Comprehensive spectral analysis of reaction of three aldehydes with ammonium sulfate and glycine

Abstract

Light-absorbing brown carbon (BrC), a major contributor to global climate change, is widely studied. Comparative analysis showed that methylglyoxal (MG) tended to form a greater amount of aqueous BrC with AS/Gly/AS-Gly in a relatively short time, whereas 1,4-dioxane-2,5-diol (DD) tended to form darker BrC in the same reaction time. The absorption Ångström exponent (Åabs) over 250–400 nm and 400–480 nm in the absorption spectra combined with the index of recent autochthonous contribution (BIX), fluorescence index (FI), and humification index (HIX) in the fluorescence spectra, provided more information on the source of aqueous BrC. The absorbance intensity order and the fluorescence intensity order were MG-AS-Gly > DD-AS-Gly > GX-AS-Gly and GX-AS-Gly > MG-AS-Gly > DD-AS-Gly, respectively. The reaction rate (K) of H2O2 oxidation and photodecomposition gradually increased from ultraviolet (UV) to visible (Vis) region. The short lifetime of aldehyde-amine mixtures indicates that imine BrC is photochemically unstable and easily dissolves under sunlight. The absorbance and light decomposition rates were lowest in the GX-AS-Gly reaction system, indicating that the small amount of BrC formed by this system was more stable. The unsaturated double bonds gradually lengthened or shortened in the Maillard-type browning or aldol condensation reactions to form different numbers of product molecules. Furthermore, as these BrC reactions are strongly pH-dependent, it is necessary to measure (or control) the pH in the specific BrC system. Extensive laboratory spectroscopic comparative studies on these aqueous BrCs can be used to better understand the evolution of atmospheric BrC and its effects on climate.