Unlike most volatile organic compounds in Earth's atmosphere, pyruvic acid (an alpha-keto carboxylic acid) is destroyed mostly by photodissociation rather than oxidation by OH radicals. Although the photodissociation of pyruvic acid has been well studied in environmental chamber experiments, such approaches don't clearly delineate between primary photodissociation and secondary chemical reactions. We have explored the photodissociation of pyruvic acid excited to its S1 state at 351 nm using Time-Resolved Multiplexed Photoionization Mass Spectrometry. Our results support previous conclusions that decarboxylation, producing CO2 as a primary product, is the major product channel. The isomer-specific detection capabilities of this experiment provide the first direct evidence that the co-fragment of CO2 is in fact methylhydroxycarbene (CH3-C-OH). In contrast with the IUPAC recommendations for product branching yields, we find that CH3COH + CO2 represents 97-100% of the primary photochemical yield at 351 nm. For d1-pyruvic acid we see a small branching (3%) to the Norrish Type I product channel forming CH3CO + DOCO. This product channel is absent in undeuterated pyruvic acid. Coupled with calculations of the potential energy surfaces, we conclude that pyruvic acid decays at least partially by tunneling through an excited state hydrogen transfer barrier. Finally, we observe a bimolecular reaction of CH3COH, implying that this carbene will be stabilized and participate in chemical reactions with O2 and H2O in Earth's troposphere, and should be considered in atmospheric modeling.

Tunneling to a carbene: The photochemistry of pyruvic acid

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