PhD Oral Exam - Cassandra Leigh Johannessen, Chemistry

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Tire wear particles (TWPs) are an increasingly recognized source of urban environmental pollution. These particles release tire-derived chemicals (TDCs), including additives and transformation products (TPs), that may pose risks to human and ecological health. This thesis presents a multidisciplinary investigation into the occurrence, transformation, and environmental fate of TDCs, integrating field measurements with computational modelling. A critical review of atmospheric tire-wear pollution identified key knowledge gaps in TDC occurrence, transformation chemistry, and physico-chemical property data that guided subsequent investigations. Passive air sampling across global megacities and source-sector-resolved locations within a large metropolitan area characterized concentrations of TDCs such as 6PPD-quinone, benzothiazoles, benzotriazoles, and hexa(methoxymethyl)melamine, demonstrating that tire pollution is ubiquitous in urban air globally. Source-sector-resolved sampling revealed distinct chemical profiles associated with traffic-heavy locations, with guidance on passive sampler application provided. Furthermore, non-targeted screening of barren-ground caribou and lichen in Northern Canada provided insight into TDC presence in remote ecosystems and corresponding risks to northern biota. Quantum chemical calculations investigated atmospheric and aquatic oxidation pathways of 2-(methylthio)benzothiazole, a benzothiazole of environmental concern. Using density functional theory, hydroxyl radical–initiated reaction mechanisms were explored to assess thermodynamic and kinetic feasibility, enabling prediction of environmental persistence and identification of key TPs. Finally, physico-chemical property estimation, fate modelling, and hazard prediction were applied to 131 synthetic rubber additives to prioritize TDCs of potential environmental concern. Overall, this research advances understanding of TDC environmental occurrence and fate while supporting evidence-based strategies for monitoring, risk assessment, and mitigation of tire-wear pollution.