pH Dependence of the OH Reactivity of Organic Acids in the Aqueous Phase

Abstract

Photochemical processing taking place in atmospheric aqueous phases serves as both a source and a sink of organic compounds. In aqueous environments, acid–base chemistry and, by extension, aqueous-phase pH, are an important yet often neglected factors to consider when investigating the kinetics of organic compounds. We have investigated the aqueous-phase OH-oxidation of pinic acid, cis-pinonic acid, limononic acid, and formic acid (FA) as a function of pH. We have also extended our studies to other organic acids (OAs) present in the water-soluble fraction of secondary organic aerosol (SOA) arising from the ozonolysis of α-pinene. Although all the OAs exhibited larger OH reactivities at pH 10, the pH dependence was dramatically different between FA, the smallest OA, and those that contained more than eight carbons. A kinetic box model was also employed to characterize our photoreactor and to provide confidence to our results. Our finding shows that the atmospheric lifetimes of small OAs (e.g., FA) are highly sensitive to cloud water pH. However, those of larger OAs and many other OAs in α-pinene SOA are affected to a much less extent. These results are of great importance for the simplification of cloud water chemistry models.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

“…Thus, in the absence of buffering, or a chemical OH − source compensating for acidification, less acidic or even slightly basic particles are rapidly acidified in the troposphere. This is also known for freshly formed sea salt particles, which rapidly become acidified within minutes after their emission, characterized by a pH drop by about 4 pH units (Angle et al, 2021). Furthermore, Fig.…”

“…Therefore, the statements of Zhao et al (2016), regarding the pH dependence in the reactivity of saturated carboxylic acids, are by far too overgeneralized. For the sake of completeness, a more recent study by the same authors (Amorim et al, 2020) shows that larger organic acids indeed do not exhibit much pH dependence.…”

T- and pH-Dependent Kinetics of the Reactions of ·OH(aq) with Glutaric and Adipic Acid for Atmospheric Aqueous-Phase Chemistry

“…Amorim et al and Ervens et al suggested that smaller alkyl groups normally show a stronger deprotonation dependence on the ·OH reactivity by up to 1 order of magnitude higher compared to larger alkyl groups. Larger alkyl groups provide more H atoms for abstraction and reduce the electronic effect of the deprotonation from the carboxyl groups …”

“…Although the rate constants used differ slightly from the compiled rate constants in Table S3, the variation trends of the ratios as a function of the carbon chain length are constant. Amorim et al 50 and Ervens et al 19 suggested that smaller alkyl groups normally show a stronger deprotonation dependence on the •OH reactivity by up to 1 order of magnitude higher compared to larger alkyl groups. Larger alkyl groups provide more H atoms for abstraction and reduce the electronic effect of the deprotonation from the carboxyl groups.…”

“…Larger alkyl groups provide more H atoms for abstraction and reduce the electronic effect of the deprotonation from the carboxyl groups. 50 Activation Parameter. The activation parameters of aqueous-phase reactions of the glutaric acid and adipic acid with •OH are determined.…”