Acoustic levitation with polarising optical microscopy (AL-POM): water uptake in a nanostructured atmospheric aerosol proxy

Laboratory studies on levitated particles of atmospheric aerosol proxies have provided significant contributions to our understanding of aerosol processes. We present an experimental method combining acoustic levitation with polarising optical microscopy (AL-POM) to probe optically birefringent particles, such as the nanostructured surfactant atmospheric aerosol proxy studied here. Birefringent particles were subjected to a step increase in humidity. A decrease in birefringence was measured over time as a result of a nanostructure change, confirmed by complementary synchrotron X-ray scattering. A multi-layer water uptake model was created and fitted to the experimental data, revealing a water diffusion coefficient increase by ca. 5-6 orders of magnitude upon phase transition. This has implications for the timescale of water uptake in surfactant-containing aerosols and their atmospheric lifetimes. This experimental setup has strong potential to be used in conjunction with other levitation methods and in different contexts concerning birefringent materials such as crystallisation. Environmental significance The phase state of atmospheric aerosols is an important factor in determining the chemical lifetime of aerosol components, some of which affect cloud droplet formation and human health. This study presents a novel, non-invasive method of probing the phase state of levitated single aerosol particles undergoing humidity change. The application of a simplified model of particle water uptake allowed us to extract the water diffusion coefficient throughout humidifying particles of a cooking aerosol emission proxy - an important parameter in determining the timescales of water uptake and cloud droplet formation. There is potential to apply this technique to other atmospherically relevant systems to determine key physical parameters such as the diffusion coefficient of molecules in the condensed phase.