Enhanced nocturnal and aqueous formation of CHON during winter haze in Beijing

Organic nitrogen aerosols influence haze pollution, climate change, and human health, while our understanding of their molecular composition and formation mechanisms in polluted environments remains limited. In this study, ultra-high performance liquid chromatography coupled with high-resolution Orbitrap mass spectrometry was employed to characterize the organic molecular composition of PM2.5 in Beijing winter. Our results revealed that CHON constituted over 40% of the total organic molecules, with molecules in the m/z range of 250–450 identified as key drivers of haze evolution. CHON molecules showed a higher O/N ratio (2.8) in the negative mode than that (1.8) in the positive mode, and more molecules were detected during polluted periods, especially at night. CHON compounds were primarily composed of anthropogenic aromatic compounds. The formula numbers and peak abundance of CHON molecules exhibited a strong correlation with ambient humidity and increased at night, indicating the critical role of nocturnal and aqueous chemistry. This study provides insights into the molecular characteristics and formation mechanism of CHON molecules in polluted environments.