Molecular self-organisation in surfactant atmospheric aerosol proxies

Aerosols are ubiquitous in the atmosphere. Outdoors, they take part in the climate system via cloud droplet formation, and they contribute to indoor and outdoor air pollution – impacting on human health and man-made environmental change. In the indoor environment, aerosols are formed by common activities such as cooking and cleaning. People can spend up to ca. 90% of their time indoors, especially in the western world. Therefore, there is a need to understand how indoor aerosols are processed in addition to outdoor aerosols.

Surfactants make significant contributions to aerosol emissions, with sources ranging from cooking to sea spray. These molecules alter the cloud droplet formation potential by changing the surface tension of aqueous droplets and thus increasing their ability to grow. They can also coat solid surfaces such as windows (“window grime”) and dust particles. Such surface films are more important indoors due to the higher surface-to-volume ratio compared to the outdoor environment, increasing the likelihood of surface film-pollutant interactions.

A common cooking and marine emission, oleic acid, is known to self-organise into a range of 3-D nanostructures. These nanostructures are highly viscous and as such can impact the kinetics of aerosol and film ageing (i.e. water uptake and oxidation). There is still a discrepancy between the longer atmospheric lifetime of oleic acid compared with laboratory experiment based predictions.

We have created a body of experimental and modelling work focussing on the novel proposition of surfactant self-organisation in the atmosphere. Self-organised proxies were studied as nanometre-to-micrometre films, levitated droplets and bulk mixtures. This access to a wide range of geometries and scales has resulted in the following main conclusions: (i) an atmospherically abundant surfactant can self-organise into a range of viscous nanostructures in the presence of other compounds commonly encountered in atmospheric aerosols; (ii) surfactant self-organisation significantly reduces the reactivity of the organic phase, increasing the chemical lifetime of these surfactant molecules and other particle constituents; (iii) while self-assembly was found in a wide range of conditions and compositions, the specific, observed nanostructure is highly sensitive to mixture composition; (iv) a “crust” of product material forms on the surface of reacting particles and films, limiting the diffusion of reactive gasses to the particle or film bulk and subsequent reactivity. These findings suggest that hazardous, reactive materials may be protected in aerosol matrices underneath a highly viscous shell, thus extending the atmospheric residence times of otherwise short-lived species.