cis-Pinonic Acid Oxidation by Hydroxyl Radicals in the Aqueous Phase under Acidic and Basic Conditions: Kinetics and Mechanism

Aqueous-phase oxidation of cis-pinonic acid (CPA) by hydroxyl radicals (OH) was studied using a relative rate technique under acidic and basic conditions. Liquid chromatography (LC) coupled to the negative electrospray ionization (ESI) quadrupole tandem mass spectrometry (MS/MS) was used to monitor the concentrations of CPA and reference compounds. The measured second order reaction rate coefficients of CPA with OH were: 3.6 ± 0.3 × 109 M–1 s–1 (pH 2) and 3.0 ± 0.3 × 109 M–1 s–1 (pH 10) - combined uncertainties are 2σ. These results indicated that the lifetimes of CPA in the atmosphere are most likely independent from the aqueous-phase pH. LC-ESI/MS/MS was also used to tentatively identify the CPA oxidation products. Formation of carboxylic acids with molecular weight (MW) 216 Da (most likely C10H16O5) and MW 214 Da (C10H14O5) was confirmed with LC-ESI/MS/MS. When the initial CPA concentration was increased from 0.3 to 10 mM, formation of additional products was observed with MW 188, 200, 204, and 232 Da....