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

Abstract

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.

Atmospheric transport is a major pathway of microplastics to remote regions

“…Yet another source of uncertainty is the efficiency with which particles are scavenged by precipitation. Plastics are generally hydrophobic and should therefore be rather inefficient cloud condensation nuclei (CCN) 91,92 . However, as known for BC, coatings may make the particles more hydrophilic with time in the atmosphere 49 .…”

Examination of the ocean as a source for atmospheric microplastics

“…Cloud coalescing nuclei do not have to be hygroscopic in nature, their presence is often enough to be a factor in the formation of clouds [56]. As illustrated in recent research by Ganguly and Ariya (2019) MP and NP have a high ice nucleation efficiency and may be important for cloud formation [54]. While this is theoretical and speculative, the potential for ocean-atmosphere exchange similar to SSA is a first step to the greater questions on the impact of atmospheric MP/NP.…”

“…The importance of the potential for MP or NP to act in a similar way to SSA is that the potential ocean-atmosphere exchange identifies the earths ocean surface as a possible atmospheric plastic pollution secondary source (a sink that becomes a source through bubble burst ejection and wave action). Once airborne (expelled from the ocean via bubble burst or jet ejection) MP or NP may function in a similar way to dust and salt particles [52], acting as cloud condensing nuclei (CCN) for ice or cloud nucleation [53,54]. Once airborne, dust, SSA, bacteria and sand are all known to be rained out (incorporation of particles within cloud droplets) or washed out (collision with precipitation below cloud level) of the atmosphere.…”

Atmospheric Micro and Nanoplastics: An Enormous Microscopic Problem

“…Figure 1 reports the main sources and transport pathways of MP and NP. Once in the atmosphere, plastic particles are transported following air masses movements [17,43] and could act as ice nuclei, facilitating the formation of ice crystals [44]. The atmospheric oxidant capacity probably has a key role in their transformation, since MP and NP are exposed to sunlight [45], gas-phase oxidants (ozone, hydroxyl and nitrate radicals) [46][47][48][49] and aqueous phase oxidants (hydroxyl and nitrate radicals) [50] with higher concentrations compared to soil and water bodies.…”