New Insights Into the Sources of Atmospheric Organic Aerosols in East China: A Comparison of Online Molecule‐Level and Bulk Measurements

Abstract Organic aerosols (OA) significantly contribute to haze pollution, threaten human health, and affect the radiation balance. However, real‐time tracking of OA evolution at the molecular level is limited, hindering a comprehensive understanding of their origins and behaviors. In this study, we investigated wintertime OA in a megacity in East China by combining simultaneous measurements from an extractive electrospray ionization time‐of‐flight mass spectrometer (EESI‐TOF) and a high‐resolution time‐of‐flight aerosol mass spectrometer (AMS) (HR‐TOF‐AMS). AMS results indicate that OA accounts for about 27% of non‐refractory submicron particulate matter (NR‐PM 1 ) on average. EESI‐TOF data reveal that C x H y O z and C x H y N 1–2 O z are the predominant OA components, contributing over 70% and 20%, respectively. Factorization analysis shows that while traffic, cooking, and biomass burning are major primary sources, most OA (>70% for EESI‐TOF, >55% for AMS) originate from secondary production. EESI‐TOF, although missing hydrocarbon‐like OA, excels in providing molecular information on oxygenated OA, identifying aromatics and aliphatics as possible key precursors. It further differentiates less oxidized secondary organic aerosols (SOA) into two factors with distinct molecular compositions, likely due to diverse source regions. Importantly, EESI‐TOF identifies two additional factors: one possibly related to plasticizers and another representing SOA formation from monoterpene oxidation by NO 3 radicals. In conclusion, EESI‐TOF complements AMS by offering valuable molecular insights into the chemical processes underlying OA formation, especially in complex urban environments.