Morning Rush-Hour Emissions Drive Peak Secondary Organic Aerosol Formation Potential at an Urban Roadside Site in Seoul during Spring.

Urban traffic emissions are a well-known source of primary particulate matter, but their contribution to secondary organic aerosol (SOA) formation in complex city environments remains under-quantified. This study investigated the oxidative evolution of fresh plumes at a traffic-impacted site in central Seoul during spring 2022, utilizing a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) coupled with an oxidation flow reactor (OFR). We found that SOA formation potential peaked distinctly during the morning rush hour (07:00-09:00 KST), driven by a surge in aromatic precursors (toluene, xylenes) from nearby traffic corridors. The abundance of reactive precursors emitted under high-NOx roadside conditions fueled rapid mass growth via carboxylic acid functionalization once exposed to OH-rich, oxidative conditions in the OFR, analogous to plume aging downwind where NOx is rapidly diluted. Furthermore, the highest SOA enhancement was observed when high relative humidity (>70%) and elevated VOC concentrations co-occurred, suggesting that precursor availability and humid atmospheric conditions may jointly favor SOA formation. These results demonstrate that fresh emissions in the urban center possess a high potential for immediate secondary transformation, particularly during humid springtime mornings. Consequently, emission control strategies must target the specific precursors emitted during traffic peaks to mitigate the downwind burden of secondary pollution.