Photo-Oxidation of α-Pinene Oxidation Products in Atmospheric Waters - pH- and Temperature-Dependent Kinetic Studies

The atmospheric α-pinene oxidation leads to three carboxylic acids: norpinonic acid (NPA), pinic acid (PA), and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). In this study, the OH radical kinetics in the aqueous phase of these carboxylic acids were investigated at different temperatures and pH values of solutions. Activation parameters and the corresponding atmospheric lifetimes of the acids in the troposphere were derived. The overall second-order rate constants for the individual speciation forms of the acids (AH and A^- for NPA; AH₂, AH^- and A^2- for PA; and AH₃, AH₂^-, AH^2- and A^3- for MBTCA) were determined. At 298 K, the rate constants for reactions of protonated forms (AH) of NPA, PA, and MBTCA with ^•OH, were (1.5 ± 0.2) × 10⁹ L mol^-1 s^-1, (2.4 ± 0.1) × 10⁹ L mol^-1 s^-1, and (4.1 ± 0.6) × 10⁸ L mol^-1 s^-1, respectively. For the fully deprotonated forms (A) of studied acids, the second-order rate constants were (2.2 ± 0.2) × 10⁹ L mol^-1 s^-1, (2.8 ± 0.1) × 10⁹ L mol^-1 s^-1, and (10.2 ± 0.7) × 10⁸ L mol^-1 s^-1 at 298 K, respectively. It was found that the reactions of NPA and PA with OH radicals are faster than with MBTCA. For MBTCA, the reaction rate depends on pH more strongly at elevated temperatures (>298 K). The atmospheric lifetimes of the acids considered due to their reactivity with ^•OH were calculated for different model scenarios at a temperature of 283 K and pH = 2 in the aqueous phase. For this purpose, liquid water content (LWC) was used for aerosols and clouds under storm conditions and at various aqueous-phase concentrations of OH radicals. The lifetimes decreased with increasing LWC (from 10^-12 m³ m^-3 in aerosol to 10^-5 m³ m^-3 in storms), indicating that the acids undergo significant aqueous processing under realistic atmospheric conditions. Besides, the aerosol systems appeared less effective in removing PA and NPA, with lifetimes ranging from hundreds of days to tens and hundreds of hours, respectively. Clouds were more effective, with lifetimes ranging from tens of hours to a single second or less. MBTCA, which dissolves better in water, was effectively removed in all systems, with the longest lifetime of approximately 90 min.