Structure-Reactivity Analyses for Epoxide, Organosulfate, and Organonitrate Reactions Relevant to Secondary Organic Aerosol Composition

Extensive studies of secondary organic aerosol (SOA) formation in the southeastern United States have identified epoxide intermediates as key species in the formation of isoprene-derived SOA. In this mechanism, SOA species are formed from the isomerization and nucleophilic addition reactions of common SOA nucleophiles (such as water and sulfate) with epoxides formed from gas phase oxidation of isoprene and related species. In this study, experimental laboratory kinetics data were analyzed to ascertain the structure-reactivity relationships that rationalize the kinetics results for a variety of systems, including isoprene-derived epoxides (IEPOX) and methacrolein-derived epoxide (MAE). Epoxide substituent effects were used to rationalize the differing nucleophilic addition mechanisms and reaction rate constants, and a nucleophilicity scale was developed to rationalize product formation observations under conditions in which two or more nucleophiles are present. A similar structure-reactivity analysis was performed for organosulfate and organonitrate species that may undergo reaction under SOA conditions. These analyses may form the basis for predicting the SOA reactivity of as-of-yet unidentified epoxides, organosulfates, and organonitrates in the presence of the variety of nucleophiles known to exist on SOA.