Experimental Study On The Reaction Of Pyruvaldehyde With Reductive Nitrogen To Generate Imidazoles

Imidazole and its derivatives are the main heterocyclic nitrogen-containing organic brown carbon components in atmospheric aerosol particles.They have a strong light absorption ability,which can reduce atmospheric visibility and endanger human health.Their component identification and formation process have become the focus of research in the field of environmental organic chemistry.Dicarbonyl compounds such as glyoxal and pyruvaldehyde produced by photooxidation of volatile organic compounds can be transferred to liquid aerosols,water droplets,clouds,and other aqueous phases through gas-liquid partitioning,and can interact with reducing nitrogen compounds（inorganic ammonium,organic amine,amino acid）to form imidazoles.In this paper,the aqueous reaction device was built,the reaction solution was atomized by the TSI 9302 atomizer,and the aqueous secondary organic aerosol（SOA）particles were generated after passing through the diffusion drying tube.The imidazole components of the aqueous SOA produced by the reaction of pyruvaldehyde and the reducing nitrogen species were characterized by off-line and online optical and mass spectrometry,and the effects of water-soluble anions,different p H,and short-chain alcohols on the formation of imidazoles were investigated.The obtained experimental results are as follows:The extraction of aqueous SOA formed from the reaction of pyruvaldehyde and ammonium sulfate was measured by off-line and online techniques.There were characteristic absorption peaks at 217 and 282 nm in the ultraviolet-visible absorption spectrum;the infrared spectrum showed the stretching vibration peaks of the C-N bond and C=N bond;the~1H-NMR resonance spectroscopy contained the chemical shift peaks of hydrogen atoms on the C=C bond,N-H bond and C-N bond on the imidazole ring;and the synchrotron radiation single-photon ionization mass spectrum presented the characteristic mass spectrum peak of imidazole compounds at m/z=68.Based on these spectral pieces of information,methyl-imidazole and methyl-imidazole-2-methyl ketone were identified as the main products of the reaction between pyruvaldehyde and ammonium sulfate.The water-soluble anions such as CO₃^2-,F^-,NO₃^-,SO₄^2-and Cl^-in the aqueous phase would promote the reaction of pyruvaldehyde and ammonium ions to produce imidazole products,resulting in the increasing of the averaged mass absorption coefficient（）of aqueous SOA in the range of200-600 nm,and the order of promotion was:F^->CO₃^2->NO₃^->SO₄^2->Cl^-.The extraction of aqueous SOA produced by the reaction of pyruvaldehyde and methylamine had fluorescence characteristics,with the characteristic absorption at 220 nm and 290 nm,and emitted fluorescence at 472 nm,its infrared spectrum contained the stretching vibration peaks of the C-N bond and C=N bond;and the spray ionization positive ion mass spectrum appeared the related mass peaks of imidazoles at m/z=154,125,111.The laser desorption/ionization mass spectra of aqueous SOA particles measured by online technology showed the characteristic mass peaks of imidazoles at m/z=28（CH₂N⁺）,m/z=41（C₂H₃N⁺）and m/z=67（C₃H₃N₂⁺）.Combining these spectral pieces of information,1,2,3,5-tetramethyl-imidazole and 1,3,5-trimethyl-imidazole-2-methyl ketone were obtained as the main products of pyruvaldehyde and methylamine.When there were organic amines with different types and numbers of functional groups in the reaction system,the functional group substituents affected the reactivity and steric effect of the nucleophilic attack of the organic amine molecules,thus affecting the formation of imidazole products.The order of the content of imidazole compounds produced by the reaction of different organic amines with pyruvaldehyde was:methylamine>ethylamine>dimethylamine>trimethylamine.Under different acid-base conditions,due to the influence of the solvent effect,the strong acid（p H=3）condition could inhibit the production of imidazole compounds,while the strong alkaline（p H=11）condition was conducive to the formation of imidazole products.The UV-visible absorption spectrum of the extraction of aqueous SOA produced by the reaction of pyruvaldehyde and glycine had characteristic absorption peaks at 220 nm and 280nm;the infrared spectrum showed the stretching vibration peaks of the C-N bond and C=N bond;and the electrospray ionization positive ions mass spectrum contained the characteristic mass peaks of imidazoles at m/z=141,183 and 213.Combining these spectral pieces of information,1,3-Bis（carboxymethyl）-2,4-dimethyl-imidazole was identified as the main product of the reaction between pyruvaldehyde and glycine.In the presence of short-chain alcohols in the reaction system,the alcohol molecules were surrounded by multiple water molecules through hydrogen bonds,forming a“cage”like an aggregate of alcohol-in-water.The aggregate could promote the hydration reaction of pyruvaldehyde and the nucleophilic attack reaction of glycine,thereby facilitating the formation of 1,3-bis（carboxymethyl）-2,4-dimethyl-imidazole.The order of promotion strength of short-chain alcohols to the formation of 1,3-Bis（carboxymethyl）-2,4-dimethyl-imidazole was:isopropanol>propanol>ethanol>methanol.These experimental results would help to clarify the formation process of atmospheric imidazole compounds,analyze the source of imidazole brown carbon,and provide an experimental basis for the study of the model mechanism of aqueous secondary organic aerosol.