• Title of thesis: Characterization of Water-soluble Metals in Urban Aerosols and Comparative Analysis of PM2.5 Oxidative Potential in a Subarctic City
• Date: November 16th
• Time: 2:00 PM - 5:00 PM
• Location: Ford ES&T L1114
• Name of advisor and committee members: Rodney Weber (advisor), Lewis G. Huey, Pengfei Liu, Nga Lee (Sally) Ng, Armistead G. Russell

Epidemiological studies have established a link between fine particulate matter (PM2.5) mass and adverse health-related issues. Particle oxidative potential (OP), referring to the redox ability of PM, is a possible unifying concept that connects a host of adverse health effects, but has not been well studied in subarctic regions. Furthermore, there is a notable absence of comparative investigations regarding OP across cities with distinct sources, as well as between indoor and outdoor environments. In this dissertation, we investigated the OP of PM2.5 in wintertime Fairbanks, Alaska, which is a subarctic city with episodic severe PM2.5 concentrations during winter associated with large residential heating emissions, and compared it to Atlanta and Los Angeles. Approximately 40 23.5-hr filter samples were collected during the 2022 winter heating season and analyzed for OP via the dithiothreitol-depletion (OPDTT) and hydroxyl-generation (OPOH) assays. Multivariate linear regression analysis, correlations with source tracers, and contrast between cold and warmer events indicated that OPDTT was sensitive to copper, elemental carbon and organic aerosol from residential wood burning, and OPOH to iron and organic aerosol from vehicles. Fairbanks exhibited higher PM2.5 mass concentrations than Atlanta and Los Angeles, while OPDTT levels were similar between Fairbanks and Atlanta. Los Angeles had the highest OPDTT and OPOH levels. Differences were due to contrasting emissions from biomass burning and vehicles. Indoor PM2.5 at the residential site was also investigated with OPDTT and compared to outdoor levels. Indoor perturbation experiments showed intrinsic OPDTT (OPDTT per PM2.5 mass, a measure of toxicity) was highly dependent on the type of activity; pellet stove emissions had high toxicity while cooking emissions were substantially lower. These findings demonstrate the importance of sources and specific aerosol components, such as transition metals and organic species, providing valuable insights beyond PM2.5 mass concentration in assessing air quality.

While OP of organic species has been investigated in a number of studies, the linkage between transition metals and aerosol adverse health effect is not very well understood. One possibility is that transition metals, especially water-soluble redox-active transition metals, such as iron and copper ions in PM, can catalyze the production of reactive oxygen species (ROS) in vivo, leading to oxidative stress and therefore are important contributors to aerosol OP and have a stronger association with adverse health outcomes than PM2.5 mass. In addition, soluble metals and metal nanoparticles are readily transported to the brain from the olfactory mucosa and have been associated with reduced cognitive function and dementia. However, water-soluble metals are operationally defined (as those species in the aqueous filter extract that can penetrate through a 0.45 µm syringe filter) and water-soluble species have not been well characterized. In this dissertation, we developed robust liquid spectrophotometric methods for measuring total and soluble Fe and Cu with a relatively inexpensive analytical system. These methods were applied to 24-hour filter samples collected throughout the year 2017 in urban Atlanta. The water-soluble components were further characterized by ultrafiltration, which showed that roughly 85% of both the Fe and Cu in the water-soluble fraction was composed of dissolved species or colloidal particles smaller than nominally 4 nm. The solubility and ultrafiltration components of Fe are possibly associated with both acid-promoted dissolution process and complexation by organic ligands, such as oxalate; oxalate-Fe ligand was mainly found in the smallest ultrafiltrate size (< nominally 2 nm), whereas pH cycling could lead to the formation of colloidal particles with sizes between nominally 2-4 nm. Size distributions of metals of PM samples collected at a roadside site in urban Atlanta showed a shift in water-soluble metals to smaller particle size relative to the size of total metal, moving to more acidic aerosol. The shift was inversely related to the solubility of the metal such that less soluble metals like iron were at smaller particle sizes than more soluble metals like copper, consistent with the large role of particle pH on the dissolution of highly insoluble emitted species. This thesis provides new insights into the specific components of PM2.5 that may be most detrimental to human health.