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-β-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.

中文翻译：

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测定硫酸盐，硫酸氢盐和有机硫酸盐与环氧在次级有机气溶胶上的相对反应性

大量的实验室和现场研究已经确定异戊二烯环氧二醇（IEPOX）的亲核加成反应是形成异戊二烯衍生的二次有机气溶胶（SOA）的关键途径。有机硫酸盐是这些过程的重要反应产物，但是尚不清楚硫酸盐和/或硫酸氢盐亲核试剂是否负责它们的形成，以及有机硫酸盐本身是否可以在低聚物形成反应中用作亲核试剂。硫酸根，硫酸氢根和甲基硫酸根阴离子的相对反应性（相对于水的亲核强度）是通过一系列使用核磁共振（NMR）光谱的环氧化物-亲核模型实验测得的。这些实验还有助于建立对不同碳取代基和环氧基官能团对硫酸根，硫酸氢根和甲基硫酸根阴离子的有效亲核体调节的影响的严格理解。经测定，硫酸氢根和甲基硫酸根阴离子的亲核体分别比所研究的大多数环氧化物弱于硫酸根约100倍和50倍，这是通过对它们的热力学碱性进行计算估算得出的。因此，对于大多数SOA酸性情况，硫酸盐-环氧反应预计将是有机硫酸盐气溶胶成分的主要来源。由于硫酸盐-环氧反应化学计量消耗了酸，因此这些反应也具有提高SOA pH值的能力，从而减慢了所有酸催化的化学过程。反-β-IEPOX，因此表明通过环氧化物-有机硫酸盐反应的低聚反应可能无法与实际SOA上的较强亲核试剂（如硫酸盐）或较丰富亲核试剂（如水）竞争。