Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol-Derived Secondary Organic Aerosol

Heterogeneous hydroxyl radical (^•OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition. It was recently demonstrated that heterogeneous ^•OH oxidation can age single-component particulate methyltetrol sulfates (MTSs), causing ∼55% of the SOA mass loss. However, our most recent study of freshly generated IEPOX-SOA particulate mixtures suggests that the lifetime of the complete IEPOX-SOA mixture against heterogeneous ^•OH oxidation can be prolonged through the fragmentation of higher-order oligomers. Published studies suggest that the heterogeneous ^•OH oxidation of IEPOX-SOA could affect the organic atmospheric aerosol budget at varying rates, depending on aerosol chemical composition. However, heterogeneous ^•OH oxidation kinetics for the full IEPOX-SOA particulate mixture have not been reported. Here, we exposed freshly generated IEPOX-SOA particles to heterogeneous oxidation by ^•OH under humid conditions (relative humidity ∼57%) for 0-15 atmospheric-equivalent days of aging and derived an effective heterogeneous ^•OH rate coefficient (k_OH) of 2.64 ± 0.4 × 10^-13 cm³ molecules^-1 s^-1. While ∼44% of particulate organic mass of nonoxidized IEPOX-SOA was consumed over the entire 15 day aging period, only <7% was consumed during the initial 10 aging days. By molecular-level chemical analysis, we determined oligomers were consumed at a faster rate (by a factor of 2-4) than monomers. Analysis of aerosol physicochemical properties shows that IEPOX-SOA has a core-shell morphology, and the shell becomes thinner with ^•OH oxidation. In summary, this study demonstrates that heterogeneous ^•OH oxidation of IEPOX-SOA particles is a dynamic process in which aerosol chemical composition and physicochemical properties play important roles.