Oxidation of Terpene-Derived Acids in Aqueous Solution

Tobias Otto, Thomas Schaefer and Hartmut Herrmann

Leibniz Institute for Tropospheric Research, Atmospheric Chemistry Department (ACD), Permoserstraße 15, 04318 Leipzig, Germany

The tropospheric aqueous phase is increasingly getting into focus to be important for the formation of aqueous secondary aerosol (aqSOA). The oxidations of biogenic volatile organic compounds (BVOCs) such as isoprene and terpenes yield a major fraction of SOA precursor in the gas phase. Aqueous-phase oxidation reactions of terpene-derived compounds have not been intensely studied.

Hence, the aqueous-phase kinetics of the reactions of hydroxyl (OH), nitrate (NO₃) and sulfate (SO₄^-) radicals with cis-pinonic acid and (+)-camphoric acid as a function of temperature and pH were investigated in this study. Both precursor compounds are well water-soluble due to their functionalization (H^cp_estimated (cis-pinonic acid) = 7.63 × 10⁷ mol L^-1 atm^-1 and H^cp_estimated ((+) camphoric acid) = 1.12 × 10⁶ mol L^-1 atm^-1. 

The kinetic measurements were conducted applying laser flash photolysis – laser long path absorption technique (LFP-LLPA) to determine reaction rate constants for the reactions of cis-pinonic acid and (+)-camphoric acid with OH, NO₃ and SO₄^- radicals. For CPA and +CA the following OH reaction rate constants at T = 298 K are determined: k_2nd (CPA, pH < 2) = (2.8 ± 0.1) × 10⁹ L mol^-1 s^-1, k_2nd (CPA, pH > 8) = (2.7 ± 0.3) × 10⁹ L mol^-1 s^-1, k_2nd (+CA, pH < 2) = (2.1 ± 0.1) × 10⁹ L mol^-1 s^-1, k_2nd (+CA, pH = 5.3) = (2.7 ± 0.3) × 10⁹ L mol^-1 s^-1, k_2nd (+CA, pH > 8) = (2.7 ± 0.1) × 10⁹ L mol^-1 s^-1. The obtained kinetic results are used to determine atmospheric aqueous-phase lifetimes of these compounds to assess the atmospheric impact of their aqueous-phase oxidation. In addition to it, the kinetic experimental findings can be implemented into the chemical aqueous-phase radical mechanism (CAPRAM), to enhance predictive capabilities of this model and the impact on the oxidative capacity of the tropospheric aqueous phase.