Formation and photochemical investigation of brown carbon by hydroxyacetone reactions with glycine and ammonium sulfate

Abstract

Increasing attention has been paid to atmospheric “brown carbon” (BrC) aerosols due to their effect on the earth's climate. Aqueous BrC aerosols were formed through aqueous reactions of hydroxyacetone (HA) with nitrogen compounds such as glycine (Gly) and/or ammonium sulfate (AS). When exposed to nitrogen compounds for several days, HA, as a type of aqueous carbonyl compound, becomes absorbent and fluorescent in the blue visible and near ultraviolet regions, which have been monitored by UV/vis and fluorescence spectroscopy. In this study, we quantified absorption and excitation-emission matrix (EEM) spectra in the formation of aqueous BrCs, which was generated from HA-Gly and HA-Gly-AS mixtures, respectively. The obtained data was used to determine the base-10 absorption coefficient (α), absorption Angstrom exponent (AAE), and effective quantum yield (QY). All of the related parameters provide further evidence for the formation of aqueous BrC. The absorbances of the as-obtained BrCs follow the order HA-Gly-AS > HA-Gly > HA-AS. In other words, HA-Gly-AS mixtures displayed the most intense absorbances, whereas HA-AS mixtures barely produced visible absorbance. It is reasonable to speculate that Gly promotes the formation of HA-Gly BrC mixtures. The experimental results are consistent with previous measurements reported by Powelson et al. BrCs from HA-Gly-AS and HA-Gly exhibit stronger fluorescence between 300 and 400 nm. Glycine plays a more important role in the formation of aqueous BrC than that of AS. Furthermore, we examined the mass absorption coefficient (MAC) by photolysis of aqueous BrCs, which resulted from the oxidation of HA-Gly and HA-Gly-AS mixtures by 5 mM H₂O₂. An effective photolysis time induced significant changes near-UV (300–400 nm) absorption intensity of HA-Gly and HA-Gly-AS mixtures. These results emphasize the dynamic nature of the corresponding atmospheric aqueous BrC. Overall, our study provides the optical properties of the corresponding atmospheric aqueous BrC and the H₂O₂ oxidation photolysis process of the as-obtained BrC in detail, which may contribute to the understanding of the important effects of aqueous BrC for atmospheric chemistry and climate.