Temperature-dependent aqueous OH kinetics of C2–C10 linear and terpenoid alcohols and diols: new rate coefficients, structure–activity relationship, and atmospheric lifetimes

Abstract

Abstract. Aliphatic alcohols (AAs), including terpenoic alcohols (TAs), are ubiquitous in the atmosphere due to their widespread emissions from natural and anthropogenic sources. Hydroxyl radical (OH) is the most important atmospheric oxidant in both aqueous and gas phases. Consequently, the aqueous oxidation of the TAs by the OH inside clouds and fogs is a potential source of aqueous secondary organic aerosols (aqSOAs). However, the kinetic data, necessary for estimating the timescales of such reactions, remain limited. Here, bimolecular rate coefficients (kOHaq) for the aqueous oxidation of 29 C2–C10 AAs by hydroxyl radicals (OH) were measured with the relative rate technique in the temperature range 278–328 K. The values of kOHaq for the 15 AAs studied in this work were measured for the first time after validating the experimental approach. The kOHaq values measured for the C2–C10 AAs at 298 K ranged from 1.80 × 109 to 6.5 × 109 M−1 s−1. The values of activation parameters, activation energy (7–17 kJ mol−1), and average Gibbs free energy of activation (18 ± 2 kJ mol−1) strongly indicated the predominance of the H-atom abstraction mechanism. The estimated rates of the complete diffusion-limited reactions revealed up to 44 % diffusion contribution for the C8–C10 AAs. The data acquired in this work and the values of kOHaq for AAs, carboxylic acids, and carboxylate ions available in the literature were used to develop a modified structure–activity relationship (SAR). The SAR optimized in this work estimated the temperature-dependent kOHaq for all compounds under investigation with much higher accuracy compared to the previous models. In the new model, an additional neighboring parameter was introduced (F≥ (CH2)6), using the kOHaq values for the homolog (C2–C10) linear alcohols and diols. A good overall accuracy of the new SAR at 298 K (slope = 1.022, R2=0.855) was obtained for the AAs and carboxylic acids under investigation. The kinetic database (kOHaq values in this work and compiled literature data) was also used to further enhance the ability of SAR to predict temperature-dependent values of kOHaq in the temperature range 278–328 K. The calculated atmospheric lifetimes indicate that terpenoic alcohols and diols can react with the OH in aerosol, cloud, and fog water with liquid water content (LWC) ≥0.1 g m−3 and LWC ≥ 10−4 g m−3, respectively. The preference of terpenoic diols to undergo aqueous oxidation by the OH under realistic atmospheric conditions is comparable with terpenoic acids, making them potentially effective precursors of aqSOAs. In clouds, a decrease in the temperature will strongly promote the aqueous reaction with the OH, primarily due to the increased partitioning of WSOCs into the aqueous phase.