Oxidative Degradation of Higher-Molecular-Weight Aromatic Amine Compounds Is a Potential Source of Anilinium in Urban Aerosols

Aminiums are significant components of organic aerosols with intense research on aliphatic aminiums. However, the mechanisms of formation of aromatic aminiums in urban aerosols remain elusive. Highly time-resolved PM2.5 samples were collected in the center of Shanghai (China) during the winter to investigate the origin and formation of aminiums. The dominant aminium groups were aliphatic (mainly dimethylaminium and monomethylaminium). Anilinium was the third most abundant aminium. The concentrations of anilinium and total aminiums showed higher levels during the daytime and on weekdays. This finding combined with source apportionment analysis suggested that the daily and weekly scale variations of anthropogenic activities (e.g., traffic for commuting) were mainly responsible for the fluctuations in aminium concentrations (particularly aromatic aminiums). The acid dependence of aliphatic aminium formation was significantly stronger than that of aromatic aminium formation. Aliphatic and aromatic aminiums were significantly negatively and positively correlated with ozone, respectively, suggesting that the oxidative processes weakened the abundance of aliphatic aminiums but promoted the formation of aromatic aminiums. The molecular characterization of aromatic aminiums suggested that the atmospheric degradation of higher-molecular-weight aromatic amine compounds was an important mechanism for anilinium formation in urban aerosols. Thus, this study provides novel insights into the formation of aromatic aminiums.