Photo-oxidation of pinic acid in the aqueous phase: a mechanistic investigation under acidic and basic pH conditions

Atmospheric aqueous phases (cloud and fog droplets, aerosol liquid water) are important reaction media for the processing of water-soluble organic acids (OAs). The photochemistry of these species is known to contribute to the formation of aqueous secondary organic aerosols (aqSOA). OAs arising from the gas-phase oxidation of α-pinene, such as cis -pinonic acid (CPA) or pinic acid (PA), are considered relevant aqSOA precursors, as they are oxygenated compounds and expected to be sufficiently water-soluble to undergo cloudwater processing. Although PA is a major oxidation product of α-pinene, the mechanism of its aqueous-phase photo-oxidation has never been studied previously. In this work, we performed the first in-depth investigation for aqueous-phase OH-oxidation of PA. Given that the pH of atmospheric aqueous phases is highly variable, and it has been increasing over the past 40 years, we were particularly interested in the impact of pH on the reaction mechanisms and product yields. Experiments were performed using a combination of offline and online MS techniques to identify PA and monitor the evolution of PA OH-oxidation products. We identified PA as a novel source of important α-pinene SOA tracers, including 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), and norpinic acid (NPA). Furthermore, we show a drastic difference in the yield of the oxidation products of PA under acidic and basic pH conditions. Specifically, we observed the yields of NPA and MBTCA at 3.4% and 2.6% under pH 2 but at 10% and 5.0%, respectively, at pH 8. These yield differences are likely due to a charge transfer reaction pathway unique to the aqueous phase. Such impact of pH on the reaction mechanism of OA has never been shown before. The mechanism of aqueous-phase OH-oxidation of pinic acid was investigated for the first time, and norpinic acid and 3-methyl-1,2,3-butanetricarboxylic acid were identified as major products. The mechanism is pH-dependent, with enhanced yields under basic conditions.