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

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 & Wide-Angle X-ray Scattering (SAXS/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 atmospheric lifetimes of surface-active organic species in aerosols are highly phase dependent, potentially impacting on climate, urban air quality and long-range transport of pollutants such as Polycyclic Aromatic Hydrocarbons (PAHs).

The impact of molecular self-organisation on the atmospheric fate of a cooking aerosol proxy

“…This lamellar phase is liquid crystalline, as opposed to the crystalline lamellar phase observed previously in levitated particles (Milsom et al, 2021a). This is due to the lack of characteristic wide-angle X-ray scattering (WAXS) peaks returned by these samples (Milsom et al, 2021b), characteristic of the crystalline form of this lamellar phase (Tandon et al, 2001;Milsom et al, 2021a). Liquid oleic acid does exhibit some order via the formation of dimers.…”

The impact of molecular self-organisation on the atmospheric fate of a cooking aerosol proxy

“…4). The formation of a surface crust has been postulated in the literature (Pfrang et al, 2011;Zhou et al, 2013) and direct experimental evidence of surface product aggregation has recently been presented in a similar proxy (Milsom et al, 2021a).…”

“…This is a long-standing discrepancy and suggests that some physical process is inhibiting the ageing of such aerosols. For these reasons, oleic acid has been the compound of choice for laboratory studies into aerosol heterogeneous oxidation (Gallimore et al, 2017;King et al, 2020;Milsom et al, 2021bMilsom et al, , 2021aPfrang et al, 2017;Woden et al, 2021;Zahardis and Petrucci, 2007).…”

Single-particle Raman spectroscopy for studying physical and chemical processes of atmospheric particles

“…The reaction of α-pinene in the droplet phase was more than 1 order faster than in bulk aqueous solutions, likely due to more effective diffusion of ozone in the gas phase and more efficient mass accommodation at the air-droplet interface (King et al, 2008). Milsom et al (2021) reported the ozonolysis of fatty-acid-soap mixed particles, using oleic acid and sodium oleate as models. In these particles, the strong interactions between the soap carboxylate and the carboxylic acid form crystalline structures (Milsom et al, 2021).…”

“…Milsom et al (2021) reported the ozonolysis of fatty-acid-soap mixed particles, using oleic acid and sodium oleate as models. In these particles, the strong interactions between the soap carboxylate and the carboxylic acid form crystalline structures (Milsom et al, 2021). Upon ozonolysis, the decay of C=C (∼ 1600 cm −1 ) was faster in liquid oleic acid droplets than crystalline oleic-acid-soap particles, although the crystalline structure was eventually broken by ozonolysis.…”