Claire Lucas

Investigation of the oxidation of organic

films on cloud dropletsFunded 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)

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MotivationI 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.

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Outline Introduction to the possible climatic effects Köhler Theory Evidence for organic films Oxidants in clouds and the role of OHOutline of research and method. The

Langmuir TroughNeutron Reflectivity TechniqueSome initial results and early stage analysisFuture directions

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IPCC Climate Change 2007 Summary for Policy Makers Pg 4

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IPCC Climate Change 2001 The Scientific Basis

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Haywood and Boucher. Estimates of Radiative ForcingReviews of Geophysics 34,2

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www-misr.jpl.nasa.gov

Köhler Theory

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Shulman et al 1996 Geophysical Research Letters

SolubilitySurface tension

Vapour pressure

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Dr Martin King. Unpublished Material

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Continental cumulousPruppacher and Klett (1980)

Median super saturation from a variety of cloud types. Warner (1968)

Kohler summarySurfactant 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.

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Evidence for Organic FilmsWyslouzil 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.

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Finlayson-Pitts and Pitts 2000

Organic Films and Aerosol

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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

Cloud droplets and organic films

Hydrophobic tail group

Hydrophilic head group

Cloud Droplet

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Oxidants in cloudsAtmospheric 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.

13Finlayson-Pitts and Pitts

OH Radical in CloudwaterFormed in 2 ways...1) H2O2 + h 2OH

2) 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

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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)

SO

-O

OOCH3

O

OH

CH3

O

OH

CH3 NH2CH3

SDS Oleic Acid

Stearic Acid Octadecylamine

POPC

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The Langmuir Trough

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Measuring Surface Tension

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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.

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Film Compression

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Isotherm

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40μl Stearic Acid on 5% H2O2 Subphase

Gas like

Solid

Liquid

Synthesis of OH

H2O2 + h (254 nm) 2OH

CH3(CH2)16CO2H (stearic acid) + OHproducts

When hydrogen peroxide is added to the subphase OH radicals are produced which

react with the film.

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Kinetic Decay

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3% H2O2 subphase

30min decay with lamp

Kinetic Decay with UV and OH

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Neutron TechniquesNeutrons 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.

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Making NeutronsNeed 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.

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Neutron scattering experimentsSURF 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

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SURF

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The difference in intensities at theMonitor and at the Detector = reflectance

Neutron DataISIS 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.

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Slow neutrons

Fast neutrons

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b

is surface excess molecules per unit area =nb scattering length densityb is the scattering length is the layer thickness.

Fit reflectivity data to

2

sin416

222

4

QR

Q

Reflectivity

Momentum transfer

Initial neutron results for Stearic acid

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3.5 hrs

Stearic Acid with UV

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Kinetic AnalysisTemporal 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

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(OH constant pseudo 1st order kinetics)

Dr Martin King June 2008

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.

Rate constantsThe 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.

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Stearic Acid (A)

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Product B

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Product C

Present AnalysisKinetics 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?

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Future techniques and experimentsTo study the structure and reactivity of the

films using further surface analysis techniques

Langmuir Blodgettry Brewster Angle MicroscopyAtomic Force Microscopy

Study of films with chloride added to the subphase

Study of mixed acid films04/21/23 49Sweden

Brewster Angle Microscopy

Meunier (2000) Colloids and Surfaces A Image: www.uni-ulm.de

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BAM Images

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Teixeira et al (2007) Chemistry and Physics of Lipids

Langmuir Blodgettry and AFMThe technique of dipping a

slide into the film and depositing the film onto it.

This allows the film to be studied with other techniques.

AFM: mechanical probe ‘feels’ the surface.

It can be used to measure the forces between atoms.

A tip oscillates above the sample. As it is effected by Van- der- Waals forces its oscillations increase.

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AFM Images

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Teixeira et al (2007) Chemistry and Physics of Lipids

Conclusions / Take Home MessagesOrganic films are not stable to OH attack.

Oxidation leads to new surface active materials.Continued oxidation leads to removal at the

surface.The effect of these films has implications for the

albedo of the Earth.

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Continental cumulousPruppacher and Klett (1980)

Median super saturation in cloud. Warner (1968)

AcknowledgementsProfessor Adrian Rennie, Uppsala University Dr. Christian Pfrang, University of ReadingDr Katherine Thompson, Birbeck University

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Köhler Curves Solute Effect

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References Meunier J. Why a Brewster Angle Microscope? Colloids and

Surfaces A: Physiochemical and Engineering Aspects 171 (2000) 33-40

IPPC Climate Change 2001: The Scientific Basis IPPC Climate Change 2007: Summary for Policy Makers ,pg 4 Haywood and Boucher. Estimates of Radiative Forcing, Reviews of

Geophysics 34.2 King et al. Laser Tweezers Ramen Study of Optically Trapped

Aerosol Droplets of Seawater and Oleic Acid Reacting with Ozone: Implications for Cloud Droplet Properties (2004) J. Am. Chem. Soc , 126, 16710-16711

Donaldson and Vaida. The Influence of Organic Films at the Air-Aqueous Boundary on Atmospheric Processes (2006) Chem.Rev, 106 1445-1461

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