Effect of oligomerization reactions of Criegee intermediate with organic acid/peroxy radical on secondary organic aerosol formation from isoprene ozonolysis

Secondary organic aerosols (SOA) have significant effects on atmospheric chemistry, human health and climate forcing, but their formation mechanisms via Criegee chemistry are still poorly understood. Here we present a comprehensive theoretical investigation on the oligomerization reaction of stabilized Criegee intermediates (SCIs) with organic acid/peroxy radical by using ab initio quantum-chemical methodologies. Our results show that the ozonolysis of isoprene easily leads to a series of C3 and C4 stable CIs due to its larger exothermicity and spontaneity. The formed SCIs have two isomers: syn- and anti-, and anti- is more stable in energy than that of syn- by about 2–5 kcal mol⁻¹. The barrier heights of oligomerization reactions are very sensitive to the size and structure of functional groups near the central carbon atom site, indicating they can be tuned by the substitutions. Reaction between SCIs and peroxy radical contributes significantly to the formation of oligomer which is the dominant component of SOA. However, the reaction between SCIs and organic acid plays an important role in aerosol nucleation in some regions where high SCI and low H₂O concentrations occur such as in terrestrial equatorial area. Such knowledge should be useful for understanding the mechanism of SOA formation from alkenes ozonolysis and for developing atmospheric chemistry models.