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While laboratory studies have demonstrated that aqueous reactions between carbonyls and reduced nitrogen species may contribute to the production of N-heterocycle brown carbon (BrC) such as imidazole, there is currently a lack of evidence for this in the atmosphere. We investigated the mixing state of carbonyls, ammonium, amines, and imidazole (as a surrogate of BrC) in cloud residual, interstitial, and cloud-free particles by single-particle mass spectrometry. The results provide the first ambient evidence of the formation of imidazole through reactions between carbonyls and ammonium/amines at the individual particle level. The key evidence for this is that 60% of the imidazole particles are internally mixed with carbonyls and ammonium/amines. The number fraction of imidazole is significantly enhanced in particles with internally mixed carbonyls and ammonium (7.8%)/amines (26.7%), compared with that (1.4%) in all of the cloud-free particles. Furthermore, a higher number fraction of imidazole is observed in all cloud residual and interstitial particles (2.9%) than in the cloud-free particles (1.4%). This is due to the enhancement of amines and/or the synergistic effect of ammonium and amines in the formation of imidazole in cloud residual and interstitial particles. These findings extend the current understanding of the formation and evolution of imidazole-based BrC.

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