Ring-opening yields and auto-oxidation rates of the resulting peroxy radicals from OH-oxidation of α-pinene and β-pinene

Atmospheric oxidation of monoterpenes (C10H16) contributes to ambient particle number and mass concentrations due, in part, to the resulting peroxy radicals undergoing auto-oxidation to low-volatility highly oxygenated molecules (HOMs). Only one of the structural isomers of the first-generation hydroxy peroxy radical (C10H17O3), that formed with an opening of the cyclobutyl ring, is thought to be responsible for the majority of HOMs derived from OH-initiated oxidation of alpha and beta-pinene. We constrain the yield and auto-oxidation rate of this isomer by using a unique combination of isotopically labeled precursors, such as D-3-alpha-pinene, and direct measurements of peroxy radicals and closed-shell products after a reaction time of only similar to 0.7 s. Using competitive rate studies and quantum chemical calculations, we find that the yields of these ring-opened peroxy radicals are 0.47 (0.2, 0.6) for alpha-pinene and 0.66 (0.4, 0.8) for beta-pinene, and the rates of auto-oxidation are 2.9 (1.1, 4.7) s(-1) and 0.8 (0.4, 1.6) s(-1), respectively, where values in the parentheses represent upper and lower bounds based on experimental and mechanistic uncertainties. These rates and yields are in general agreement with HOM yields determined from independent methods and in a range where OH driven monoterpene HOM formation will be atmospherically important for a wide set of conditions.

Automated summary

The atmospheric oxidation of monoterpene compounds (C10H16) contributes to the concentration of ambient particles and mass, mainly due to the auto-oxidation of resulting peroxy radicals to low-volatility highly oxygenated molecules (HOMs). The isomer of the first-generation hydroxy peroxy radical, formed by opening the cyclobutyl ring, is believed to be responsible for the majority of HOMs derived from OH-initiated oxidation of alpha and beta-pinene. By combining isotopically labeled precursors and direct measurements, the yield and auto-oxidation rate of this isomer were constrained, showing their significant atmospheric importance for a wide range of conditions.