Concentrations and formation pathways of nitrogen-containing organic compounds in PM_2.5 from Seoul and Beijing

Organic nitrogen is an important constituent of organic aerosols, yet our understanding of the specific compounds, their concentrations, and atmospheric distribution remains limited. In this study, three nitrosamines (N-Nitrosodimethylamine, N-Nitrosodietylamine, and N-Nitrosodibutylamine) and three amides (N,N-Dimethylformamide, N-Hexadecanamide, and N-Octadecanamide) in atmospheric particulate matter with an aerodynamic diameter of equal to or less than 2.5 μm (PM_2.5), collected in Seoul, South Korea, and Beijing, China, during winter in 2020 and summer in 2021, were analyzed to characterize their concentrations and major contributors. Nitrosamines and amides were found to have distinct distributions under different atmospheric conditions and aerosol properties. Total nitrosamine concentrations were higher in Seoul (5.11 ± 10.13 ng/m³ (winter); 4.15 ± 3.91 ng/m³ (summer)) than in Beijing (0.75 ± 1.59 ng/m³ (winter); 1.09 ± 1.50 ng/m³ (summer)). Total amide concentration was slightly higher in Beijing (2.73 ± 1.84 ng/m³) than in Seoul (2.54 ± 2.84 ng/m³) during winter, while in summer, the concentration in Beijing (0.65 ± 0.49 ng/m³) was much lower than in Seoul (1.82 ± 0.90 ng/m³). Seoul appeared to show higher relative humidity (RH) and aerosol liquid water content concentration compared to Beijing, which may have contributed to elevated nitrosamine concentrations, potentially through enhanced aqueous-phase nitrosation reactions. Meanwhile, amides seemed to demonstrate increased uptake into PM_2.5 and possibly participated in accretion reactions yielding particulate products, particularly under the lower RH conditions observed in Beijing. These findings suggest that atmospheric conditions, such as RH, may influence the distribution patterns of nitrogen-containing organic compounds in the atmosphere.