Keto-acids such as glyoxylic acid (GA) and pyruvic acid (PA) are ubiquitous in the atmosphere as a result of direct emissions and oxidation of volatile organic compounds in the gas, aerosol, and aqueous phases. The dicarbonyl chromophore of these molecules is spectroscopically active at wavelengths prevalent in the troposphere (λ &gt; 290 nm) making photochemistry a major aspect of their atmospheric processing. Consequently, keto-acid photochemistry has been indicated as an important contributor to photochemical formation of secondary organic aerosol in both urban and remote environments. However, uncertainty remains in the multiphase photochemical mechanism extending from the gas phase to aqueous aerosols and liquid cloud water. In the gas phase, the major photochemical pathways for GA and PA are two decarboxylation reactions: β-hydrogen transfer followed by CO2 loss and direct C-C bond cleavage (Norrish Type I reaction), and the presence of a single water molecule can significantly alter the transition state barriers associated with each reaction. For instance, water can directly mediate the hydrogen transfer reaction and enhance the relative rate of this pathway. Although, direct photolysis pathways will be slower in cluster or bulk conditions than in the gas-phase which increases the likelihood of secondary reactions of the photo-excited molecules. Indeed, experimental results for both species indicate intermolecular hydrogen transfer and oligomerization reactions in T1 to be important in the condensed phase. In this presentation, the photochemistry of GA and PA is explored in the near UV in both the singlet (S1/ S0) and triplet (T1) manifolds using ground and excited state density functional theory and transition state theory kinetics calculations. An overall mechanistic picture of the atmospheric photochemistry in the gas-phase will be presented by comparing the computational results and recent experimental findings. In addition, the impact of water on the photochemistry is also explored using both explicit solvent molecules and an implicit solvent model to better understand the photochemistry of keto-acids in aqueous aerosols and cloud water.

Photochemistry of keto-acids in the atmosphere from the gas phase to aqueous aerosols to liquid water

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