Impact of pH and NaCl and CaCl₂Salts on the Speciation and Photochemistry of Pyruvic Acid in the Aqueous Phase

Recent studies have shown that pyruvic acid can produce higher-molecular-weight compounds upon irradiation in the aqueous phase. These compounds can contribute to the formation of secondary organic aerosols. There have been several previous studies on the effect of ionic strength on the photochemistry of pyruvic acid; however, few of them investigated the effects of marine relevant salts such as NaCl and CaCl2. In this study, we examine the effect of NaCl and CaCl2, namely, containing the coordinating cations Na+ and Ca2+, on the speciation, absorption properties, and photoreactivity of pyruvic acid in aqueous solutions of varying pH. NMR shows that both Ca2+ and Na+ further deprotonate pyruvic acid and decrease the diol to ketone ratio of pyruvic acid than in pure water at the same pH, especially at more acidic pH (pH less than 4). The absorption spectrum shows a strong red shift in the peak maxima for the n → π∗ transition of pyruvic acid in NaCl/CaCl2 solutions. This dependence is much more pronounced for divalent cations (Ca2+) compared to monovalent cations (Na+). Vertical excitation energy calculations of the anionic ketone form of pyruvic acid confirm the same red shift on the n → π∗ transition peak in the presence of Ca2+. In addition, the presence of NaCl/CaCl2 suppresses the photolysis rate of pyruvic acid, which could be due to the deprotonation of pyruvic acid by the cations and the lower photochemical reactivity for pyruvate, the deprotonated form.