Determining the Relative Reactivity of Sulfate, Bisulfate, and Organosulfates with Epoxides on Secondary Organic Aerosol
Extensive laboratory and field studies have identified nucleophilic addition reactions of isoprene epoxydiols (IEPOX) as key pathways for the formation of isoprene-derived secondary organic aerosol (SOA). Organosulfates are important reaction products of these processes, but it is unclear whether sulfate and/or bisulfate nucleophiles are responsible for their formation and whether the organosulfates themselves can serve as nucleophiles in oligomer-forming reactions. The relative reactivities (nucleophilic strengths relative to water) of sulfate, bisulfate, and methyl sulfate anion were measured through a series of model epoxide-nucleophile experiments using nuclear magnetic resonance (NMR) spectroscopy. These experiments also helped establish a rigorous understanding of the effects of differing carbon substitution and functional groups of epoxides on the modulation of the effective nucleophilicites of sulfate, bisulfate, and methyl sulfate anions. It was determined that the nucleophilicites of bisulfate and methyl sulfate anions were about 100 and 50 times, respectively, weaker than sulfate toward most of the epoxides studied, which was rationalized by computational estimates of their thermodynamic basicities. Therefore, for most SOA acidity situations, sulfate-epoxide reactions are expected to be the main source of organosulfate aerosol constituents. Because sulfate-epoxide reactions stoichiometrically consume acid, these reactions also have the capability of raising the pH of SOA, thus slowing down all acid-catalyzed chemical processes. No evidence for the reaction of the methyl sulfate anion was observed with the abundant atmospherically relevant epoxide, trans-beta-IEPOX, thus suggesting that oligomerization reactions via epoxide-organosulfate reactions may not be able to compete with stronger (such as sulfate) or more abundant (such as water) nucleophiles on actual SOA.
Aoki, Erika, Jon N. Sarrimanolis, Sophie A. Lyon, and Matthew J. Elrod. 2020. "Determining the Relative Reactivity of Sulfate, Bisulfate, and Organosulfates with Epoxides on Secondary Organic Aerosol." ACS Earth and Space Chemistry 4(10): 1793-1801.
American Chemical Society
ACS Earth and Space Chemistry
Chemistry and Biochemistry
Atmosphere, Air pollution, Isoprene, Thermodynamics, Basicity, Kinetics, Nucleophilicity, Nuclear magnetic resonance