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

Abstract. Aliphatic alcohols (AAs), including terpenoic alcohols (TAs), are ubiquitous in the atmosphere due to their widespread emissions from natural and man-made 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 to estimate the time scales of such reactions is limited. Here, bimolecular rate coefficients (k_OHaq) for the aqueous oxidation of twenty-nine, C₂-C₁₀ AAs by hydroxyl radicals (OH) were measured with the relative rate method in the temperature range 278–328 K. k_OHaq values for the fifteen AAs were measured for the first time after validating the experimental approach. The k_OHaq values measured for the C₂-C₁₀ AAs at 298 K were between 1.80×10⁹ and 6.5×10⁹ M^-1s^-1. The values of activation parameters (activation energy (7–17 kJ/mol) and average Gibbs free energy of activation=20 kJ/mol) 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 to the measured k_OHaq values for the C₈-C₁₀ AAs.

The kinetic data acquired here, and the literature data, for AAs, carboxylic acids, and carboxylate ions were used to develop a modified structure-activity relationship (SAR). The new SAR developed in this work predicted the temperature-dependent k_OHaq for all compounds under investigation with higher accuracy as compared with the previous models. In the modified model, an additional neighboring parameter was introduced (F≥(CH₂)₆, using the measured k_OHaq values for the homolog (C₂-C₁₀) linear alcohols and diols. Good accuracy of the new SAR at 298 K (slope=1.05, R²=0.75) was achieved for the AAs and carboxylic acids under investigation. The kinetic database k_OHaq values in this work and compiled literature data) was also used to further enhance the ability of SAR to predict temperature-dependent values of k_OHaq in the temperature range 278–328 K.

The calculated atmospheric lifetimes indicate that terpenoic alcohols and diols will react with the OH in aerosol, cloud, and fog water with (LWC≥0.1 g/m³) and (LWC≥10^-4 g/m³), respectively. The preference of terpenoic diols to undergo aqueous oxidation by the OH under realistic atmospheric conditions is comparable with terpenoic acids, which makes them potentially effective precursors of _aqSOAs. In clouds, a decrease in the temperature will strongly favor the aqueous reaction with the OH, primarily due to the increased partitioning into the aqueous phase following Henry’s law equilibrium.