Kinetics and Mechanism of OH Oxidation of Small Organic Dicarboxylic Acids in Ice: Comparison to Behavior in Aqueous Solution

To better understand the oxidation of organic species by OH within frozen solutions, experiments were conducted with a simplified model system consisting of succinic acid (2.5, 5, and 10 mM), using H2O2 (30 rnM) as a condensed-phase OH precursor. Frozen (-20 degrees C) and aqueous (0 degrees C and room temperature) solutions were irradiated using a xenon arc lamp to study the differences between frozen and nonfrozen reaction media. Formation of products and decay of reactants were measured using ion chromatography (IC) and gas chromatography-mass spectrometry (GC-MS). Malonic acid was observed as the dominant product, with malic acid formed in significantly smaller amounts, in both frozen and unfrozen substrates, suggesting a common mechanism independent of phase. Notably, a large decrease was observed in the succinic acid first-order decay rate constant when moving from aqueous to frozen samples. This is due not to temperature-dependent second-order kinetics or different OH production rates between the samples, but instead probably arises from physical separation of the succinic acid and H2O2 upon freezing or precipitation of succinic acid upon freeze concentration. The effect is not nearly as pronounced for the decay kinetics of a much more soluble species, namely, malonic acid. From an environmental perspective, this work is the first experimental demonstration that dicarboxylic acids present in the cryosphere might be subject to photochemical degradation into smaller organic acids and carbon dioxide.