Reactivity of selected model microplastics with ozone

Microplastics (MPs) are nowadays observed in all environmental compartments. However, in the atmosphere, the presence of this new pollutant is not well documented and not well understood. So far, atmospheric microplastic particles have been observed in megacities,¹ in remote mountain areas² and even over the open ocean,³ albeit in low concentrations. Microplastics exposed to the atmosphere, undergo ageing under the influence of atmospheric oxidants and sunlight. As the MPs are altered over time, their physicochemical properties are modified. In particular, the polymer structure of MPs can become more porous increasing their exposed surface compared to their new counterparts. This in turn can lead to an increased reactivity between atmospheric oxidants / pollutants and polymers. Furthermore, their physicochemical changes can ultimately lead to fragmentation, giving rise to the formation of more and smaller particles. It therefore seems important to quantify the reactivity of these new pollutants with atmospheric oxidants, as this can affect the role of MPs in the atmosphere, and their environmental fate. The present study provides the first results of a laboratory study on the reactivity of model microplastics, PEEK, PTFE, LDPE, PET, with ozone in a in a DRIFT (Diffuse Reflectance Infrared Fourier Transform) optical cell for the in-situ monitoring of MPs surface aging, coupled with an ozone analyser and a soft ionization Mass spectrometer (SIFT-MS), for the real-time monitoring of the gas-phase. The in-situ monitoring of MPs properties with DRIFTs revealed important chemical changes on their surface due to their exposition to ozone. We suggest that carboxylic acids and/or esters are amongst the most prominent reaction products. We also determine the ozone uptake coefficient (γss) of the MPs, and the evolution of their specific surface area before and after exposure to ozone. The values of γss measured are in the range of 10-8 to 10-9, indicating that MPs are not an important sink of gaseous O₃ in the atmosphere. Further analysis of the gas-phase with SIFT-MS, evidenced the formation of VOCs and particularly carboxylic acids amongst the most prominent reaction products of MPs reaction with ozone. The impact of the modification of chemical functionalities at the surface of these microplastic on their environmental behaviour (e.g. hygroscopicity) and the reactivity of microplastics on the composition of the atmosphere is further discussed.
Ref. : [1] Dris et al., 2015, Environ. Chem / [2] Allen et al., 2015, Nat. Geosci. / [3] Trainic et al., 2020, Commun. Earth Environ.