New insights into the sources of atmospheric organic aerosols in East China: a comparison of online molecule-level and bulk measurements

Organic aerosols (OA) are of great concern because they contribute to haze pollution, threaten human health and affect the radiation balance. However, tracking OA evolution in real time at the molecular level is still 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 time-of-flight mass spectrometer (EESI-TOF) and a high-resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS). AMS results show that the OA mass concentration account for about 27% of non-refractory submicron particulate matters (NR-PM₁) on average during the measurement. Speciated-organic data from EESI-TOF further reveals that C_xH_yO_z and C_xH_yN_1-2O_z are the predominant components of OA, contributing over 70% and 20%, respectively. By performing factorization analysis of data obtained from both instruments, we found that traffic, cooking and biomass burning are major primary sources of OA, but most of OA (>70% for EESI-TOF, >55% for AMS) come from secondary production. Compared to AMS, EESI-TOF misses hydrocarbon-like OA but owns advances in providing molecular information on oxygenated OA, revealing that aromatics and aliphatics are important precursors. Specifically, EESI-TOF further splits the less oxidized secondary organic aerosols (SOA) into two factors with distinct molecular compositions, possibly resulted from diverse source regions. Importantly, EESI-TOF additionally identifies two factors based on the tracer molecules, one possibly related to plasticizers and the other representing the SOA formation from the oxidation of monoterpenes by NO₃ radicals. In conclusion, our findings suggest that EESI-TOF is highly complementary to the widely used AMS, providing valuable molecular information that aids in uncovering chemical processes underlying the formation of OA, especially in the highly complex urban environment.