Session: Aquatic Photochemistry:

Oxaloacetic acid (OAA) is present in atmospheric environment, and its low vapor pressure and high Henry constant make it a good representative molecule for secondary organic aerosol (SOA) found in aqueous phase of atmospheric particles. OAA is known to undergo a spontaneous decarboxylation reaction, which can be catalyzed by ammonium. Understanding the decarboxylation of OAA both with and without catalysts is crucial to studying the formation and aging of SOA. Using UV-Vis spectroscopy, the absorption spectra of OAA in pure water and in ammonium sulfate solution were obtained, from which the first-order rate constant and lifetime of OAA in both decarboxylation reactions were calculated. In order to further analyze and validate the experimental result, electronic structure calculations with density functional theory (DFT) were performed to generate the energy diagrams of both uncatalyzed and catalyzed decarboxylation of OAA. The main goals of these calculations were to establish the mechanism for decarboxylation using resource-efficient methodology, rather than quantitatively predict the rate constants. The DFT calculations suggested that the activation energy of catalyzed decarboxylation is significantly lower than that of the uncatalyzed decarboxylation, which supports experimental observations. This combined experimental-theoretical framework can be extended to study other 3-oxocarboxylic acids to help better understand the fate of carboxylic acids in atmospheric particles.

Sergey Nizkorodov

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