Wintertime VOC distributions, emission signatures, and transboundary transport over the Seoul Metropolitan Area and the Yellow Sea from airborne observations during ASIA-AQ

Abstract. Volatile organic compounds (VOCs) are key precursors of ozone and secondary organic aerosol, yet wintertime constraints on how local emissions, chemical lifetime, and regional transport jointly shape VOC distributions over Korea and the Yellow Sea remain limited based on airborne measurements. We analyzed proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) measurements from 17 Beechcraft 1900D flights during the 2024 Airborne and Satellite Investigation of Asian Air Quality (ASIA-AQ) campaign. The sum of seven VOC species or groups was 2.4 times higher over Seoul than over the Yellow Sea. Toluene, C₈ aromatics, and a methyl ethyl ketone (MEK)-equivalent signal were strongly enhanced in low-altitude, source-influenced air masses, whereas benzene, acetone, and acetonitrile showed weaker regional and vertical contrasts. Seoul-to-Yellow Sea VOC-to-benzene slope ratios generally increased with chemical reactivity, but toluene and C₈ aromatics showed positive deviations, indicating an additional influence of urban aromatic source composition. Yellow Sea observations, supported by NASA DC-8 measurements, indicated continental outflow. An order-of-magnitude transport analysis estimated distance scaled Yellow Sea transport equivalents of approximately 30 % of Seoul benzene emissions in the Clean Air Policy Support System inventory and 6 % for toluene. Wintertime VOC variability is thus species dependent: urban and industrial emissions dominate reactive aromatic VOCs, whereas longer-lived benzene is more sensitive to regional transport. Effective wintertime VOC management requires species-specific strategies considering domestic emissions and regional inflow.