T- and pH-Dependent Kinetics of the Reactions of ·OH<sub>(aq)</sub> with Glutaric and Adipic Acid for Atmospheric Aqueous-Phase Chemistry
Liang Wen, Thomas Schaefer, Lin He, Yimu Zhang, Xiaomin Sun, Oscar N. Ventura, Hartmut Herrmann
Abstract
Glutaric acid and adipic acid are dicarboxylic acids (DCAs) that are commonly found in atmospheric aerosols and cloud droplets. Within this study, the temperature- and pH-dependent rate constants of aqueous-phase OH radical reactions of these two DCAs were determined through the competition kinetics method. The following Arrhenius expressions were derived for the temperature dependency of the OH radical reaction with glutaric acid: k(T, H2A) = (3.9 ± 0.1) × 1010 × exp[(−1270 ± 200 K)/T], k(T, HA–) = (2.3 ± 0.1) × 1011 × exp[(−1660 ± 190 K)/T], k(T, A2–) = (1.4 ± 0.1) × 1011 × exp[(−1400 ± 170 K)/T]. Similarly, for adipic acid: k(T, H2A) = (7.5 ± 0.2) × 1010 × exp[(−1210 ± 170 K)/T], k(T, HA–) = (9.5 ± 0.3) × 1010 × exp[(−1200 ± 200 K)/T], k(T, A2–) = (8.7 ± 0.2) × 1010 × exp[(−1100 ± 170 K)/T] (in units of L mol–1 s–1). Density functional theory (DFT) calculations were performed to calculate the energy barrier of the H atom abstraction. The calculation results show that the energy barriers at the Cβ-atoms of the two DCAs are much lower than those at the Cα-atoms, indicating that the H atom abstractions predominantly occur at the Cβ-atoms. The increased ·OH rate constant in the case of the deprotonated form can be explained by the reduction of energy barrier, which is predominately caused by the variation of the inductive effect of the carboxyl group. As an important sink in the atmosphere, the degradation of glutaric acid and adipic acid by ·OH under atmospheric conditions can be accurately described by the Arrhenius expressions obtained.