**Thomas Schaefer, Liang Wen, Arne Estelmann , Joely Maak and Hartmut Herrmann \***

Atmospheric Chemistry Department (ACD), Leibniz-Institute for Tropospheric Research (TROPOS), Permoserstrasse 15, 04318 Leipzig, Germany

**\*** Correspondence: herrmann@tropos.de; Tel.: +49-341-2717-7024

Received: 4 March 2020; Accepted: 24 March 2020; Published: 26 March 2020

**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 H2O<sup>2</sup> 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−<sup>1</sup> s −1 ): succinic acid, k(T, AH2) (2.1 <sup>±</sup> 0.1) <sup>×</sup> <sup>10</sup><sup>10</sup> exp[(−<sup>1530</sup> <sup>±</sup> 250 K)/T], k(T, AH−) (1.8 <sup>±</sup> 0.1) <sup>×</sup> <sup>10</sup><sup>10</sup> exp[(−<sup>1070</sup> <sup>±</sup> 370 K)/T], k(T, A2−) (2.9 <sup>±</sup> 0.2) <sup>×</sup> <sup>10</sup><sup>11</sup> exp[(−<sup>1830</sup> <sup>±</sup> 350 K)/T] and pimelic acid, k(T, AH2) (7.3 <sup>±</sup> 0.2) <sup>×</sup> <sup>10</sup><sup>10</sup> exp[(−<sup>1040</sup> <sup>±</sup> 140 K)/T], k(T, AH−) (1.8 <sup>±</sup> 0.1) <sup>×</sup> <sup>10</sup><sup>11</sup> exp[(−<sup>1200</sup> <sup>±</sup> 240 K)/T], k(T, A2−) (1.4 <sup>±</sup> 0.1) <sup>×</sup> <sup>10</sup><sup>12</sup> exp[(−<sup>1830</sup> <sup>±</sup> 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.

**Keywords:** OH radicals; rate constants; dicarboxylic acids; succinic acid; pimelic acid; aqueous phase chemistry
