UC Merced

The chemistry of copper (Cu) in ambient aerosols is complex, constrained partially by corrosive weathering nuanced by local aerosol chemistry and overtly controlled by source factors which influence the chemical speciation of Cu in emissions. Without a natural source to the atmosphere the bulk of Cu in atmospheric systems is localized near emission sources which are most commonly due to abrasive wear of moving parts in a mechanical system. A prominent system for Cu is traffic emissions, of which dust from brake wear comprises the majority. Fine and ultra-fine particulate matter (PM2.5 and PM0.25), often enriched in Cu, is considered the greatest environmental hazard to human health, especially in urban locations where health outcomes are notably worse. Inadequate and often simplified characterization of Cu in ambient PM and automotive brake wear prevents full realization of this hazard and handicaps modelling and remediation efforts. This dissertation focusses on the chemical speciation of Cu in ambient PM in urban environments and seeks to elucidate the chemical formation pathways through which Cu is emitted and environmentally weathered. The overall goal was to determine a clear connection from source to sink through chemical analyses of Cu phases. Three main experiments were conducted with the following aims: 1.) Characterize the Cu phases present in weathered and fresh ambient size-fractionated PM; 2.) Investigate whether mechanical wear affects the phase of Cu emitted from car brake pads; 3.) Determine whether the Cu from brake dust forms species similar to those found in ambient PM. Ambient PM2.5 and PM0.25 were collected from several locations in California including at two separate times (2016 and 2020) in Los Angeles. Samples from 2016 were allowed to weather under ambient conditions in a laboratory setting while samples from 2020 were frozen immediately upon collection and protected from air to inhibit weathering/aging. Weathered ambient PM2.5 samples from three locations in the San Joaquin Valley were also studied for comparison. Speciation of Cu phases in bulk samples was performed via X-ray absorption spectroscopy to investigate composition of Cu in PM upon emission and its chemical fate. To investigate the effect of mechanical wear on Cu phases emitted from automotive brake pads a pin-on-disc tribometry experiment was employed. Utilizing commercially available brake pads, the effect of nominal contact pressure on Cu speciation in size fractionated (PM2.5 and PM0.25) samples was examined. Brake pad PM samples were collected during wear using a cascade impactor which allowed for separation of particles based on aerodynamic diameter. Finally, a closed chamber experiment utilizing tribometer-generated brake wear samples probed the effects of various weathering treatments on Cu speciation. Environmentally relevant conditions including high relative humidity, gaseous sulfur dioxide exposure, ultraviolet light, and free radical exposure (∙OH and ∙NOx) were progressively utilized to explore the production and development of Cu species from the parent brake material. Overall, the coupled nature of these experiments illustrates the causal links between automotive brake wear and urban ambient PM and elucidates the major Cu species relevant for environmental and human health considerations.