Aqueous Phase Oligomerization of Methyl Vinyl Ketone by Atmospheric Radical Reactions

Aqueous phase oxidation reactions in atmospheric particles can yield high molecular weight products and create secondary organic aerosol (SOA) upon droplet evaporation. Oxidation by hydroxyl radicals to create oligomers in solution that form SOA has been previously investigated; however, mixed organic solutions that can initiate radical chemistry have been largely overlooked. In aqueous solution, pyruvic acid (PA), an α-keto acid found in both the gas and aqueous phases in the atmosphere, photolyzes via a radical mechanism. Here, we use this photochemistry of pyruvic acid to trigger oligomerization of methyl vinyl ketone (MVK), an α,β-unsaturated compound generated by the atmospheric oxidation of isoprene. We closely compare the reaction products and mechanism to a recent work in which the radical oligomerization of MVK initiated by hydroxyl radical is studied in depth. Using mass spectrometry, it is shown that the two reactions create oligomers of similar molecular weights, up to m/z 1200 for initial MVK concentrations of 20 mM. In the MVK and PA photolysis, exploring initial reactant concentrations demonstrates that the same oligomer series are produced regardless of the initial reactant or dissolved oxygen concentrations. However, the size of the oligomers formed increases with increasing initial reactant concentrations, and the oligomerization process is slowed when dissolved oxygen is present. Finally, using a Langmuir trough, that measures the surface tension as a function of liquid surface area, it is shown that these oligomer photoproducts are surface active. These results indicate the importance of mixed organic systems to understanding secondary organic aerosol formation and growth. Consequently, this chemistry may affect gas–particle mass transfer of water and semivolatile aerosol components and, therefore, the way that aerosol interacts with its environment.