EGU25-18595, updated on 15 Mar 2025
https://doi.org/10.5194/egusphere-egu25-18595
EGU General Assembly 2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
Impact of nanostructure on hygroscopicity and reactivity of fatty acid atmospheric aerosol proxies.
Christian Pfrang1, Adam Milsom1, Ben Woden2, Maximilian Skoda3, Yizhou Su1, Andy Ward4, Andy Smith5, Adam Squires6, and Ben Laurence6
Christian Pfrang et al.
  • 1University of Birmingham, School of Geography, Earth and Environmental Sciences, Birmingham, United Kingdom of Great Britain – England, Scotland, Wales (c.pfrang@bham.ac.uk)
  • 2University of Reading, Department of Chemistry, United Kingdom of Great Britain – England, Scotland, Wales
  • 3ISIS Neutron and Muon Source, Didcot, United Kingdom of Great Britain – England, Scotland, Wales
  • 4Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot, United Kingdom of Great Britain – England, Scotland, Wales
  • 5Diamond Light Source, Diamond House, Harwell Science and Innovation Campus, Didcot, United Kingdom of Great Britain – England, Scotland, Wales
  • 6Department of Chemistry, University of Bath, South Building, Soldier Down Ln, Claverton Down, Bath, United Kingdom of Great Britain – England, Scotland, Wales

Atmospheric aerosol hygroscopicity and reactivity play key roles in determining the aerosol’s fate and are strongly affected by its composition and physical properties. Fatty acids are surfactants commonly found in organic aerosol emissions. They form a wide range of different nanostructures dependent on water content and mixture composition. We follow nano-structural changes in mixtures frequently found in urban organic aerosol emissions, i.e. linoleic acid (LOA), oleic acid (OA), sodium oleate and fructose, during humidity change and exposure to the atmospheric oxidant ozone. Small-Angle X-ray Scattering (SAXS) was employed (Milsom et al., 2024) to derive the hygroscopicity of each nanostructure by measuring time- and humidity-resolved changes in nano-structural parameters. We found that hygroscopicity is directly linked to the specific nanostructure. Reaction with ozone revealed a clear nanostructure-reactivity trend, with notable differences between the individual nanostructures investigated. Simultaneous Raman microscopy complementing the SAXS studies revealed the persistence of oleic acid even after extensive oxidation. Our findings demonstrate that self-assembly of fatty acid nanostructures can significantly impact water uptake and chemical reactivity, thus directly affecting the atmospheric lifetime of these materials.

Another focus of our studies are one-molecule thin layers of LOA and their behaviours when exposed to ozone in multi-component films at the air–water interface (Woden et al., 2024). LOA’s two double bonds allow for ozone-initiated autoxidation, a radical self-oxidation process, as well as traditional ozonolysis. Neutron reflectometry was employed to follow the kinetics of these films in real time in a temperature-controlled environment. We oxidised deuterated LOA (d-LOA) as a monolayer, and in mixed two-component films with either oleic acid (h-OA) or its methyl ester, methyl oleate (h-MO), at room temperature and atmospherically more realistic temperatures of 3 ± 1 °C. We found that the temperature change did not notably affect the reaction rate which was similar to that of pure OA. Kinetic multi-layer modelling using our Multilayer-Py package showed that neither temperature change nor introduction of co-deposited film components alongside d-LOA consistently affected oxidation rates, but the deviation from a single process decay behaviour (indicative of autoxidation) at 98 ppb is clearest for pure d-LOA, weaker for h-MO mixtures and weakest for h-OA mixtures. As atmospheric surfactants will be present in complex, multi-component mixtures, it is important to understand the reasons for these different behaviours even in two-component mixtures of closely related species. Our work demonstrates that it is essential to employ atmospherically realistic ozone levels as well as multi-component mixtures to understand LOA behaviour at low O3 in the atmosphere. Residue formation may be affected by the temperature change, potentially impacting on the persistence of the organic character at the surface of aqueous droplets. Our findings could have impacts on both urban air quality (e.g. protecting harmful urban emissions from atmospheric degradation and therefore enabling their long-range transport), and climate (e.g. affecting cloud formation), with implications for human health and wellbeing.

Milsom et al., Atmos. Chem. Phys., 2024, 24, 13571–13586, DOI: 10.5194/acp-24-13571-2024, 2024.

Woden et al., Faraday Discuss., 2024, DOI: 10.1039/D4FD00167B.

How to cite: Pfrang, C., Milsom, A., Woden, B., Skoda, M., Su, Y., Ward, A., Smith, A., Squires, A., and Laurence, B.: Impact of nanostructure on hygroscopicity and reactivity of fatty acid atmospheric aerosol proxies., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18595, https://doi.org/10.5194/egusphere-egu25-18595, 2025.