Urban emissions of fine and ultrafine particulate matter in Los Angeles: Sources and variations in lung-deposited surface area

Airborne particulate matter (PM) in urban environments poses significant health risks by penetrating the respiratory system, with concern over lung-deposited surface area (LDSA) as an indicator of particle exposure. This study aimed to investigate the diurnal trends and sources of LDSA, particle number concentration (PNC), elemental carbon (EC), and organic carbon (OC) concentrations in Los Angeles across different seasons to provide a comprehensive understanding of the contributions from primary and secondary sources of ultrafine particles (UFPs). Hourly measurements of PNC and LDSA were conducted using the DiSCmini and Scanning Mobility Particle Sizer (SMPS), while OC and EC concentrations were measured using the Sunset Lab EC/OC Monitor. The results showed distinct diurnal trends in PNC and EC, with peaks occurring in the early morning and evening, which were consistent with periods of increased traffic volume. During warmer periods, a midday increase in PNC was observed, attributed to photochemical reactions. In contrast, a nighttime peak during colder months suggested the formation of secondary aerosols through aqueous-phase chemistry. Additionally, the DiSCmini consistently reported higher LDSA values than SMPS, indicating the presence of irregularly shaped UFPs, particularly during periods of heavy traffic flow. Positive Matrix Factorization (PMF) analysis identified three primary sources. Factor 1 (photochemically influenced processes), driven by secondary organic aerosol formation during warmer periods, contributed to 19% of LDSA. Factor 2, in which primarily traffic influenced emissions were the dominant contributor, accounting for 70% of LDSA and associated with high loadings of OC (61%), EC (78%), and NO x (94%). Factor 3 (aqueous phase secondary process influenced emissions) during colder months, accounted for 11% of LDSA. Both Factor 1 and 3 sources exhibited comparable contributions of OC 4 (52% and 48%, respectively), underscoring their roles in secondary aerosol formation. These findings emphasize the need to address both primary and secondary emissions to mitigate health risks associated with UFP exposure. • Traffic emissions cause ∼70% of lung-deposited surface area (LDSA) in Los Angeles. • Diurnal LDSA peaks were observed at traffic hours. • Higher LDSA was observed during the cold season. • Photochemical, aqueous phase, and traffic emissions sources were identified. • Nighttime aqueous phase processes significantly increase LDSA in colder months.