Temperature- and pH- Dependent OH Radical Reaction Kinetics of Tartaric and Mucic Acids in the Aqueous Phase.

Tartaric acid and mucic acid are dicarboxylic acids (DCAs), a substance class often found in atmospheric aerosols and cloud droplets. The hydroxyl radical (OH)-induced oxidation in the aqueous phase is known to be an important loss process of organic compounds such as DCAs. However, the study of OH kinetics of DCAs in the aqueous phase is still incomplete. In the present study, the rate constants of the OH reactions of tartaric acid and mucic acid in the aqueous phase were determined by the thiocyanate competition kinetics method as a function of temperature and pH. The following T-dependent Arrhenius expressions (in units of L mol s) were first derived for the OH reactions with tartaric acid─(T, HA) = (3.3 ± 0.1) × 10 exp[(-1350 ± 110 K)/T], (T, HA) = (3.6 ± 0.1) × 10 exp[(-580 ± 110 K)/T], and (T, A) = (3.3 ± 0.1) × 10 exp[(-1190 ± 170 K)/T]─as well as mucic acid─(T, HA) = (2.2 ± 0.1) × 10 exp[(-1140 ± 150 K)/T], (T, HA) = (4.8 ± 0.1) × 10 exp[(-1280 ± 170 K)/T], and (T, A) = (2.1 ± 0.1) × 10 exp[(-970 ± 70 K)/T]. A general trend of the OH rate constant is found as . The pH- and temperature-dependent rate constants of the OH radical reactions allow an accurate description of the source and sink processes in the tropospheric aqueous phase.