New insights into traffic emissions: the role of hydrocarbons and oxygenated organic species in traffic-derived aerosol

Abstract. A substantial fraction of submicron particles originates from vehicle emissions in urban environments. This study investigated the chemical characteristics and sources of submicron organic aerosol (OA) at a traffic site in Helsinki, Finland, using four datasets collected in 2018–2024. Measurements were conducted using an Aerodyne Aerosol Mass Spectrometer, and source apportionment was performed using Positive Matrix Factorization. The results showed that vehicular traffic contributed to several types of OA. Hydrocarbon-like OA (HOA) typically peaked during morning traffic, whereas more oxygenated OA, referred to here as traffic-related OA (TrOA), also peaked in the morning but remained elevated for a longer duration. The mass spectra of TrOA resembled those of HOA and biomass burning OA, however, TrOA had distinct fractions of C2H4O2+ (at m/z 60), C2H5O2+ (at m/z 61) and C3H5O2+ (at m/z 73) in OA. The exact origin of TrOA remains uncertain, however, delayed morning peaks suggest that TrOA is processed in the atmosphere or emitted from modern vehicles, which typically operate later than heavy-duty vehicles. Semi-volatile OOA also appeared to be partially traffic-related, although due to its secondary nature, it was not directly linked to daily traffic patterns. This study highlights that traffic-associated OA encompasses both hydrocarbons and oxygenated POA and SOA. Relying solely on HOA to estimate traffic POA can result in a 50 % underestimation, as HOA and TrOA often have similar magnitudes. The characteristics of OA linked to vehicular emissions are likely to evolve in future as the vehicle fleet changes.