Oxygenated Organic Molecules in Urban Beijing: Contrasting Summer–Winter Composition, Sources, and Roles in Secondary Organic Aerosol Formation

Oxygenated organic molecules (OOMs) are important precursors of secondary organic aerosol (SOA), yet their molecular characteristics and sources in megacities are not fully understood. Here, we characterize the molecular composition, summer-winter differences, and sources of gaseous OOMs in urban Beijing using an iodide chemical ionization mass spectrometer. Our results show that C₄-C₅ molecules, especially isoprene oxidation products, accounted for 48% of measured OOMs in summer, while CHO molecules and nitrophenols dominated in winter (∼60%). Most OOMs, particularly organic nitrates in the summer and organic acids in the winter, showed significant increases during polluted episodes. OOMs also increased significantly during summer heatwaves due to enhanced volatilization and precursor emissions. Source apportionment identified ten OOM factors, with isoprene-related factors contributing 43% of OOMs in summer, and nitrophenols and O₄ organic acids prevailing in winter. The relationship between the OOMs and SOA indicated that more than half of summertime SOA is linked to isoprene-related oxidation, while nitrophenols and highly oxidized organic acids contributed most under elevated PM_2.5 levels in winter. Emission controls during the 2022 Winter Olympic Games effectively reduced several OOMs groups, particularly organic acids, while increasing the formation of nitrogen-containing species. Overall, this work reveals the distinct summer-winter behavior of OOMs in Beijing and their varied roles in SOA formation under changing atmospheric and regulatory conditions.