Formation and kinetics of terpenoic acids from α-pinene and δ-3-carene-derived SOA components via aqueous-phase OH radicals

Bimolecular reaction rate coefficients (k_OH, L mol⁻¹s⁻¹) for the aqueous oxidation of pinic, caric, caronic, norpinic, and norpinonic acids by OH radicals were measured within the presented study. Furthermore, a quantitative analysis was performed for selected products formed in the studied OH reactions, including those known in the literature, SOA components of α-pinene and δ-3-carene. Values of k_OH were measured for the neutral (pH = 2) and deprotonated (pH = 10) forms of the terpenoic acids under investigation in the temperature range between 288 and 308 K. The reactions of caronic and caric acids with OH radicals appear slightly faster (k_OH^{298K pH2} = 3.3 ± 0.1 × 10⁹ L mol⁻¹s⁻¹ and k_OH^{298K pH2} = 3.3 ± 0.1 × 10⁹ L mol⁻¹s⁻¹, respectively) than those of norpinic (k_OH^{298K pH2} = 1.2 ± 0.1 × 10⁹ L mol⁻¹s⁻¹), norpinonic (k_OH^{298K pH2} = 1.1 ± 0.1 × 10⁹ L mol⁻¹s⁻¹) and pinic acids (k_OH^{298K pH2} = 2.1 ± 0.1 × 10⁹ L mol⁻¹s⁻¹). The calculated activation energies (E) fall in a 9–21 kJ mol⁻¹ range, explaining the very weak temperature dependence of the measured reaction rates. The calculated lifetimes decreased with increasing LWC (from 10⁻¹² m³ m³ in aerosol to 10⁻⁵ m³ m⁻³ in storms), indicating that the acids undergo significant aqueous processing under realistic atmospheric conditions. The analysis of extremely low-volatility products allowed proposing reaction paths from earlier– to later–generation products (only carboxylic acid) of α-pinene. We measured that terebic acid can be formed through the aqueous phase OH reaction of norpinic acid, but also through terpenylic acid oxidation. Moreover, we found that the reaction of terpenylic acid + OH radicals is almost 10 times slower than the oxidation of pinic and pinonic acids. Therefore, observed in the atmosphere, terebic acid formation is mainly through direct oxidation of an early-generation oxidation product of α-pinene (pinic and pinonic acids) rather than from the secondary reaction of terpenylic acid.