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 -1 s -1 ) were first derived for the • OH reactions with tartaric acid─ k (T, H 2 A) = (3.3 ± 0.1) × 10 10 exp[(-1350 ± 110 K)/T], k (T, HA - ) = (3.6 ± 0.1) × 10 10 exp[(-580 ± 110 K)/T], and k (T, A 2- ) = (3.3 ± 0.1) × 10 10 exp[(-1190 ± 170 K)/T]─as well as mucic acid─ k (T, H 2 A) = (2.2 ± 0.1) × 10 10 exp[(-1140 ± 150 K)/T], k (T, HA - ) = (4.8 ± 0.1) × 10 10 exp[(-1280 ± 170 K)/T], and k (T, A 2- ) = (2.1 ± 0.1) × 10 10 exp[(-970 ± 70 K)/T]. A general trend of the • OH rate constant is found as k A 2 - > k H A - > k H 2 A . 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.