Ice Nucleation of Model Nanoplastics and Microplastics: A Novel Synthetic Protocol and the Influence of Particle Capping at Diverse Atmospheric Environments

Little is known about airborne atmospheric aerosols containing emerging contaminants such as nano- and microplastics. A novel, minimum energy usage, synthetic protocol of plastic micro/nanoparticles was herein developed. Stable plastic hydrosols were synthesized and characterized using three different types of plastics. The ice nucleation efficiency (INE) was investigated in both normal and synthetic seawater to mimic environmental ice nucleation. Among the three tested plastic precursors (low-density polyethylene, high-density polyethylene, and polypropylene), polypropylene produced the highest particle density with narrow particle size distribution. The change of size, shape, surface charge, and electronic behavior of the plastic nano- and microparticles accounted for the altered INE. The effects of environmental factors such as particle acidity and temperature on ice nucleation were also examined. An increase in pH increased INE due to an increased particle density (number of particles per unit volume), whereas increased temperature decreased INE significantly due to aggregation (attaching particles to produce a larger particle). Four types of capping were used on the surfaces of nano- and microplastics to investigate how the plastics act to nucleate ice when mixed with different particles. They include (a) ZnO as an emerging metal contaminant, (b) kaolin as a clay mineral, (c) HgCl2 as a toxic ionic water pollutant, and (d) phenanthrene as a polycyclic aromatic hydrocarbon. Capping by ZnO and HgCl2 decreased the INE of plastic nano- and microparticles, whereas kaolin and phenanthrene enhanced INE significantly. The association of contaminants to micro- and nanoplastics changes INE likely due to water affinity, surface buckling, and lattice mismatch energy of ice, affecting ice nuclei formation processes. The observed differential physicochemical behaviors of nano- and microplastics, with and without co-contaminant cappings, provide further insights to understand natural environmental ice nucleation and precipitation events. Our work shows that future emissions of nano- and microplastics may become important for cloud formation and thus anthropogenic climate change.