Aqueous OH kinetics of sugar acids: new rate coefficients and atmospheric lifetimes

Sugar acids such as gluconic, glucuronic, and galacturonic acids are emitted directly by biomass burning, vegetation, and microbiota, and are formed through the oxidation of sugars and sugar alcohols. These compounds are commonly found in atmospheric particulate matter (PM), particularly in biomass burning and biogenic secondary organic aerosols. Hydroxyl radical (OH) is a major daytime atmospheric oxidant, formed by photolysis of ozone (gas phase) and by Fenton or Fenton-like reactions in water-containing particles, thereby dominating the oxidative capacity of the atmosphere. 
Due to their extremely low volatilities and high water solubility, the aqueous reaction with the OH radicals inside the different hydrometeors can contribute to the transformation and removal of saccharides. As such, aqueous OH radicals' reaction with polyols influences particle aging, secondary oxidation processes, and changes in aerosol chemical and optical properties relevant to regional climate. 
The values of bimolecular reaction rate coefficients (k_OH, M^-1s^-1) for the atmospherically abundant, water-soluble organic compounds (WSOCs) are needed to estimate their atmospheric lifetimes and develop kinetic predictive models, particularly structure-activity relationships (SARs). Kinetic SARs are widely used in atmospheric chemistry to predict k_OHaq for the atmospheric-abundant water-soluble organics; they are also frequently embedded in atmospheric models and automated mechanism generators. At the same time, the number of k_OH values for many (poly)functional, highly polar organic compounds found in the atmospheric multiphase system remains limited.
In this work, k_OH values for gluconic, glucuronic, and galacturonic acids were systematically measured at temperatures using a laser flash photolysis-laser long-path spectroscopy. Measurements were conducted over a temperature range 278 to 318K under acidic (pH=2) and neutral (pH=7) conditions. The k_OH values were determined at five reactant concentrations, ranging from 5.0×10^-5 to 2.0×10^-4 M, using potassium thiocyanate (KSCN, 2×10^-5M), as a kinetic reference compound. The resulting k_OH values ranged from 10⁸ to 10⁹ M^-1 s^-1, consistent with available literature data for similar polyols. All three sugar acids exhibit a clear temperature dependence of the measured k_OH values, following Arrhenius behavior. 
Arrhenius analysis yielded activation energies (E_A, kJ mol^-1), pre-exponential factors (A, M^-1 s^-1), activation enthalpies (∆H^‡, kJ mol^-1), activation entropies (∆S^‡, J K^-1 mol^-1), and Gibbs free energies of activation (∆G^‡, kJ mol^-1). These results provide mechanistic insights into the OH reaction with sugar acids. Lastly, the performances of different kinetic SARs for highly oxygenated WSOCs were evaluated using the newly acquired data.