Aqueous-Phase Oxidation of Terpene-Derived Acids by Atmospherically Relevant Radicals.

Terpene-derived acids formed through the atmospheric gas-phase oxidation of terpenes are able to efficiently undergo a phase transfer into the aqueous phase. The subsequent aqueous-phase oxidation of such compounds has not been intensely studied. Accordingly, the aqueous-phase second-order rate constants of the oxidation reactions of cis-pinonic acid (CPA) and (+)-camphoric acid (+CA) with hydroxyl radicals ( ^• OH), nitrate radicals (NO ₃ ^• ), and sulfate radicals (SO ₄ ^•- ) were investigated as a function of temperature and pH in the present study. For CPA and +CA the following ^• OH reaction rate constants at T = 298 K are determined: k _second (CPA, pH<2) = (2.8 ± 0.1) × 10 ⁹ L mol ^-1 s ^-1 , k _second (CPA, pH>8) = (2.7 ± 0.3) × 10 ⁹ L mol ^-1 s ^-1 , k _second (+CA, pH<2) = (2.1 ± 0.1) × 10 ⁹ L mol ^-1 s ^-1 , k _second (+CA, pH=5.3) = (2.7 ± 0.3) × 10 ⁹ L mol ^-1 s ^-1 , k _second (+CA, pH>8) = (2.7 ± 0.1) × 10 ⁹ L mol ^-1 s ^-1 . In order to assess the atmospheric impact of the aqueous-phase oxidation of such compounds, atmospheric aqueous-phase lifetimes were calculated for two model scenarios based on CAPRAM 3.0i. The aqueous-phase oxidation under remote conditions emerges to be the most favored pathway with lifetimes of 5 ± 1 h.