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.

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Published in Environmental Science & Technology, v. 53, issue 21.

Copyright © 2019 American Chemical Society

This ACS article is provided to You under the terms of this ACS AuthorChoice/Editors’ Choice via Creative Commons Attribution Non-Commercial No Derivative Works agreement between You and the American Chemical Society (“ACS”).

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National Science Foundation awards 1903744 and 1255290.

NASA Earth and Space Science Fellowship (NESSF) Program.

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The Supporting Information is available free of charge on the ACS Publications website at https://doi.org/10.1021/acs.est.9b03742. It is also available for download as the additional file listed at the end of this record.

es9b03742_si_001.pdf (194 kB)
Additional experimental details on HO• monitoring; decay of FFA during the photolysis; and concentration of the hydroxyl radical trap