T- and pH-Dependent Hydroxyl-Radical Reaction Kinetics of Lactic Acid, Glyceric Acid, and Methylmalonic Acid in the Aqueous Phase.

Abstract

Carboxylic acids are a common class of compounds found in atmospheric aerosols and cloud droplets. In this study, the oxidation kinetics of several carboxylic acids in the aqueous phase by the atmospherically relevant ^•OH radical were investigated to better understand the loss processes for this class of compounds. The rate constants for the reactions of the ^•OH radical were determined using the thiocyanate competition kinetics method for lactic acid, glyceric acid, and methylmalonic acid as a function of temperature and pH. The Arrhenius equations for oxidation by the ^•OH radical are as follows (unit in L mol^-1 s^-1): For lactic acid: k(T, HA) = (1.3 ± 0.1) × 10¹⁰ × exp[(-910 ± 160 K)/T] and k(T, A^-) = (1.3 ± 0.1) × 10¹⁰ × exp[(-800 ± 80 K)/T]; for glyceric acid: k(T, HA) = (6.0 ± 0.2) × 10¹⁰ × exp[(-1100 ± 170 K)/T] and k(T, HA^±) = (3.6 ± 0.1) × 10¹⁰ × exp[(-1500 ± 100 K)/T]; and for methylmalonic acid: k(T, H₂A) = (5.5 ± 0.1) × 10¹⁰ × exp[(-1760 ± 100 K)/T], k(T, HA^-) = (1.4 ± 0.1) × 10⁹ × exp[(-530 ± 80 K)/T] and k(T, A^2-) = (9.6 ± 0.4) × 10¹⁰ × exp[(-1530 ± 270 K)/T]. The general trend of the ^•OH rate constant was observed k_A^2- > k_HA^- > k_H2A. The energy barriers of the ^•OH radical reaction and thus the most probable site of H atom abstraction were calculated using density functional theory simulations in Gaussian with the M06-2X method and the 6-311++G(3df,2p) basis set. Kinetic data predicted from structure-activity relationships were compared to the measured ^•OH radical rate constants. ^•OH radical oxidation in the aqueous phase could serve as an important sink for carboxylic acids, and the pH- and T-dependent rate constants of ^•OH radical reactions provide a better description of the aqueous-phase sink processes. Hence, the atmospheric lifetime as well as the partitioning of the investigated carboxylic acids was calculated.