Kinetics and Products of the Reaction of the First-Generation Isoprene Hydroxy Hydroperoxide (ISOPOOH) with OH

Abstract

The atmospheric oxidation of isoprene by the OH radical leads to the formation of several isomers of an unsaturated hydroxy hydroperoxide, ISOPOOH. Oxidation of ISOPOOH by OH produces epoxydiols, IEPOX, which have been shown to contribute mass to secondary organic aerosol (SOA). We present kinetic rate constant measurements for OH + ISOPOOH using synthetic standards of the two major isomers: (1,2)- and (4,3)-ISOPOOH. At 297 K, the total OH rate constant is 7.5 ± 1.2 × 10^(–11) cm^3 molecule^(–1) s^(–1) for (1,2)-ISOPOOH and 1.18 ± 0.19 × 10^(–10) cm^3 molecule^(–1) s^(–1) for (4,3)-ISOPOOH. Abstraction of the hydroperoxy hydrogen accounts for approximately 12% and 4% of the reactivity for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. The sum of all H-abstractions account for approximately 15% and 7% of the reactivity for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. The major product observed from both ISOPOOH isomers was IEPOX (cis-β and trans-β isomers), with a ∼ 2:1 preference for trans-β IEPOX and similar total yields from each ISOPOOH isomer (∼70–80%). An IEPOX global production rate of more than 100 Tg C each year is estimated from this chemistry using a global 3D chemical transport model, similar to earlier estimates. Finally, following addition of OH to ISOPOOH, approximately 13% of the reactivity proceeds via addition of O_2 at 297 K and 745 Torr. In the presence of NO, these peroxy radicals lead to formation of small carbonyl compounds. Under HO_2 dominated chemistry, no products are observed from these channels. We suggest that the major products, highly oxygenated organic peroxides, are lost to the chamber walls. In the atmosphere, formation of these compounds may contribute to organic aerosol mass.

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