Abstract

The variable composition of secondary organic aerosols (SOA) contributes to the large uncertainty for predicting radiative forcing. A better understanding of the reaction mechanisms leading to aerosol formation such as for the photochemical reaction of aqueous pyruvic acid (PA) at λ ≥ 305 nm can contribute to constrain these uncertainties. Herein, the photochemistry of aqueous PA (5-300 mM) continuously sparged with air is re-examined in the laboratory under comparable irradiance at 38° N at noon on a summer day. Several analytical methods are employed to monitor the time series of the reaction, including (1) the derivatization of carbonyl (C═O) functional groups with 2,4-dinitrophenylhydrazine (DNPH), (2) the separation of photoproducts by ultrahigh pressure liquid chromatography (UHPLC) and ion chromatography (IC) coupled to mass spectrometry (MS), (3) high resolution MS, (4) the assignment of (1)H NMR and (13)C gCOSY spectroscopic features, and (5) quantitative (1)H NMR. The primary photoproducts are 2,3-dimethyltartaric acid and unstable 2-(1-carboxy-1-hydroxyethoxy)-2-methyl-3-oxobutanoic acid, a polyfunctional β-ketocarboxylic acid with eight carbons (C8) that quickly decarboxylates into 2-hydroxy-2-((3-oxobutan-2-yl)oxy)propanoic acid. Kinetic isotope effect studies performed for the first time for this system reveal the existence of tunneling during the initial loss of PA. Thus, the KIEs support a mechanism initiated by photoinduced proton coupled electron transfer (PCET). Measured reaction rates at variable initial [PA]0 were used to calculate the sum of the quantum yields for the products, which displays a hyperbolic dependence: ∑Φproduct = 1.99 [PA]0/(113.2 + [PA]0). The fast photochemical loss of aqueous PA with an estimated lifetime of 21.7 min is interpreted as a significant atmospheric sink for this species. The complexity of these aqueous phase pathways indicates that the solar photochemistry of an abundant α-ketocarboxylic acid can activate chemical processes for SOA formation.

Document Type

Article

Publication Date

3-31-2017

Notes/Citation Information

Published in The Journal of Physical Chemistry A, v. 121, no. 15, p. 2924-2935.

Copyright © 2017 American Chemical Society

ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Digital Object Identifier (DOI)

https://doi.org/10.1021/acs.jpca.6b11916

Funding Information

M.I.G. thanks research funding from the National Science foundation under NSF CAREER award CHE-1255290. A.J.E. acknowledges support by the NASA Earth and Space Science Fellowship (NESSF) Program.

Related Content

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b11916.

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Additional experimental details, additional figures including mass spectra, UHPLC-MS, and NMR, and data tables