Online molecular insights into sources and formation of organic aerosol in Shanghai

Abstract Organic aerosols (OA) constitute a large portion of atmospheric aerosols. The detailed sources of OA, especially precursors and formation processes of secondary OA (SOA) in megacities remain unclear, largely due to lack of molecular composition. We characterized the molecular composition of OA using online extractive electrospray ionization time-of-flight mass spectrometer in winter Shanghai. Eleven OA sources were identified using Positive Matrix Factorization (PMF), combined with gas-phase organic precursors and other auxiliary measurements. We identified a plasticizer-related OA source and a primary biomass burning OA source containing likely oxidation products of biogenic volatile organic compounds (VOCs). SOA from photo-oxidation and aged biomass burning contributed substantially (~48%) to OA and dominated in SOA. Possible precursors of photo-oxidation SOA included aromatics and oxygenated VOCs. The nighttime oxidation of aldehydes and ketones by NO 3 radicals were important sources of SOA. Aqueous SOA derived from aged biomass burning emissions also contributed markedly to OA. Our results here highlight the previously under-appreciated sources and formation processes of OA in megacities, improving our understanding of OA sources and providing support for more accurate air pollution control strategies.