Oligomer formation from cross-reaction of Criegee intermediates in the styrene-isoprene-O3 mixed system

Abstract

Alkene ozonolysis can produce stabilized Criegee intermediates (SCIs), which play a key role in oligomers' formation. Though styrene and isoprene coexist in the ambient atmosphere as important anthropogenic and biogenic secondary organic aerosol (SOA) precursors, respectively, their cross-reactions have not received attention. This study investigated the interactions of SCIs from styrene and isoprene ozonolysis for the first time. The high-resolution Orbitrap mass spectrometer was used to determine the unique ion mass spectra of the isoprene-styrene-O3 mixture. The results show that the signal intensities of new ions account for >8.4% of total ions in the mass spectra of the styrene-isoprene-O3 mixed system. Styrene and isoprene ozonolysis can produce characteristic C7–SCI and C4–SCI, respectively. C7–SCI and C4–SCI can be involved in the cross-reactions, and the results of tandem mass spectra directly confirmed both C7–SCI and C4–SCI as chain units. The O/C and H/C ratios of cross-products are in the range of 0.38–1.07 and 1.00–1.50, respectively, which are consistent with cross-reaction products. Adding a C7–SCI unit reduces the oligomer's volatility by 1.3–1.4 orders of magnitude lower than adding a C4–SCI unit. Thus, C4–SCI can compete with C7–SCI to react with styrene-derived RO2/RC(O)OH to produce more volatile cross-products, while the less volatile cross-products can be formed when isoprene-derived RO2/RC(O)OH reacted with C7–SCI instead of C4–SCI. The SOA yield of the mixed system is lower than that of the single styrene-O3 system but higher than that of the single isoprene-O3 system. Ambient particles were also collected, and 5 possible SCI-related cross-products were identified. This study illustrates the effects of SCI-related cross-reactions on SOA components and physicochemical properties, providing a basis for future research on SCI-related cross-reactions that frequently occur in the ambient atmosphere.