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

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