An organic crystalline state in ageing atmospheric aerosol proxies: spatially resolved structural changes in levitated fatty acid particles
<p>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...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-10-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/15003/2021/acp-21-15003-2021.pdf |
| Summary: | <p>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 <span class="inline-formula">±</span> 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).</p> |
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| ISSN: | 1680-7316 1680-7324 |