Atmospheric imidazole formation: Unraveling the roles of precursors, humidity, and aerosol acidity

To analyze the physicochemical properties and formation mechanisms of atmospheric imidazole-containing particles, a single-particle aerosol mass spectrometer (SPAMS) was employed to sample particulate matter in a major city in eastern China from December 19, 2018, to January 19, 2019. Throughout the monitoring period, particles containing carbonyls (183,952), amines and carbonyls (64,019), and imidazole (120,429) were collected. Four pollution episodes (E1-E4) and three clean periods (C1-C3) were identified based on increases in dominant precursor carbonyl particle concentrations. During pollution episodes, carbonyl-containing, amine-carbonyl mixed and imidazole-containing particles increased by 79.4 %, 43.3 %, and 61.9 % on average, respectively. Relative humidity between 50 % and 70 % and aerosol acidity in the range of 20–30 facilitated the conversion of carbonyl and amine/ammonium precursors into imidazole. Notably, glyoxal and methylglyoxal exhibited higher uptake rates under RH < 70 %. Precursor particles peaked during the daytime. Imidazole particle concentrations were elevated at night, reflecting reduced photolysis and contributions from secondary formation processes. Principal component analysis (PCA) identified four factors, explaining 81.0 % of the total variance. PC1 (41.0 %) revealed C₂HO₃⁻, NH₄HSO₄, and DEA as key precursors. Factor loading analysis across different periods revealed that imidazole formation is a multi-factor process influenced by precursor composition and environmental parameters.