Occurrence of Traffic-Derived Microplastics in Different Matrices in the Road Environment

Abstract: The prevalence of microplastic contamination has raised concerns about the potential risk and impact on the global environment. Traffic-derived microplastics, i.e., tire wear particles (TWP), polymer-modified bitumen, and road markings contribute to the emissions, and TWP are assumed to be one of the largest sources of microplastic emissions. Due to analytical difficulties, there is still a knowledge gap regarding transport routes, environmental concentrations, and toxicity. This thesis aims to investigate the occurrence of traffic-derived microplastics in several traffic environments and thereby increase the understanding of the particles. Samples were collected on the road surface (Paper I‒IV), in the stormwater (Paper I, II, and IV), in the air (Paper IV), and in material collected by a street sweeper (Paper I‒II). In addition to environmental samples, a road-simulator was used to generate TWP in a controlled environment enabling comparison between tire types and brands resulting in a deeper understanding of the characteristics and physicochemical properties of TWP (Paper V). Different sample preparation steps such as optimised density separation, solvent cleaning, and size fractionation have been assessed, and several analytical methods light microscopy, SEM/EDX, FTIR, and pyr-GC/MS have been evaluated and used for the analysis of traffic-derived particles. Further, a novel method, automated SEM/EDX single particle analysis coupled to a machine learning classifier, has been developed for the analysis of TWP and other traffic-derived particles (Paper III, implemented in Paper IV). The automated SEM/EDX determined the size, shape, surface texture, and elemental composition of the different particles, and was able to categorize the particles into several subclasses: TWP, bitumen, road markings, metals, organics, and minerals. The estimated absolute masses showed that the fine fraction (2‒20 µm) corresponds to more than 50w% of the TWP and bitumen wear particles independently of the sample matrix indicating that TWP can both affect the PM10 concentrations and be transported long distances through water and air (Paper IV). Further, it was concluded that the stormwater system is an important transport route for traffic-derived particles, especially since the road runoff in Sweden is not commonly treated prior to release to recipients. Street sweeping as a potential measure to prevent the spreading of TWP was evaluated in Paper I‒II. Even though the street sweeper collects considerable amounts of material containing high concentrations of TWP, metals, and organic pollutants, no clear reduction was detected neither on the road surface nor in the stormwater. Besides traffic-derived microplastics, Paper II analysed metals and organic pollutants. The results showed concentrations of metals, PAH, phthalates, and aliphatic hydrocarbons exceeding the national guidelines. The result from this thesis contributes to an increased knowledge about the properties and composition of TWP as well as the occurrence of traffic-derived microplastics in different environments. The results can be used as validation against theoretical emissions and transport models. The results have also highlighted the importance of including fine particles (<20 µm) in forthcoming works.