NOx Dependent Formation Pathways of Particulate Organic Nitrogen Compounds in Urban Seoul

This study examined how particulate nitrosamines and nitramines form in the urban atmosphere during spring over Seoul. These organic nitrogen compounds are recognized carcinogens and necessitate systematic investigation in metropolitan areas characterized by high population density and elevated exposure risks. We collected 17 daily particulate matter with an aerodynamic diameter equal or less than 2.5 μm (PM₂.₅) samples from May to June 2019 and analyzed them using gas chromatography-mass spectrometry to determine concentrations and formation mechanisms. Measurements showed total nitroso compound levels of 17.51 ± 16.74 ng/m³, markedly higher than previous spring observations, with nitroso-dibutylamine dominating at 7.86 ± 8.59 ng/m³. This represents a notable shift from prior seasonal patterns where nitrosodimethylamine typically predominated, suggesting changes in either emission sources or secondary formation processes. Correlation analysis revealed positive associations with both primary emission markers such as carbon monoxide and polycyclic aromatic hydrocarbons, as well as factors indicative of secondary formation including liquid water content, indicating multiple pathways contribute to ambient concentrations. Box model simulations incorporating comprehensive gas-phase and aqueous-phase reaction mechanisms revealed that secondary atmospheric reactions contributed substantially to measured concentrations, accounting for approximately 24% of nitrosodimethylamine and 55% of N-nitrodimethylamine formation. Examining compound responses to nitrogen oxide variations revealed distinct patterns: nitrogen dioxide increases enhanced both compounds through elevated N₂O₃ and N₂O₄ production, whereas nitrogen monoxide selectively promoted only nitrosodimethylamine formation via the formation of dimethylamino radicals. Our findings demonstrate the complex NOx chemistry governing carcinogenic nitro(so) compound formation in urban environments and suggest that effective mitigation requires coordinated strategies targeting both NOx emissions and precursor amine sources rather than singular approaches.