After Rubber Meets the Road: a Comprehensive Geochemical Analysis of Tire- and Road-wear Particles in Urban Environments

Tire- and road-wear particles (TRWPs) are an increasingly recognized form of non-exhaustemissions and primary microplastic pollution in urban environments. Despite their widespread presence, TRWPs remain difficult to characterize due to their heterogeneous composition, variable density, and interactions with road dust and stormwater systems. This dissertation presents a multi- method investigation into the physical characteristics and chemical behavior of TRWPs in the urban environment, combining particle-level morphology, mineralogy, and density data with chemical analysis of associated organic compounds. Across three integrated research chapters, I developed and applied new analytical approaches to isolate and characterize TRWPs in Charleston, SC and Philadelphia, PA. In Chapters 2 and 3, I used density separation and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) to quantify TRWP size, shape, elongation, and mineral encrustation. Findings show that higher-density TRWPs tend to exhibit greater encrustation and elongation than their low-density counterparts, and that mineral coatings (particularly quartz, feldspar, and iron oxides) play a key role in modifying particle density and environmental behavior. In Philadelphia, I applied this method to a network of urban sites, linking TRWP properties to traffic volume, land use, and street location, and pairing particle-level data with bulk road dust analysis. Results revealed consistent density distributions of TRWPs across sites, along with strong correlations between TRWP concentration and organic content. viii Chapter 4 expanded the investigation to include the chemical behavior of TRWPs under wetting conditions. Using stormwater sampling during a rain event and laboratory leaching experiments, I tracked the release of four tire-associated organic compounds—benzothiazole (BT), 2-hydroxybenzothiazole (2-OHBT), 2-methylthiobenzothiazole (MTBT), and diphenylguanidine (DPG)—alongside elemental analysis. These compounds were detected consistently during the early stages of runoff and continued to leach under extended water exposure, reinforcing the dual role of TRWPs as both physical particles and sources of dissolved contaminants in urban runoff. Together, this dissertation contributes new tools, datasets, and perspectives to the field of TRWP research. By integrating morphological, mineralogical, and chemical approaches, it offers a more complete understanding of TRWP transport, transformation, and fate in the environment, while highlighting the importance of site-specific data and interdisciplinary methods for addressing non-exhaust vehicle emissions in urban systems.