A New Potential Atmospheric Accretion Mechanism: Acid-Catalyzed Hetero-Michael Addition Reactions

The mechanistic basis for the formation of low volatility secondary organic aerosol (SOA) commonly invokes the formation of larger molecules from two or more smaller molecules, which are generically termed as accretion reactions. The previously unconsidered acid-catalyzed hetero-Michael addition reaction (ACHMAR) is shown to be a potential accretion mechanism route for atmospherically relevant α,β unsaturated carbonyls and alcohols, which are both common atmospheric constituents. The kinetics of ACHMARs for a series of carbonyls and alcohols were measured with bulk aqueous phase experiments using nuclear magnetic resonance (NMR) spectroscopy. The rates of these reactions are shown to be very sensitive to the structures of both the carbonyl (only ketones showed measured reactivity) and the alcohols, with both the less substituted carbonyl and alcohol species exhibiting faster rates of reaction. For example, these results suggest that the major isoprene primary oxidation product methyl vinyl ketone could undergo ACHMARs, while isoprene’s other major oxidation product, methacrolein, is not expected to be similarly reactive. Overall, these results indicate that ACHMARs involving atmospheric α,β ketones are a plausible accretion mechanism in SOA.