High chemodiversity of nitrogen-containing organic compounds in vehicle emissions driven by transformative reactions

Nitrogen-containing organic compounds (NOCs) threaten air quality and public health by contributing to secondary organic aerosol (SOA) formation and atmospheric toxins. However, despite their importance, NOCs present in vehicle emissions remain poorly understood. To bridge this knowledge gap, this study deployed ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in positive electrospray ionization (ESI+) mode to characterize the molecular composition and dynamic evolution of NOCs in traffic-related aerosols from the entrance and exit of a tunnel in Yichang, China. Two main NOCs groups, CHN+ (13.3-22.6 %) and CHON+ (44.1-55.2 %), were identified. CHN+ compounds, mainly aromatic nitrogen-containing heterocyclics, were light-absorbing and might significantly contribute to SOA. CHON+ species showed high reactivity, with over 70 % undergoing oxidation (42-45 %) or hydrolysis (14-15 %) or both (14-21 %) within the tunnel. After release, they evolved via methylation or hydroxylation, generating high-molecular-weight oxidized derivatives. Moreover, tunnel-specific transformations included dealkylation, oxygenation, and reactions with amines, nitro, and nitroso groups, without decarboxylation due to limited hydroxyl radicals in the dark environments. These findings highlight vehicle emissions as a major NOCs source and provide molecular-level evidence for integrating vehicular NOCs into air quality models and emission control strategies, addressing their impacts on public health and the global nitrogen cycle.