Size-segregated chemical composition and oxidative potential of ambient quasi-ultrafine and accumulation mode particles in Los Angeles

Quasi-ultrafine particles (q-UFPs; 30 nm < d_p < 170 nm) are increasingly recognized as potent contributors to air pollution-related health effects due to their physicochemical characteristics and deep lung penetration. This study investigated the size-segregated chemical composition and oxidative potential of ambient particles in the q-UFPs and accumulation mode ranges (30 nm - 2.5 μm) in central Los Angeles. A Low-Pressure Impactor (LPI), coupled with a Versatile Aerosol Concentration Enrichment System (VACES), was employed alongside a Sioutas Personal Cascade Impactor (PCIS) to collect size-fractionated PM for mass, elemental, ionic, carbonaceous, and toxicological analysis. Mass concentrations in q-UFPs (30–170 nm) were significantly lower than those in the accumulation mode (250–2500 nm), with the latter mode contributing >83 % of total PM mass. Dithiothreitol (DTT) activity was observed to increase across q-UFP size bins, peaking in the 108–170 nm range at 25.98 pmol/min/m³ in winter and 44.31 pmol/min/m³ in summer/fall. In the accumulation mode, slightly lower DTT activity levels were measured (23.95 and 39.98 pmol/min/m³, respectively). Strong positive correlations were identified between DTT activity and elemental carbon (r = 0.95), organic carbon (r = 0.94), ammonium (r = 0.93), and sulfate (r = 0.91), suggesting contributions from both primary emissions and secondary atmospheric processes. Respiratory deposition modeling using the International Commission on Radiological Protection (ICRP) method showed that although q-UFPs comprised a smaller fraction of the total mass, they were found to contribute 29 % of the cumulative alveolar-region dose, with the 108–170 nm fraction alone delivering 31.5 pmol/min. These results highlight the toxicological importance of UFPs and support the need for continued monitoring and research, consistent with the World Health Organization's good practice statements, which recommend the integration of UFP metrics into air quality monitoring frameworks in the absence of formal guideline values.