Photooxidation of dicarboxylic acids—Part II: Kinetics, intermediates and field observations

Among C2–C9 dicarboxylic acids (DCAs), succinic acid (C4 DCA) exhibits the lowest photooxidation rate, while oxalic acid has the highest photooxidation rate in a liquid-phase reaction system. Twenty-seven identified intermediates are classified into six classes: secondary DCAs, oxo-DCAs, methyl-DCAs, hydroxy-DCAs, hydroxy-monocarboxylic acids (hydroxy-MCAs), and oxo-MCAs. Concentration profiles of the identified intermediates affirm two hypotheses: (1) longer DCAs can be the precursors of shorter DCAs and (2) succinic acid can be oxidized to malonic acid with malic acid as an intermediate. Ambient DCAs can also be precursors of substituted DCAs and MCAs; methyl-, oxo-, and hydroxy-substituents consistently position at the center carbon of the base DCA molecules, while hydroxy- and oxo-substituents favor the ω or (ω−1) position of the base MCAs. A comparison of field observations with the experimental data obtained in this study indicates that oxo-DCAs, having a higher concentration than methyl- and hydroxy-DCAs in ambient particulates, may characterize an atmosphere under prevailing photooxidation with little influence of anthropogenic pollution.