Radical mechanisms of methyl vinyl ketone oligomerization through aqueous phase OH-oxidation: on the paradoxical role of dissolved molecular oxygen

Abstract

It is now accepted that one of the important pathways of secondary organic aerosol (SOA) formation occurs through aqueous phase chemistry in the atmosphere. However, the chemical mechanisms leading to macromolecules are still not well understood. It was recently shown that oligomer production by OH radical oxidation in the aerosol aqueous phase from α-dicarbonyl precursors, such as methylglyoxal and glyoxal, is irreversible and fast. <br><br> Methyl vinyl ketone (MVK) was chosen in the present study as it is an α,β-unsaturated carbonyl that can undergo radical oligomerization in the aerosol aqueous phase. We present here experiments on the aqueous phase OH-oxidation of MVK, performed under various conditions. Using NMR and UV absorption spectroscopy, high and ultra-high resolution mass spectrometry, we show that the fast formation of oligomers up to 1800 Da is due to radical oligomerization of MVK, and 13 series of oligomers (out of a total of 26 series) are identified. The influence of atmospherically relevant parameters such as temperature, initial concentrations of MVK and dissolved oxygen are presented and discussed. In agreement with the experimental observations, we propose a chemical mechanism of OH-oxidation of MVK in the aqueous phase that proceeds via radical oligomerization of MVK on the olefin part of the molecule. This mechanism highlights in our experiments the paradoxical role of dissolved O₂: while it inhibits oligomerization reactions, it contributes to produce oligomerization initiator radicals, which rapidly consume O₂, thus leading to the dominance of oligomerization reactions after several minutes of reaction. These processes, together with the large range of initial concentrations investigated show the fundamental role that radical oligomerization processes likely play in polluted fogs and atmospheric aerosol

Structure–activity relationship for the estimation of OH-oxidation rate constants of carbonyl compounds in the aqueous phase

“…It is believed that these processes need the presence of water. In the aqueous phase, it has been recently suggested that radical polymerization, initiated by OH-oxidation of partially (or fully) hydrated carbonyl compounds, are responsible for the formation of oligomers (Lim et al, 2010;Tan et al, 2012;Renard et al, 2013). While the database of experimental kinetic parameters relevant to the atmospheric aqueous phase is still limited, the number of compounds which can be potentially involved in such processes is tremendous.…”

Key parameters controlling OH‐initiated formation of secondary organic aerosol in the aqueous phase (aqSOA)

“…However, observed SOA properties such as the average oxygenation level (oxygen‐to‐carbon ratio) and specific molecular composition (e.g., dicarboxylic acids and oligomers) cannot be explained by such models and might imply missing SOA sources and processing mechanisms. Many recent laboratory, model, and field studies suggest that water‐soluble gases undergo chemical reactions in the aqueous phase of cloud droplets and aerosol particles that efficiently produce compounds that remain in the particle phase upon water evaporation (aqSOA) [e.g., Loeffler et al , ; Sareen et al , ; Lim et al , ; Ervens et al , ; Galloway et al , ; Nguyen et al , ; Renard et al , ]. These products include highly oxygenated and high molecular weight species that have been identified in aerosol particles and whose atmospheric abundance is not captured by gas‐phase chemistry models.…”

Oligomer formation in the troposphere: from experimental knowledge to 3-D modeling

“…First of all, the fate of isoprene in the condensed phase shall be examined, as isoprene has been recognized as a major SOA precursor through its first generation products methacrolein (MACR) and methyl vinyl ketone (MVK) (Pandis et al, 1991;Carlton et al, 2009), which were found to be important oligomer precursors in the condensed phase (El Haddad et al, 2009;Liu et al, 2012b;Renard et al, 2013). On this specific point, the study of Renard et al (2015), based on the photo-oxidation of MVK into a photo-reactor, revealed that considering only a first-order rate constant to represent the formation of oligomers is not appropriate, as the oxidation of MVK by OH was (in the condensed phase of the deliquescent aerosol) governed by a kinetic competition between functionalization and oligomerization, which depends on the precursor initial concentration. Furthermore, the branching ratio in favor of highly oxidized monomers seems to be more important in the condensed phase than in the gas phase (Kroll and Seinfeld, 2008), thus favoring the formation of a stable OA.…”

“…Furthermore, the branching ratio in favor of highly oxidized monomers seems to be more important in the condensed phase than in the gas phase (Kroll and Seinfeld, 2008), thus favoring the formation of a stable OA. A multiphase box model study conducted by Ervens et al (2015) based on the laboratory experiments of Renard et al (2015) underlined a potential key role of the MVK-to-oxygen concentration ratio in the oligomerization rate under atmospherically relevant conditions. Thus, in the case of isoprene, the formation of oligomers via the oxidation by OH (aq) in the condensed phase may well be represented by a kinetic approach based on a second-order rate constant.…”