Assessing the Potential for Oligomer Formation from the Reactions of Lactones in Secondary Organic Aerosols

Laboratory and field measurements have demonstrated that 2-methyl glyceric acid (2-MG) is the base component of a wide range of chemical species found in methacrolein-derived secondary organic aerosol (SOA). In order to explore the recently proposed hypothesis that a lactone oxidation intermediate is the origin of 2-MG and its derivatives in SOA, nuclear magnetic resonance techniques were used to study kinetics and reaction products of the aqueous phase reactions of a model lactone, β-propiolactone (BPL). BPL was found to react with a lifetime of 4-10 h (depending on solution conditions) via a general acid catalyzed mechanism, which suggests that lactones similar to BPL are reactive on an atmospherically relevant time scale. BPL was also shown to form a variety of nucleophilic addition products (organosulfates and nitrates and oligomers) similar to the 2-MG-based species observed in previous experiments involving the photooxidation and SOA processing of methacrolein. While many of the BPL reaction products could be rationalized via an epoxide-like nucleophilic addition mechanism, evidence for ester-like nucleophilic addition was suggested through the observation of inorganic ion-catalyzed oligomer formation. The formation of oligomers was found to depend strongly on the proportion of organic acid nucleophile present in its deprotonated form. Therefore, due to the nature of the general acid catalysis and importance of deprotonated acids for efficient BPL oligomerization, it is suggested that oligomerization from lactone intermediates will be more efficient at higher SOA pH values. This result may help explain why overall isoprene-derived SOA formation has been observed to be largely pH-independent. Overall, the results strongly support the previous conclusion that a lactone intermediate is responsible for the formation of 2-MG-related species found in methacrolein-derived SOA.