Production of Singlet Oxygen (¹O₂) during the Photochemistry of Aqueous Pyruvic Acid: The Effects of pH and Photon Flux under Steady-State O₂(aq) Concentration

The photochemistry of pyruvic acid (PA) in aqueous atmospheric particles contributes to the production of secondary organic aerosols. This work investigates the fate of ketyl and acetyl radicals produced during the photolysis (λ ≥ 305 nm) of 5-100 mM PA under steady state [O₂(aq)] = 260 μM (1.0 ≤ pH ≤ 4.5) for photon fluxes between 1 and 10 suns. The radicals diffuse quickly into the water/air interface of microbubbles and react with dissolved O₂ to produce singlet oxygen (¹O2∗). Furfuryl alcohol is used to trap and bracket the steady-state production of 2 × 10^-12 ≤ [¹O2∗] ≤ 1 × 10^-11 M. Ion chromatography mass spectrometry shows that 2,3-dimethyltartaric acid (DMTA), 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic acid (oxo-C₇ product), and 2-(1-carboxy-1-hydroxyethoxy)-2-methyl-3-oxobutanoic acid (oxo-C₈ product) are formed under all conditions investigated. The sigmoidal dependence of initial reaction rates with pH resembles the dissociation curve of PA. For increasing photon fluxes, the branching ratio of products shifts away from the radical recombination that favors DMTA toward multistep radical chemistry forming more complex oxocarboxylic acids (oxo-C₇ + oxo-C₈). The large steady-state production of ¹O₂ indicates that PA in aerosols can be a significant source of atmospheric oxidants on par with natural organic matter.