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

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 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.

Thermodynamics and Kinetics of Atmospherically Relevant Acetalization Reactions

“…The proxy aldehydes and polyols chosen for study are likely not significant SOA components owing to their low atmospheric abundance or limited ability to partition to the particle phase. However, the polyols used in the present study are quite similar to 2-methyltetrol, a major isoprene-related species . Therefore, the weak dependence of the thermodynamics and kinetics on the specific polyol structure indicates that the present results are probably a reasonable approximation for reactions involving 2-methyltetrol.…”

Deactivating Effect of Hydroxyl Radicals Reactivity by Sulfate and Sulfite Functional Groups in Aqueous Phase─Atmospheric Implications for Small Organosulfur Compounds

“…Chemical transformations of organosulfur compounds in the atmosphere are highly complex. One crucial process that can significantly affect the abundance and fate of these compounds in atmospheric aerosols and cloud droplets is their oxidation by hydroxyl ( • OH) radicals in aqueous phase. − In aqueous phase, like many other organic compounds, organosulfur compounds can react with • OH radicals by different mechanisms, including hydrogen abstraction from saturated compounds, electrophilic addition to unsaturated compounds, and electron transfer. − These reactions play a vital role in both the removal and formation of water-soluble organosulfur compounds in the atmosphere . For example, aqueous-phase processes result in the formation of aqueous secondary organic aerosols (aqSOA), which are a diverse group of oxygenated organic compounds being less explored .…”