Experimental study on the imidazoles of aqueous secondary organic aerosol formed from reaction of methylglyoxal and ammonium sulphate

Imidazole and its derivatives are the main heterocyclic nitrogen-containing brown carbon constituents in aerosol particles. Imidazoles of aqueous secondary organic aerosol (aqSOA) particles formed from the reaction of methylglyoxal and ammonium sulphate ((NH4)2SO4) are detected by vacuum ultraviolet photoionization aerosol mass spectrometer (VUV-PIMS) in real-time and verified off-line with optical spectrometry in current study. The experimental results showed that the single photon ionization mass spectrum of aqSOA particles measured by VUV-PIMS emerged the molecular ion peaks at m/z = 68, 82 and 110. While, the extraction solution of aqSOA particles detected by off-line techniques demonstrated that the characteristic peaks at 217 and 282 nm appeared in the UV–Vis spectrum; the strong emission in 405–480 nm with maximum emission wavelength at 435 nm contained in the 3D fluorescence spectra; and the chemical shift peaks of hydrogen atoms on N–H, CC and CN bond on imidazole ring and methyl group arised in the 1H-NMR spectrum. In view of these spectral information, 4-methyl-imidazole and 4-methyl-imidazole-2-carbaldehyde are determined as the principal products of the aqueous reaction between methylglyoxal and (NH4)2SO4. The possible reaction mechanism leading to the generation of imidazole compounds are also discussed. These will help to understand the formation process of imidazole compounds in atmospheric aerosol particles, and provide experimental basis for the model mechanism study of aqSOA particles.