Abstract
Abstract. Laboratory experiments of efficient oligomerization from methyl vinyl ketone (MVK) in the bulk aqueous phase were simulated in a box model. Kinetic data are applied (if known) or fitted to the observed MVK decay and oligomer mass increase. Upon model sensitivity studies, in which unconstrained rate constants were varied over several orders of magnitude, a set of reaction parameters was found that could reproduce laboratory data over a wide range of experimental conditions. This mechanism is the first that comprehensively describes such radical-initiated oligomer formation. This mechanism was implemented into a multiphase box model that simulates secondary organic aerosol (SOA) formation from isoprene, as a precursor of MVK and methacrolein (MACR) in the aqueous and gas phases. While in laboratory experiments oxygen limitation might occur and lead to accelerated oligomer formation, such conditions are likely not met in the atmosphere. The comparison of predicted oligomer formation shows that MVK and MACR likely do negligibly contribute to total SOA as their solubilities are low and even reduced in aerosol water due to ionic strength effects (Setchenov coefficients). Significant contribution by oligomers to total SOA might only occur if a substantial fraction of particulate carbon acts as oligomer precursors and/or if oxygen solubility in aerosol water is strongly reduced due to salting-out effects.
Highlights
Organic aerosol particles in the atmosphere comprise about 50 % of the total particulate matter mass (Zhang et al, 2007)
We have derived a comprehensive chemical mechanism of the oligomerization of methyl vinyl ketone (MVK) in the aqueous phase, based on bulk aqueous-phase laboratory studies that are described in previous work (Renard et al, 2013, 2015, Part I)
The oligomerization rates for high and low aqueous-phase concentrations of oxygen, respectively, can be reproduced by the model. This branching of reaction pathways occurs because alkyl radicals that are formed by OH oxidation of MVK can react either with oxygen forming peroxy radicals or with another MVK molecule, which leads to oligomers
Summary
Organic aerosol particles in the atmosphere comprise about 50 % of the total particulate matter mass (Zhang et al, 2007). SOA formation from such products (termed “gasSOA” by Ervens et al (2011) since the chemical reactions leading to condensable species occur in the gas phase) is often described by the two-product model (Odum et al, 1996) or, more recently, by the volatility basis set (VBS) (e.g., Donahue et al, 2006, 2011; Trump and Donahue, 2014) While this concept can explain a large amount of observed ambient SOA mass, specific SOA properties (e.g., high oxygen-to-carbon (O / C) ratio) and individual compounds (e.g., dicarboxylic acids, oligomers) cannot be predicted. The question is explored under what atmospheric conditions aqSOA formation by oligomerization might be of importance as an efficient SOA source For this estimate, we include similar reaction patterns in the aqueous phase for MACR as for MVK and imply the existence of additional oligomer precursors
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