Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol-Derived Secondary Organic Aerosol
Jinpei Yan, Yue Zhang, Yuzhi Chen, N. Cazimir Armstrong, Nicolas A. Buchenau, Ziying Lei, Yao Xiao, Zhenfa Zhang, Andrew T. Lambe, Man Nin Chan, Barbara J. Turpin, Avram Gold, Andrew P. Ault, Jason D. Surratt
Abstract
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 cm 3 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.