Mechanistic Insights into Isoprene SOA Formation Utilizing Measurements and Modeling

Isoprene (C₅H₈) has the highest global emission rate of any non-methane volatile organic compound. As a result, isoprene has the potential to produce large quantities of secondary organic aerosol (SOA), which has implications for climate, air quality, and human health. However, the mechanisms by which isoprene forms SOA remain uncertain. We use a series of chamber experiments together with a detailed multiphase chemical model and quantum chemical calculations to improve our understanding of SOA formation from isoprene when IEPOX multiphase chemistry is suppressed. Isoprene photochemical oxidation experiments were conducted in the Pacific Northwest National Laboratory (PNNL) 10.6 m³ environmental chamber with dry ammonium sulfate seed, thus suppressing IEPOX reactive uptake and allowing for the study of the recently discovered dihydroxy dihydroperoxide (C₅H₁₂O₆) formation pathway. Concentrations of radicals in the chamber, such as OH, HO₂, RO₂, and NO were systematically varied. We coupled a dynamic gas-particle partitioning module to the University of Leeds Master Chemical Mechanism with custom updates to reflect the non-IEPOX C₅H₁₂O₆ pathway. The model reproduces the approximate quantity and time evolution of SOA in the PNNL chamber, as well as previous studies performed in different chambers. However, to explain the observed trends of C₅H₁₂O₆ versus HO₂ and NO concentrations, a fast isomerization rate (0.3-0.8 s^-1) of the C₅H₁₁O₆ peroxy radical is required and well supported by quantum chemical calculations of the corresponding rate coefficient. These calculations also suggest epoxides are a major product of the RO₂ isomerization, indicating that epoxides and their related multiphase chemistry may be a general feature of the isoprene SOA system. The implications for the ability of isoprene oxidation to contribute to SOA under atmospheric conditions will be discussed.