Aqueous Carbonyl-Amino Aggregation Drives Atmospheric Nitrogen-Containing Oligomer Formation in China Haze

Nitrogen-containing organic compounds (NOCs) are ubiquitous in the troposphere and contribute to the reactive nitrogen cycle and aerosol light absorption, yet their formation mechanisms remain unclear, especially in polluted regions. Here, we characterized NOCs in PM2.5 during winter haze in the North China Plain (NCP), with a comparison to those in the Sichuan Basin (ScB). Reduced-nitrogen NOCs (Re-NOCs) accounted for 40% of organics detected in the ESI+ mode, highlighting their importance within atmospheric NOCs. Compared to dry haze (DH) in NCP, high-molecular-weight (HMW, m/z > 250) Re-NOCs in humid haze (HH) were enhanced, which positively correlated with NH4+ and the mass ratio of aqueous-phase secondary organic aerosols to total water-soluble organic aerosols, along with increases in aliphatic chain length and molecular unsaturation, indicating carbonyl-amino-driven oligomerization and cyclization. Aliphatic Re-NOCs dominated the Re-NOCs during HH, together with strong correlations of a typical NOC oligomer (C8H13N3O2) with water-soluble organic nitrogen and HMW Re-NOCs, further suggesting oligomerization as a key formation pathway for NOCs. We found that stronger aerosol acidity and higher temperature favored oligomerization of Re-NOCs compared to cyclization, resulting in enhanced C8H13N3O2 in ScB relative to that in NCP. Our work revealed oligomerization as a critical mechanism shaping the NOC composition, particularly HMW species.