Airborne Microplastics: Emission Mechanisms and Exposure Factors.

다국어 초록 (Multilingual Abstract)

Microplastics are released from terrestrial surfaces into the atmosphere where they are conveyed via wind and may be deposited, remain in the atmosphere, or disperse across long ranges. Because of this high atmospheric contamination, inhalation is one of the primary exposure pathways for microplastics. Inhalation exposure is controlled by two key factors: a) the quantity of microplastics available to be emitted from terrestrial environments to the atmosphere, and b) the factors that influence the potential of this emission occurring. The reported concentrations of microplastics on terrestrial surfaces vary widely based on monitoring methodology and characteristics of the capture region, making it difficult to estimate potentially emitted microplastics. Concentrations of traditional atmospheric aerosols such as particulate matter (PM 10, PM 2.5), NOx, SOx, and heavy metals, vary based on region characteristics such as climate, land use, socioeconomic vulnerability, and urban sources including landfills, wastewater treatment plants, and hazardous waste sites. However, microplastics have unique properties: low density and high hydrophobicity, which may alter their emission mechanisms and spatial distribution. Thus, it is unknown how fluctuation in these exposure factors could affect the emission potential of microplastics, influencing concentration levels in the atmosphere. Additionally, the fundamental emission mechanisms and factors that could affect the emission potential of microplastics into the atmosphere are unclear. A wide body of literature models the atmospheric emission of particulate pollutants; however, no model to date adapts these traditional emission models by accounting for the unique properties of microplastics. The overall objective of the dissertation is to improve the mechanistic understanding of microplastic emission into the atmosphere, and the factors which may affect the exposure of human populations to higher levels of emitted microplastics in the atmosphere. The dissertation consists of six research chapters. Chapter 2 analyzes the global distribution of deposited airborne microplastics in terrestrial environments to estimate the relative importance of climate and land use, proving that the climate has a larger impact than land use on the abundance of deposited airborne microplastics. Chapter 3 compares the abundance, composition, and size of microplastics found in sediment cores and the water column in the Gulf of Mexico, proving that most microplastics have been deposited in the last two decades. Chapter 4 estimates the potential variability of the abundance of airborne microplastics on leaves in Los Angeles as a function of leaf height, leaf surface hydrophobicity, and land use, confirming a limited scope to use leaves as biomonitoring systems for urban atmospheric microplastics due to high uncertainty. Chapter 5 estimates the relative importance of socioeconomic status or proximity to known sources of microplastics on deposited airborne microplastics, revealing the difficulty of predicting exposure risk and the ubiquitous spatial distribution of microplastic abundance in the atmosphere. Chapter 6 analyzes wind-borne sediments collected from wind tunnel experiments on biosolid-applied agricultural fields, finding preferential emission of microplastics from the soil when compared to the background soil. A theoretical model was constructed considering microplastics' low density and high hydrophobicity which verifies this experimental data and predicts a substantial underestimation of the wind events which exceed the fluid threshold necessary to emit microplastic from biosolid-amended solids. Chapter 7 quantifies the adhesion force between plastics and sand revealing a significant inverse relationship between ultraviolet (UV) weathering time and relative humidity (RH), which implies microplastic's emission potential into the atmosphere could be increased as residence time in the environment increases, or in more humid environments. Overall, the results help to: a) quantify the fundamental forces affecting microplastic emission, b) outline the factors that may influence the emission potential of microplastics, and c) identify research needs to better predict microplastic inhalation exposure. .