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Instruments / Technical Area	Publication Details	Published
I22-Small angle scattering & Diffraction	The persistence of a proxy for cooking emissions in megacities: a kinetic study of the ozonolysis of self-assembled films by simultaneous Small & Wide Angle X-ray Scattering (SAXS/WAXS) and Raman microscopy Adam Milsom , Adam M. Squires , Ben Woden , Nicholas J. Terrill , Andrew D. Ward , Christian Pfrang Faraday Discussions DOI: 10.1039/D0FD00088D Diamond Proposal Number(s): [21663, 23096] Open Access Show Abstract ▼ Abstract: Cooking emissions account for a significant proportion of the organic aerosol emitted into the urban environment and high pollution events have been linked to an increased organic content on urban particulate matter surfaces. We present a kinetic study on surface coatings of self-assembled (semi-solid) oleic acid-sodium oleate cooking aerosol proxies undergoing ozonolysis. We found a clear film thickness-dependent kinetic behaviour and measured the effect of the organic phase on the kinetics for this system. In addition to the thickness-dependent kinetics, we show that significant fractions of unreacted proxy remain at the end of extensive ozone exposure and that this effect scales approximately linearly with film thickness, suggesting that a late-stage inert reaction product may form to inhibit reaction progress – effectively building up an inert crust. We determine this by using a range of simultaneous analytical techniques; most notably Small-Angle X-ray Scattering (SAXS) has been used for the first time to measure reaction kinetics of films of a wide range of thicknesses from ca. 0.59 to 73 µm with films < 10 µm thick being of potential atmospheric relevance. These observations have implications for the evolution of particulate matter in the urban environment, potentially extending the atmospheric lifetimes of harmful aerosol components and affecting the local urban air quality and climate.	Sep 2020
I22-Small angle scattering & Diffraction	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 Atmospheric Chemistry And Physics 21, 15003 - 15021 DOI: 10.5194/acp-21-15003-2021 Diamond Proposal Number(s): [20541, 21663] Open Access Show Abstract ▼ 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).	Oct 2021
I22-Small angle scattering & Diffraction	The impact of molecular self-organisation on the atmospheric fate of a cooking aerosol proxy Adam Milsom , Adam Squires , Andrew D. Ward , Christian Pfrang Atmospheric Chemistry And Physics 22, 4895 - 4907 DOI: 10.5194/acp-22-4895-2022 Diamond Proposal Number(s): [21663] Open Access Show Abstract ▼ Abstract: Atmospheric aerosols influence the climate via cloud droplet nucleation and can facilitate the long range transport of harmful pollutants. The lifetime of such aerosols can therefore determine their environmental impact. Fatty acids are found in organic aerosol emissions with oleic acid, an unsaturated fatty acid, being a large contributor to cooking emissions. As a surfactant, oleic acid can self-organise into nanostructured lamellar bilayers with its sodium salt, and this self-organisation can influence reaction kinetics. We developed a kinetic multi-layer model-based description of decay data we obtained from laboratory experiments of the ozonolysis of coated films of this self-organised system, demonstrating a decreased diffusivity for both oleic acid and ozone due to lamellar bilayer formation. Diffusivity was further inhibited by a viscous oligomer product forming in the surface layers of the film. Our results indicate that nanostructure formation can increase the reactive half-life of oleic acid by an order of days at typical indoor and outdoor atmospheric ozone concentrations. We are now able to place nanostructure formation in an atmospherically meaningful and quantifiable context. These results have implications for the transport of harmful pollutants and the climate.	Apr 2022

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