Measurement report: Occurrence of aminiums in PM _2.5 during winter in China – aminium outbreak during polluted episodes and potential constraints

Abstract. Amines and aminiums play an important role in particle formation, liquid-phase reactions, and climate change and have attracted considerable attention over the years. Here, we investigated the concentrations and compositions of aminiums in PM2.5 in 11 Chinese cities during the winter, focusing on the characteristics of aminiums during the polluted days and the key factors influencing aminium outbreak. Monomethylaminium was the dominant aminium species in most cities, except Taiyuan and Guangzhou, followed by dimethylaminium. Diethylaminium dominated the total aminiums in Taiyuan and Guangzhou. Thus, the main amine sources in Taiyuan and Guangzhou were significantly different from those in other cities. The concentrations of the total aminiums (TAs) in most cities increased significantly during the polluted days, while relatively weak aminium outbreaks during the polluted days occurred in Xi'an and Beijing. Additionally, the concentrations of TAs in Xi'an and Beijing were insignificantly correlated with those of PM2.5 and the major acidic aerosol components, while the opposite pattern was observed in nine other cities. Thus, acid–base chemistry was significantly associated with the formation of aminiums in PM2.5 in all cities, except Xi'an and Beijing. Based on the sensitivity analysis of the aminiums : ammonium ratio to ammonium changes, as well as excluding the effects of relative humidity and atmospheric oxidation, we proposed the possibility of the competitive uptake of ammonia versus amines on acidic aerosols or the displacement of aminiums by ammonia in Xi'an and Beijing (constraining aminium outbreaks). Overall, this study deepens the understanding of the spatiotemporal differences in aminium characteristic and formation in China. However, the uptake of amines on particles to form aminiums and the relevant influencing factors require further mechanistic research.