Catalysis of Ferric Ions and Ferrous Ions on the Reaction of Methylglyoxal and Ammonium Sulfate

Abstract

Secondary brown carbon is an important component of the atmosphere. The contribution of secondary atmospheric brown carbon from the reaction of methylglyoxal (MG) with ammonium sulfate (AS) has been demonstrated in the laboratory. However, the mechanism of the reaction is still unclear, especially influenced by other factors, for example, inorganic salt. In this study, the effect of soluble iron salt on the reaction between MG and AS was investigated through chemical analysis methods, including optical molecular probes. The results indicate that iron sulfate and ferrous sulfate may promote the MG–AS reaction and accelerate the formation of brown carbon. Ferrous sulfate exhibits a higher catalytic activity than ferric sulfate does. Moreover, the reaction mechanism indicates that Fe(II) is the true catalyst, with a rate constant 13 times higher than that of the uncatalyzed reaction, while the catalytic effect of Fe(III) is due to its reduction to Fe(II) by MG. In addition, the complex of Fe(II) and MG in the aqueous phase was identified.