An organic crystalline state in ageing atmospheric aerosol proxies: Spatially resolved structural changes in levitated fatty acid particles

Adam Milsom, Adam M. Squires, Jacob A. Boswell, Nicholas J. Terrill, Andrew D. Ward, Christian Pfrang

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17 Citations (SciVal)

Abstract

Organic aerosols are key components of the Earth's atmospheric system. The phase state of organic aerosols is known to be a significant factor in determining aerosol reactivity, water uptake and atmospheric lifetime - with wide implications for cloud formation, climate, air quality and human health. Unsaturated fatty acids contribute to urban cooking emissions and sea spray aerosols. These compounds, exemplified by oleic acid and its sodium salt, are surface-active and have been shown to self-assemble into a variety of liquid-crystalline phases upon addition of water. Here we observe a crystalline acid-soap complex in acoustically levitated oleic acid-sodium oleate particles. We developed a synchrotron-based simultaneous small-angle and wide-angle X-ray scattering (SAXS and WAXS)-Raman microscopy system to probe physical and chemical changes in the proxy during exposure to humidity and the atmospheric oxidant ozone. We present a spatially resolved structural picture of a levitated particle during humidification, revealing a phase gradient consisting of a disordered liquid crystalline shell and crystalline core. Ozonolysis is significantly slower in the crystalline phase compared with the liquid phase, and a significant portion (34±8%) of unreacted material remains after extensive oxidation. We present experimental evidence of inert surface layer formation during ozonolysis, taking advantage of spatially resolved simultaneous SAXS-WAXS experiments. These observations suggest that atmospheric lifetimes of surface-active organic species in aerosols are highly phase-dependent, potentially impacting climate, urban air quality and long-range transport of pollutants such as polycyclic aromatic hydrocarbons (PAHs).

Original languageEnglish
Pages (from-to)15003-15021
Number of pages19
JournalAtmospheric Chemistry and Physics
Volume21
Issue number19
DOIs
Publication statusPublished - 8 Oct 2021

Bibliographical note

Funding Information:
Financial support. This research has been supported by the Nat-

Funding Information:
Acknowledgements. Adam Milsom is grateful for the support from NERC SCENARIO DTP and CENTA DTP; Christian Pfrang wishes to thank the Royal Society and NERC for support in developing the acoustic levitation system; Jacob A. Boswell was funded by the EPSRC Centre for Doctoral Training in Sustainable Chemical Technologies. Staff on the I22 beamline at the Diamond Light Source including Andy Smith and Tim Snow are acknowledged; Niclas Johansson and Esko Kokkonen are acknowledged for their help with the beamtime experiments. Ben Woden is acknowledged for helping to calibrate the ozoniser. This work was carried out with the support of the Diamond Light Source, instrument I22 (proposals SM20541 and SM21663). Joanne M. Elliott is acknowledged for providing access to the polarising microscope.

ASJC Scopus subject areas

  • Atmospheric Science

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