Investigation of the aqueous oxidation of terpenoic alcohols by OH as a potential source of secondary organic aerosols

      The oxygenated volatile organic compounds, including terpenoic alcohols (TAs) are emitted into the atmosphere mainly from vegetation; from plant resins, and to communicate with neighbouring plants and insects. Furthermore, biogenic alcohols, including menthol, borneol, fenchol, pinanediol and camphanediol are widely utilized in industries. Consequently, terpenoic alcohols are present in all environmental compartments: air, surface and cloud water, fog, and atmospheric particulate matter. Therefore, water provides an important reaction medium for the environmental processing of  terpenoic alcohols.

     Hydroxyl radical (OH) is the most important daytime atmospheric oxidant, and plays a fundamental role in the advanced oxidation processes in the gas phase as well as in the atmospheric waters. In the atmosphere, TAs partition into the clouds and fog droplets, where they can undergo aqueous oxidation by free radicals, mainly OH, which yields low-volatility products. These low-volatility, highly-oxygenated molecules contribute to secondary organic aerosols (SOA) and tropospheric ozone following their aqueous and multiphase oxidation. Hence, terpenoic alcohols are the potential precursors of aqueous SOAs (_aqSOAs). Atmospheric aerosols are important climate forcing agents and also have a negative impact on human health. According to recent estimates, SOAs, including _aqSOAs, contribute significantly to the global budget for fine particulate matter (PM).

     The goal of this work is to investigate kinetics and mechanism of the aqueous phase oxidation of menthol, borneol, fenchol, pinanediol and camphanediol by the OH radicals. The rate coefficients measurements for reaction of TAs with OH radicals were carried out in the custom-designed aqueous photoreactor using relative rate method approach. The mechanism of the oxidation reactions under investigation were studied with gas and liquid chromatography coupled with the mass spectrometry. Such an approach provided detailed insights into the molecular structures and distribution of products, including neutral molecules (alcohols, carbonyls) and functionalized carboxylic acids. Yields of formation of the major products were also measured using a commercially available and synthesized standards (terpenoic acids). The detailed reaction mechanism of the OH-initiated reaction for five TA was proposed using the experimental data acquired. Subsequently, a box-model for the aqueous OH oxidation of the five terpenoic alcohols was developed and compared to the experimental data; the reaction channels and branching ratios of the major detected by-products were proposed. Atmospheric implications of the results obtained are discussed in the connection with the possible in-cloud formation following the OH oxidation of menthol, borneol, fenchol, pinanediol and camphanediol. Additionally, the rate coefficients and reaction mechanism information for TA reaction with OH radicals in water could be utilized for wastewater treatment.