Measurement report: Molecular insights into organic aerosol sources and formation at a regional background site in South China

Abstract. Understanding the chemical composition and quantifying the sources of organic aerosols (OA) are crucial for assessing their formation and human-related effects, yet individual mass spectrometry techniques still struggle to reveal the effects of sources and atmospheric processes on OA composition at the molecular level. In this study, we combined for the first time a high-resolution time-of-flight aerosol mass spectrometer (AMS), a thermal-desorption aerosol gas-chromatograph-mass spectrometer (TAG-MS), and an electrospray ionization high-resolution orbitrap mass spectrometer (HR-MS) to analyze OA at a background site in South China from bulk to molecular levels. Positive matrix factorization (PMF) analysis based on AMS data and organic tracers from TAG-MS showed that the low-oxidized oxygenated OA (LO-OOA1) was mainly contributed by biomass burning-related OA (BB-OA) and gas-phase secondary OA (gas-pSOA), while the high-oxidization degree of more-oxidized oxygenated OA (MO-OOA) was mainly due to isoprene-derived secondary OA. Using a non-negative matrix factorization (NMF) approach constrained by PMF source contributions to offline HR-MS data, we identified molecules associated with each PMF-resolved factor. The NMF resolved cooking-related OA (C-OANMF) factor exhibited the highest O/C ratio but the lowest double bond equivalent (DBE) value, whereas the BB-OANMF factor was characterized by the greatest aromaticity and a high abundance of nitroaromatics. For secondary processes, sulfur additions played a more significant role in gas-pSOA than in secondary inorganic aerosol-related OA (SIA-OA). Overall, this study enhances our understanding of the sources and formation of different AMS components, reveals the impact of different sources on the molecular composition of OA, and underscore the prominent impact of anthropogenic emissions and their photo-oxidation on ambient OA in areas with low particulate matter pollution but high O3 levels.