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) × 10(10) × exp[(-910 ± 160 K)/T] and k(T, A(-)) = (1.3 ± 0.1) × 10(10) × exp[(-800 ± 80 K)/T]; for glyceric acid: k(T, HA) = (6.0 ± 0.2) × 10(10) × exp[(-1100 ± 170 K)/T] and k(T, HA(±)) = (3.6 ± 0.1) × 10(10) × exp[(-1500 ± 100 K)/T]; and for methylmalonic acid: k(T, H(2)A) = (5.5 ± 0.1) × 10(10) × exp[(-1760 ± 100 K)/T], k(T, HA(-)) = (1.4 ± 0.1) × 10(9) × exp[(-530 ± 80 K)/T] and k(T, A(2-)) = (9.6 ± 0.4) × 10(10) × exp[(-1530 ± 270 K)/T]. The general trend of the (•)OH rate constant was observed k(A(2-)) > k(HA(-)) > k(H(2)A). 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.