This study examines long-term trends in fine particulate matter (PM_2.5) composition and oxidative potential in Los Angeles based on data from the University of Southern California’s Particle Instrumentation Unit, with chemical composition retrieved from the EPA’s Air Quality System (AQS). While regulatory interventions have reduced PM_2.5 mass concentration and primary combustion-related components, our findings reveal a more complex toxicity pattern. From 2001 to 2008, the PM_2.5 oxidative potential, measured via the dithiothreitol (DTT) assay, declined from ~0.84 to ~0.16 nmol/min/m³ under stringent tailpipe controls. However, after this initial decline, PM_2.5 DTT stabilized and gradually increased from ~0.35 in 2012 to ~0.97 nmol/min/m³ by 2024, reflecting the growing influence of non-tailpipe emissions such as brake/tire wear. Metals, such as iron (Fe, ~150 ng/m³) and zinc (Zn, ~10 ng/m³), remained relatively stable as organic and elemental carbon (OC and EC) declined, resulting in non-tailpipe contributions dominating PM_2.5 toxicity. Although PM_2.5 mass concentrations were effectively reduced, the growing contribution of non-tailpipe emissions (e.g., brake/tire wear and secondary organic aerosols) underscores the limitations of mass-based standards and tailpipe-focused strategies. Our findings emphasize the need to broaden regulatory strategies, targeting emerging sources that shape PM_2.5 composition and toxicity and ensuring more improvements in public health outcomes.

Airborne microplastics (MPs) are increasingly recognized as atmospheric pollutants with potential implications for human exposure; however, their occurrence, temporal variability, and associations with urban particulate matter remain poorly understood. This study investigated MPs associated with PM₁₀ in central Los Angeles across two seasonal sampling campaigns by integrating MP abundance, morphology, size distribution, PM₁₀ chemical composition, and meteorological observations. MPs were detected in all samples, with concentrations ranging from 2.2-4.0 MP/m³ during the cool-season campaign and 2.2-3.4 MP/m³ during the warm-season campaign. Fibers remained the dominant morphology, while particles <100 µm, particularly the 5-50 µm and 50-100 µm fractions, consistently dominated the airborne MP population. Strong positive correlations (generally ρ ≥ 0.79) were observed between total MP concentrations and major crustal (Fe, Si, Mn, Ca, Al, and Mg), traffic-related (Ba, Cu, and Ni), and carbonaceous (OC and EC) tracers, indicating that MPs are closely associated with urban particulate mixtures influenced by resuspension and non-tailpipe traffic emissions. In contrast, short-term meteorological variability showed no consistent relationship with MP concentrations. These findings indicate that airborne MPs are a persistent component of the urban PM10 mixture rather than isolated contaminants, and provide new insight into their sources and behavior in densely populated urban environments.