Heterogeneous Ice Nucleation of Microplastics before and after Oxidation

Many recent studies point to the environmental threat posed by microplastic pollution, both in waterways and as transmitted globally in the atmosphere.1,2 Airborne microplastics impact the climate by the direct absorption and scattering of radiation3 and may act indirectly to influence cloud formation and precipitation by means of heterogeneous ice nucleation.4 But, the true efficiency of microplastics as ice-nucleating particles and its implications for cloud formation remain largely unknown.Here, we present evidence for ice nucleation in immersion freezing mode induced by various microplastics suspended in water. This study focuses on seven distinct microplastic morphologies in substances composed of polypropylene (PP), polyethylene (PE) and polyethylene terephthalate (PET). For each polymer type, we analyzed at least one commercially-available microplastic sample and one generated from the breakdown of a commonly used commercial product. PP needles, PP fibers and PET fibers nucleated ice at temperatures relevant for mixed-phase cloud formation, with T50 values of -20.88 °C ± 0.52, -23.24°C ± 0.21 and -21.93°C ± 0.51, respectively. The number of ice nucleation sites per surface area (ns(T)) ranged from 10-1 to 104 cm-2 in a temperature interval of -15 to -25°C. In addition, we conducted oxidation experiments, exposing the samples to ozone and UV light, resulting in a decrease of nucleation temperatures among the ice-active microplastics. The presented data holds significant potential for integration into climate models, facilitating estimations of their impact on cloud formation. (1) Dris, R.; Gasperi, J.; Rocher, V.; Saad, M.; Renault, N.; Tassin, B. Microplastic Contamination in an Urban Area: A Case Study in Greater Paris. Environ. Chem. 2015, 12 (5), 592–599. https://doi.org/10.1071/EN14167.(2) Allen, S.; Allen, D.; Baladima, F.; Phoenix, V. R.; Thomas, J. L.; Le Roux, G.; Sonke, J. E. Evidence of Free Tropospheric and Long-Range Transport of Microplastic at Pic Du Midi Observatory. Nat Commun 2021, 12 (1), 7242. https://doi.org/10.1038/s41467-021-27454-7.(3) Revell, L. E.; Kuma, P.; Le Ru, E. C.; Somerville, W. R. C.; Gaw, S. Direct Radiative Effects of Airborne Microplastics. Nature 2021, 598 (7881), 462–467. https://doi.org/10.1038/s41586-021-03864-x.(4) Ganguly, M.; Ariya, P. A. Ice Nucleation of Model Nanoplastics and Microplastics: A Novel Synthetic Protocol and the Influence of Particle Capping at Diverse Atmospheric Environments. ACS Earth Space Chem. 2019, 3 (9), 1729–1739. https://doi.org/10.1021/acsearthspacechem.9b00132.