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

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‐PM1) on average. EESI‐TOF data reveal that CxHyOz and CxHyN1–2Oz 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 NO3 radicals. In conclusion, EESI‐TOF complements AMS by offering valuable molecular insights into the chemical processes underlying OA formation, especially in complex urban environments. Plain Language Summary: Organic aerosols (OA) are significant atmospheric constituents that profoundly influence weather patterns, climate dynamics, and public health, yet traditional techniques are difficult to effectively characterize the components and sources of organic aerosols. Here, we applied an extractive electrospray ionization time‐of‐flight mass spectrometer (EESI‐TOF) to explore the relevant characteristics of atmospheric organic aerosols during wintertime in the Yangtze River Delta region of eastern China and compared the results with traditional aerosol mass spectrometer (AMS) techniques. Near‐molecular level measurements of EESI‐TOF reveal that nitrogen‐free components dominate the total OA, and nitrogen‐containing components, which may represent organic nitrates, contribute less than 25%. Source apportionment indicates that traffic, cooking activity, and biomass burning are major primary sources of OA, while secondary production is the dominant OA source. Beyond the source attributions discernible from an AMS‐derived data set, EESI‐TOF furnishes enhanced perspectives on OA provenance due to its molecular‐level measurement. It enables the identification of primary OA sources potentially linked to plasticizer emissions and characterizes secondary sources tied to nocturnal monoterpene reactions. In summary, EESI‐TOF provides new insights into the components and sources of atmospheric OA in East China. Key Points: EESI‐TOF was applied to investigate the sources of atmospheric organic aerosols during wintertime in western YRDCxHyOz and CxHyN1–2Oz are the predominant components of OA, contributing over 70% and 20%Being highly complementary to AMS, EESI‐TOF provides a more comprehensive understanding of OA by capturing detailed molecular information [ABSTRACT FROM AUTHOR]