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Claire Lucas

Claire Lucas. Investigation of the oxidation of organic films on cloud droplets Funded by the Natural Environment Research Council and the Science and Technology Facilities Council The Rutherford Appleton Laboratory

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Claire Lucas

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  1. Claire Lucas Investigation of the oxidation of organic films on cloud droplets Funded by the Natural Environment Research Council and the Science and Technology Facilities Council The Rutherford Appleton Laboratory Supervisors: Dr Martin King (Royal Holloway University of London) and Dr Arwel Hughes (Rutherford Appleton Laboratory) Sweden

  2. Motivation • I am interested in the climatic implications of oxidising the organic films found on cloud droplets. • Is the film resistant to oxidation or will it change? • If there are changes in the film then this could influence climate by changing cloud properties and thus the albedo of the Earth. • Changes to clouds would have implications in the global water cycle. • Chemical information on interfaces is needed for modelling. Sweden

  3. Outline • Introduction to the possible climatic effects • Köhler Theory • Evidence for organic films • Oxidants in clouds and the role of OH • Outline of research and method. The Langmuir Trough • Neutron Reflectivity Technique • Some initial results and early stage analysis • Future directions Sweden

  4. IPCC Climate Change 2007 Summary for Policy Makers Pg 4 Sweden

  5. IPCC Climate Change 2001 The Scientific Basis Sweden

  6. Haywood and Boucher. Estimates of Radiative Forcing Reviews of Geophysics 34,2 Sweden

  7. www-misr.jpl.nasa.gov Sweden

  8. Köhler Theory Vapour pressure Solubility Surface tension Shulman et al 1996 Geophysical Research Letters Sweden

  9. Dr Martin King. Unpublished Material Sweden

  10. Continental cumulous Pruppacher and Klett (1980) Median super saturation from a variety of cloud types. Warner (1968) Sweden

  11. Kohler summary • Surfactant films will effect cloud droplet growth by dissolution and by lowering the surface tension . • Lowering the surface tension of the droplet will decrease the critical supersaturation allowing droplet growth. • This could make droplet activation more likely and allow a greater number of cloud condensation nuclei. Sweden

  12. Evidence for Organic Films • Wyslouzil et al (2005) synthesised a series of droplets suspended in a carrier gas and added organics. A shell like structure was observed around the droplet. • Shu et al (1975) collected aerosol from Los Angeles. Scanning Electron Microscopy dehydrated the aerosol and a wrinkly coating was imaged. Sweden

  13. Finlayson-Pitts and Pitts 2000 Sweden

  14. Organic Films and Aerosol Organics in the atmosphere form a surface film on aerosol particles by aligning on the surface. Components have been found in rainwater, cloudwater and fog water. Sources include the sea surface micro-layer, anthropogenic emissions and biogenic emissions. Tervahattu et al (2002) Finlayson-Pitts and Pitts Sweden

  15. Cloud droplets and organic films Cloud Droplet Hydrophobic tail group Hydrophilic head group Sweden

  16. Oxidants in clouds • Atmospheric O3 (Ozone) • OH (Radical) daytime atmospheric chemistry formed within the droplet. • Nitrate nightime atmospheric chemistry, being studied at the University of Reading on trough at RHUL NO2 by-product studied so far. NO3 future. Finlayson-Pitts and Pitts 13

  17. OH Radical in Cloudwater Formed in 2 ways... • H2O2 + h2OH • NO3- + h NO2 + O- O- + H2O OH + OH- NO3-+ h NO2- + O (3p) NO2- + h NO + O- O- + H2O OH + OH- • OH acts as a “detergent”. by breaking down carbon bonds Sweden

  18. Investigation of 5 representative organic films... • Sodium dodecyl sulphate (soluble surfactant) • Oleic acid (insoluble, unsaturated surfactant) • Stearic acid (insoluble, saturated surfactant) • Octadecylamine (insoluble, unsaturated basic surfactant) • POPC Palmitoyl-2-Oleoyl- sn-Glycero-3-Phosphocoline (phosolipid found in lung lining) Sweden

  19. Oleic Acid SDS Stearic Acid Octadecylamine POPC Sweden

  20. The Langmuir Trough Sweden

  21. Sweden

  22. Measuring Surface Tension Sweden

  23. Primary technique • Changes in the films molecular packing affect the surface tension as measured on the Langmuir Trough. • The film decays over time as it sits in the trough. The area between the barriers remains constant. • UV light effects the reactions which are determining the packing of the layer in the space between the barriers. This effect can be seen when comparing runs without UV input. • The gas above the trough can be changed to determine the effect on the film. Sweden

  24. Film Compression Sweden

  25. Isotherm Solid 40μl Stearic Acid on 5% H2O2 Subphase Liquid Gas like Sweden

  26. Synthesis of OH H2O2 + h (254 nm)2OH CH3(CH2)16CO2H (stearic acid) + OH products When hydrogen peroxide is added to the subphase OH radicals are produced which react with the film. Sweden

  27. Kinetic Decay 3% H2O2 subphase 30min decay with lamp Sweden

  28. Kinetic Decay with UV and OH Sweden

  29. Neutron Techniques • Neutrons interact with the nucleus not with the electrons as light does. • Moderated neutrons are used. • They can detect the positions and movements of atoms. Thus you can probe interiors not just the surface of a material. • They are non destructive as the interaction with the nucleus is weak. Sweden

  30. Making Neutrons • Need a wavelength of a similar order of magnitude to the interatomic separations in the material being studied. • A particle accelerator (synchrotron) is used to create a beam of charged particles (protons). • The beam is fired at a target (tantalum). The beam interacts with the target producing high energy neutrons (spallation). • These are slowed down by a moderator and pulsed to allow time of flight calculations. Sweden

  31. Sweden

  32. Neutron scattering experiments • SURF measures intensity therefore we need to maximise the reflection of neutrons. • Deuterium scatters very differently to Hydrogen. The strong contrast gives good reflection. The deuterated monolayer is very strong and the subphase is weak. Air H2O Air D2O/H2O Sweden

  33. SURF Sweden

  34. Sweden

  35. The difference in intensities at the Monitor and at the Detector = reflectance Sweden

  36. Neutron Data • ISIS is a pulsed neutron source. • The flux of neutrons is such that more fast momentum neutrons are produced. • Each neutron hitting the detector is counted and added up. Slow neutrons Fast neutrons Sweden

  37. Fitting Neutron Data  is surface excess molecules per unit area  =nb scattering length density b is the scattering length  is the layer thickness. Fit reflectivity data to Momentum transfer Reflectivity Sweden

  38. Initial neutron results for Stearic acid 3.5 hrs Sweden

  39. Stearic Acid with UV Sweden

  40. Sweden

  41. Kinetic Analysis Temporal profile of Γ suggests degradation, with surface active products i.e. Γ=[A]surface + [B]surface + [C]surface The concentration of deuterated material at the surface depends on A,B and C. Over-simplification probably involving many more steps as OH cleaves off 1 Carbon per step Sweden

  42. (OH constant  pseudo 1st order kinetics) The rate of loss of A,B and C as differential equations, solving gives concentrations. Adding together the concentrations gives surface coverage. The bottom equation is fitted to the data. Dr Martin King June 2008 Sweden

  43. Rate constants • The fitted equation implies • kA = 0.00034 s-1 • kB = 0.00048 s-1 • kC = 0.00051 s-1 • i.e. similar values that would fit with OH kinetics as OH is an unselective radical.

  44. Sweden

  45. Stearic Acid (A) Sweden

  46. Product B Sweden

  47. Product C Sweden

  48. Present Analysis • Kinetics to fit to the decay of the products. • Where is the OH attacking. Tails? Could be cleaving the Carbon bonds in the tails. • 2+ Products, what are they and how do they behave? Sweden

  49. Future techniques and experiments To study the structure and reactivity of the films using further surface analysis techniques Langmuir Blodgettry Brewster Angle Microscopy Atomic Force Microscopy Study of films with chloride added to the subphase Study of mixed acid films Sweden

  50. Brewster Angle Microscopy Meunier (2000) Colloids and Surfaces A Image: www.uni-ulm.de Sweden

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