Ozonolysis of β-Caryophyllonic and Limononic Acids in the Aqueous Phase: Kinetics, Product Yield, and Mechanism.

Ozonolysis of β-caryophyllonic (BCA) and limononic (LA) acids in the aqueous-phase was investigated. The rate coefficients (kozone) measured for the BCA + ozone (O3) reaction at 295 ± 2 K were 4.8 ± 0.6 × 105 M-1 s-1 at pH = 2 and 6.0 ± 0.3 × 105 M-1 s-1 at pH = 8. The UV-vis absorption cross sections (σ, cm2 molecule-1) for BCA and LA in water were also measured. Atmospheric lifetimes of BCA and LA due to reactions with O3, hydroxyl radicals (OH), and due to photolysis were calculated. Lifetime estimates indicate that the aqueous-phase processing of both terpenoic acids studied in this work would be relevant in the atmosphere. In cloudwater, BCA is more likely to react with O3 with some possible contribution from the oxidation by OH, whereas the opposite is true for LA. Products of BCA and LA ozonolysis were quantified with LC-MS as well as with the UV-vis assays for quantification of formaldehyde and hydroperoxides. Oxygenated derivatives of BCA and LA that were produced following aqueous ozonolysis were identified as keto-BCA and keto-LA, respectively. Additionally, large quantities of intramolecular secondary ozonides and α-acyloxyhydroperoxy aldehydes were tentatively identified as products of aqueous ozonolysis of the two unsaturated terpenoic acids investigated.