Kinetics and products of the aqueous-phase oxidation of β-caryophyllonic acid by hydroxyl radicals

Abstract

Kinetics and mechanism of β-caryophyllonic acid (BCA) oxidation by hydroxyl radicals (OH) in the aqueous-phase were investigated at 298 ± 2 K using liquid chromatography (LC) coupled to the electrospray ionization (ESI) mass spectrometry (MS). Triple quadrupole mass spectrometer operating in the multiple reaction monitoring (MRM) was used to follow concentrations of organic compounds during the photooxidation at cloud-relevant concentrations (several μM). The rate coefficients obtained for BCA + OH reaction were: 4.2 ± 0.5 × 1010 M−1s−1 at pH = 2 and 6.5 ± 0.7 × 109 M−1s−1 at pH = 8. The kinetic data obtained in this study was used to estimate atmospheric lifetimes of BCA in the aqueous-phase: between 24 min and 12 h depending on pH. The results indicated that aqueous-phase oxidation is a relevant removal pathway for this compound in clouds, fogs and wet aerosols. The products of BCA + OH reaction were studied with the high resolution tandem MS. The two major, first-generation products identified were keto-BCA and hydroxyl-hydroperoxy BCA; these two molecules were formed by OH addition to the terminal C=C bond of the precursor. The rest of the products were identified as oxygenated BCA derivatives with O:C ratios higher than the precursor although some fragmentation of the original carbon backbone was also observed. Thus, the data acquired provided insights into the mechanism of BCA + OH reaction.