Soot, also known as black carbon (BC) or elemental carbon (EC) (Buseck et al., 2014), is deeply concerning because it has a multitude of effects on the global atmosphere and Earth's surface. It is the second largest contribution to warm the atmosphere after carbon dioxide (Jacobson, 2001). Soot particles in the atmosphere exhibit complexity in size, composition, morphology, and mixing structure, which results in the high uncertainties in

As a potential fraction of brown carbon, particulate imidazole compounds may initiate photosensitive reactions and have substantial radiative effects. However, our knowledge of the atmospheric processing of imidazole compounds is still in its nascent stage. On the basis of a single-particle aerosol mass spectrometer measurement, the mixing state of imidazole-containing particles and high-time-resolved variations of imidazole compounds were investigated in Qingdao, China, in November and December 2019. Five imidazole compounds (methylimidazole, ethylimidazole, dimethylimidazole, imidazole-2-carboxaldehyde, and 2,2′-biimidazole) were identified, overall accounting for ∼10% of all of the detected particles. They are tightly correlated and internally mixed with enhanced carbonyls, amines, and ammonium, supporting their secondary formation from these precursors. The number fraction of imidazole-containing particles exhibited predominant diurnal variations, especially on sunny days. A sharp decrease in the number fraction from morning to noon is most likely attributed to photochemical degradation. This is also confirmed by the reverse correlation (r = −0.77; p < 0.01) with photochemical indicators (temperature and O3) and our laboratory experiment by exposure of imidazole compounds to sunlight. Multiple linear regression and random forest analysis further support the hypothesis, with precursors (i.e., carbonyls and amines/ammonium) and O3 being the most important factors (∼70%) regulating the variations of imidazole compounds.