Evolution • Not full quantitative agreement Comparison Mechanism - Experiments

  10. OIO is dominant species Predicted Aerosol Composition

  11. Calculate PREDICTED gas-phase concentrations at the time of EXPERIMENTAL nucleation as f(CH2I2) Pre-Nucleation Species Buildup

  12. Most likely: OIO, then I2O2 Nucleating Species

  13. PARFORCE Caltech Sub-10 nm Particle Formation Rate (cm-3s-1) 1-7 x 105 106 Growth Rates of Nucleation Particles (nm s-1) 0.1 - 0.36 0.08 • Nucleation and Growth Rates Field and lab rates are similar Comparison to Coastal Nucleation • Ambient CH2I2 Concentration? • Condensable vapor needed: 240-400 ppt • Compare with Caltech 500 ppt

  14. Likely that CH2I2 explains nucleation on West Coast of Ireland Conclusions • CH2I2 + hv + O3Þ Nucleation • down to 15 ppt CH2I2 • Mechanism captures parametric dependence • Composition: I2O4 and I2O2, HIO3 • Nucleation: OIO and/or I2O2 • Comparison to field data • Similar hygroscopicity • Similar nucleation and growth rates • H2SO4-NH3 - H2O clusters not needed

  15. BACKUP SLIDES

  16. Chamber Nucleation

  17. Fractal Particles • More compact as RH increases Particle Fractal Dimension • Agglomerates: • Df : • 3.0 Sphere • 1.8 Soot • 1.0 Chain

  18. Particle Fractal Dimension II

  19. No particles w/o UV or O3 Experimental Design • CH2I2 (ppt): 15, 50, 500, 5000, 50000 • RH: < 2%, 65% • Light Intensity: Full, ¼, 0 • O3 (ppb): 100, 0

  20. Particle density decreases with time Mobility vs. Aerodynamic Diameter

  21. PARFORCE (Field) Vakeva et al., JGR, 2002, in press RH and Particle Hygroscopicity Caltech

  22. Kinetic Simulations: Model I Aerosol a = 0.02 HI a = 0.02 HOI OH HO2 hv (94s) a = 0.036 a = 0.036 OH HO2 hv (70 s) O3 IO CH2I2 I IO OIO hv (2.8s) hv (12 h) IO hv (85 s) IO I2O2 Photolysis: Reaction: Aerosol Uptake:

  23. Kinetic Simulations: Model II Aerosol a = 0.036 a = 0.036 hv (70 s) O3 IO CH2I2 I IO OIO hv (2.8s) IO hv (85 s) IO I2O2 Photolysis: Reaction: Aerosol Uptake:

  24. Effect of Initial CH2I2 (I)

  25. Likely I2O4 Summary of Aerosol Chemistry

  26. Nucleating Species (II)

  27. Size Distribution Evolution

  28. Chamber Experiment Overview

  29. O3: Predicted vs. Measured

  30. Inconsistency in the model • Gas-phase bottleneck? Nucleation Flux Check Condensation Flux in Free-Molecular Regime: aOIO = 0.04 (McFiggans et al., 2000) = 0.0045 to match tcond experimental ~ 1 to explain particle growth rate

  31. Kinetic Simulation: Results

  32. Kinetic Simulations:O3 O(1D) O(3P) H OH HO2 I O2 O(3P) O3 O(1D) hv Photolysis: Reaction:

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