Aqueous-phase chemistry of glyoxal with multifunctional reduced nitrogen compound: A potential missing route of secondary brown carbon

Abstract. Aqueous-phase chemistry of glyoxal (GL) with reduced nitrogen compounds (RNCs) plays a significant source of secondary brown carbon (SBrC), which is one of the largest uncertainties in climate predictions. However, few studies have revealed that SBrC formation is affected by multifunctional RNCs, which has a non-negligible atmospheric abundance. Hence, we performed theoretical and experimental approaches to investigate the reaction mechanisms and kinetics of the mixtures for ammonium sulfate (AS), multifunctional amine (monoethanolamine, MEA) and GL. Our experiments indicate that the light-absorption and the growth rate are enhanced in MEA-GL mixture relative to AS-GL and MEA-AS-GL mixtures, and MEA reactions of the chromophores by more efficiently than the analogous AS reactions. Quantum chemical calculations show that the formation and propagation of oligomers proceed via four-step nucleophilic addition reactions in three reaction systems. The presence of MEA provides the extra two branched chains to affect the natural charges and steric hindrance of intermediates, facilitated the formation of chromophores. Molecule dynamics simulations reveal that the interfacial and interior attraction on the aqueous aerosols with MEA is more pronounced for small α-dicarbonyls, to facilitate the further engagement in the aqueous-phase reactions. Our results show a possible missing source for SBrC formation on urban, regional and global scales.