THE HYGROSCOPICITY OF PLASTIC AEROSOLS

Abstract

Polymeric nanoparticles affect many aspects of human life. They directly absorb or scatter sunlight, or indirectly act as cloud condensation nuclei (CCN) to change the Earth’s climate. Additionally, micro-plastics released into the environment have the potential to degrade into nano-size particles. Plastic nanoparticles' sizes, number concentration, and hygroscopicity are important properties to understanding nano-plastics’ fates. In this work, I explored aerosol measurement techniques, aerosol hygroscopicity, and polymer nanoparticles to understand subsequent effects in the environment and on human health. The project was divided into three objectives:For the first objective, I developed the single-parameter hygroscopicity model for polymeric aerosols with Flory-Huggins Köhler theory. Traditional hygroscopicity, derived from Raoult’s law, depends on the molecular volume of the solute. For polymers with a high molecular volume, the predicted hygroscopicity from traditional Köhler theory is zero. However, the experimental results showed that polymers could take up water and readily act as CCN. I developed the expression of the hygroscopicity for polymers and showed the relation between the polymer-water interaction parameter and the water-uptake ability. I also considered water-insoluble polymers and the water-adsorption model combined with Köhler theory to define water-uptake. Thus the CCN activity of polystyrene and surface modified polystyrene particles were also measured. For the second objective, I predicted the fraction of the multiply charged particles, showing that the extinction cross section measured by Cavity Ring Down Spectroscopy (CRD) was influenced by a small amount of multiply charged particles using a Differential Mobility Analyzer (DMA). The initial results indicated that ~4% to ~6% of the total number concentration are triply and quadruply charged particles at 200 nm electrical mobility. This small percentage if neglected could induce errors greater than 5% in subsequent extinction cross section measurements. Thus, the errors induced with commercially available DMAs in the extinction cross section measurement were evaluated. For the third objective, I studied the fate of the nano-plastics in the environment. Results showed that low density polyethylene (LDPE) powders generated particles less than 100 nm at temperatures above 40 oC. I quantified the number concentration of 5 materials in water via traditional atmospheric aerosol measurement techniques. The five materials are cellulose, SiO2, LDPE, polyethylene terephthalate (PET), and polyvinyl chloride (PVC). They were all common materials used for food packaging. Furthermore, the hygroscopicities of the nano-plastics were measured. I demonstrated that the nano-plastics could act as CCN under a supersaturated environment and hence affect the climate. The results showed that the plastic materials (LDPE, PVC, PET) were more hygroscopic than cellulose. The nano-plastics could travel further and be found in remote and cold areas like Antarctica, the Arctic, and high mountains. The work in this objective provided evidence of wet deposition being a possible route for nano-plastics to come to the ground. Plastics are relatively new materials compared to papers, clays, and glasses, but have already been massively produced. The work in this thesis contributed to our understanding of the impact on nano-plastics to the environment. The interaction of the water and nano-plastics in the environment was studied. The measurements of size distribution and hygroscopicity of nano-plastics can be applied in the climate model to reduce the uncertainties in the indirect effect of the aerosols in future studies.