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https://doi.org/10.5194/acp-2020-717
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-717
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  12 Aug 2020

12 Aug 2020

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This preprint is currently under review for the journal ACP.

Ozonolysis of fatty acid monolayers at the air–water interface: organic films may persist at the surface of atmospheric aerosols

Ben Woden1,2, Max W. A. Skoda2, Adam Milsom3, Armando Maestro4, James Tellam2, and Christian Pfrang3,5 Ben Woden et al.
  • 1Department of Chemistry, University of Reading, Whiteknights, Reading, RG6 6AD, UK
  • 2ISIS Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK
  • 3School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
  • 4Institut Laue Langevin (ILL), 71 Avenue des Martyrs, Grenoble, 38000, France
  • 5Department of Meteorology, University of Reading, Whiteknights Road, Reading, RG6 6BG, UK

Abstract. Ozonolysis of fatty acid monolayers was studied to understand the fate of organic-coated aerosols under realistic atmospheric conditions. Specifically, we investigated the effects of temperature and salinity on the degradation of oleic acid at the air–water interface and the persistence of the aged surfactant film at the surface. The presence of a residual film is of atmospheric importance, as surface monolayers affect the physical properties of the droplets and because of the role they play in cloud formation. This occurs via several effects, most notably via surface tension reduction. The interplay between atmospheric aerosol loading and the formation, nature, and persistence of clouds is a key uncertainty in climate modelling.

Our data show that a residual surface film, which we suspect to be formed of nonanoic acid and a mixture of azelaic and 9-oxononanoic acids, is retained at the interface after ozonolysis at near-freezing temperatures, but not at room temperature. Given the low temperature conditions used here are atmospherically realistic, the persistence of a product film must be considered when assessing the impact of unsaturated fatty acid partitioned to the air–water interface. The presence of stable (non-oxidisable) reaction products also opens the possibility of build-up of inert monolayers during the aerosol life-cycle with potential implications for cloud formation. We also measured the kinetic behaviour of these films and found that the reactions are not significantly affected by the shift to a lower temperature with rate coefficients determined to be (2.2 ± 0.4) × 10−10 cm2 s−1 at 21 ± 1 °C, and (2.2 ± 0.2) × 10−10 cm2 s−1 at 2 ± 1 °C.

Ben Woden et al.

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Short summary
Atmospheric aerosols contain a large amount of organic compounds, whose oxidation affects their physical properties through a process known as ageing. We have simulated atmospheric ageing experimentally to elucidate the nature and behaviour of residual surface films. Our results clearly show an increasing amount of residue, demonstrating that an inert product film may build up during droplet ageing, even if only ordinarily short-lived reactive species are initially emitted into the atmosphere.
Atmospheric aerosols contain a large amount of organic compounds, whose oxidation affects their...
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