Aqueous-phase oligomerization of methyl vinyl ketone through photooxidation – Part 2: Development of the chemical mechanism and atmospheric implications
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.