Weathering influences the ice nucleation activity of microplastics.

Microplastics are being increasingly detected in the atmosphere at altitudes relevant to mixed-phase cloud formation. However, the extent to which microplastics, along with their dynamic surface properties resulting from environmental weathering, could influence atmospheric microphysical processes remains largely unexplored. Here, through a series of ice nucleation experiments and droplet freezing assays, we highlight the capability of model polyethylene microplastics to induce heterogeneous ice nucleation via immersion freezing under atmospherically relevant conditions. We find that sunlight-induced weathering of the microplastic surface influences the structure of surface-bound water molecules and dictates the ice nucleation activity of the microplastics. Using polyethylene, polypropylene, polystyrene, and polyethylene terephthalate as models, we demonstrate that the ice nucleation ability of microplastics is intrinsically linked to their underlying chemistry. Our findings underscore the need to establish a connection between microplastics and atmospheric processes, as the behavior of microplastic pollutants in the atmosphere holds the potential to influence their environmental transport as well as atmospheric microphysical processes.