Abstract
Rate constants for the aqueous-phase reactions of the hydroxyl radical with the dicarboxylic acids, succinic acid and pimelic acid were determined using the relative rate technique over the temperature range 287 K ≤ T ≤ 318 K and at pH = 2.0, 4.6 or 4.9 and 8.0. OH radicals were generated by H2O2 laser flash photolysis while thiocyanate was used as a competitor. The pH values were adjusted to obtain the different speciation of the dicarboxylic acids. The following Arrhenius expressions were determined (in units of L mol−1 s−1): succinic acid, k(T, AH2) (2.1 ± 0.1) × 1010 exp[(−1530 ± 250 K)/T], k(T, AH−) (1.8 ± 0.1) × 1010 exp[(−1070 ± 370 K)/T], k(T, A2−) (2.9 ± 0.2) × 1011 exp[(−1830 ± 350 K)/T] and pimelic acid, k(T, AH2) (7.3 ± 0.2) × 1010 exp[(−1040 ± 140 K)/T], k(T, AH−) (1.8 ± 0.1) × 1011 exp[(−1200 ± 240 K)/T], k(T, A2−) (1.4 ± 0.1) × 1012 exp[(−1830 ± 110 K)/T]. A general OH radical reactivity trend for dicarboxylic acids was found as k(AH2) < k(AH−) < k(A2−). By using the pH and temperature dependent rate constants, source and sinking processes in the tropospheric aqueous phase can be described precisely.
Highlights
Dicarboxylic acids such as oxalic, succinic, and pimelic acid are important trace constituents in the troposphere where they exist predominately in the particle phase as demonstrated by the Henry’sLaw constant Hcp = (2–4.1) × 107 mol m−3 Pa−1 for succinic acid and Hcp = 8.1 × 107 mol m−3 Pa−1 for pimelic acid [1,2]
Dicarboxylic acids are ubiquitous and have been found in remote and polluted areas as well as in pristine areas such as the Arctic and Antarctic region [3,4,5,6,7,8]. These compounds reach the pristine areas through long-range transport or originate from the photochemical oxidation of volatile organic compounds (VOCs), biogenic unsaturated fatty acids emitted from the surface ocean [4], or biomass burning or vehicle emissions [3,4]
The second order OH radical rate constants of the reaction with organic acids were determined by applying the thiocyanate competition kinetics method [27,28,29] using a temperature-controlled laser flash photolysis long path absorption (LP-LPA) setup [28,29]
Summary
The aqueous-phase oxidation kinetics of carboxylic acids and dicarboxylic acid towards hydroxyl radicals have been investigated in several studies in the past [14,17,18,19,20,21,22,23,24] These investigations clearly show an influence of the speciation of the dicarboxylic acid on the OH radical rate constant, a systematic investigation of the monoanionic form of the carboxylic acid is still missing. This motivates further investigations of the temperature- and pH-dependent of hydroxyl radical reaction
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