Study of the kinetics and equilibria of the oligomerization reactions of 2-methylglyceric acid
The presence of a variety of chemical species related to the gaseous precursor isoprene in ambient secondary organic aerosol (SOA) has stimulated investigations of the nature of SOA-phase chemical processing. Recent work has demonstrated that 2-methylglyceric acid (2-MG) is an important isoprene-derived ambient SOA component and atmospheric chamber experiments have suggested that 2-MG may exist in oligomeric form (as oligoesters) under conditions of low SOA water content. In order to better understand the thermodynamic and kinetic parameters of such oligomerization reactions, nuclear magnetic resonance techniques were used to study the bulk phase acid-catalyzed aqueous reactions (Fischer esterification) of 2-MG. While the present results indicate that 2-MG oligoesters are formed in the bulk phase with similar water content equilibrium dependences as observed in atmospheric chamber SOA experiments, the acid-catalyzed rate of the Fischer esterification mechanism may be too slow to rationalize the 2-MG oligoester production timescales observed in the atmospheric chamber experiments. Furthermore, it appears that unrealistically high ambient SOA acidities would also be required for significant 2-MG oligoester content to arise via Fischer esterification. Therefore, the present results suggest that other, more kinetically facile, esterification mechanisms may be necessary to rationalize the existence of 2-MG oligomers in atmospheric chamber-generated and ambient SOA.