Evolution of nitroaromatic compounds in urban aerosols during China’s Blue Sky Protection Campaign: molecular characteristics, sources, and formation pathways

Abstract. Nitrogen-containing organic compounds (NOCs), particularly nitroaromatic compounds (NACs), are important light-absorbing components of atmospheric brown carbon. However, their molecular characteristics, sources, and formation pathways under sustained emission reductions remain poorly understood in Chinese megacities. Herein, we investigated particulate NOCs in urban Shanghai during winter 2018, winter 2023, and summer 2023, identifying 2530, 2685, and 2150 CHON molecular formulas, respectively. Approximately 64 %−75 % of the detected CHON formulas had O/N ratios ≥3, indicating substantial contributions from potential nitro- and/or nitrooxy-containing species. Compared with winter 2018, wintertime CHON species in 2023 exhibited lower aromaticity and a higher contribution of putative oxidized-N species, suggesting a marked reorganization of N-containing organic aerosol composition. The mean wintertime concentration of quantified NACs decreased by approximately 36 % from 44.4 ng·m⁻³ in 2018 to 28.4 ng·m⁻³ in 2023, whereas their contribution to organic matter increased from 0.24±0.11 % to 0.36±0.23 %. NAC concentrations were substantially higher in winter than in summer, with 4-nitrophenol and 4-nitrocatechol being the dominant species. The pronounced daytime enhancement of NACs in summer was closely associated with daytime photooxidation, whereas aqueous-phase processing and nighttime chemistry likely played relatively greater roles in winter. Tracer-based estimates suggested that secondary contribution to particulate NACs increased from 63 % in winter 2018 to 87 % in winter 2023. These findings reveal an evolution in molecular composition and potential formation pathways of urban N-containing organic aerosols and highlight the increasing relative importance of secondary atmospheric processing in regulating particulate NACs under sustained emission reductions.