Efficient formation of aqueous secondary organic aerosols from the hydroxyl radical reaction with fenchol, borneol, and menthol

Abstract. Aqueous-phase oxidation of oxygenated monoterpenes can contribute to the formation of secondary organic aerosol (SOA) from biogenic volatile organic compounds. Still, the quantitative and mechanistic data for such precursors are limited. In this study, the aqueous-phase reactions of three atmospherically relevant, saturated terpenoic alcohols (TAs) – fenchol, borneol, and menthol – with hydroxyl radicals (OH) were investigated using a photochemical reactor combined with GC/MS and LC/ToF-MS analyses and kinetic modeling. The objectives were to elucidate reaction mechanisms, identify oxidation products, and quantify yields of aqueous SOA (_aqSOA) under atmospherically relevant conditions. Comprehensive product analysis revealed that oxidation proceeds via H-atom abstraction, yielding a wide range of multifunctional products. In addition to previously reported products, this work first identified functionalized terpenoic acids formed as second-generation products, providing new evidence for the formation of low-volatility compounds from aqueous OH reaction with TAs. The molar yields of quantified products approached unity (0.88–0.99), indicating near-complete closure of the carbon balance. Based on these data, explicit kinetic box models were developed that successfully reproduced the measured temporal evolution of reactants. Modelled _aqSOA yields ranged from 10% to 70%, depending on liquid water content and reaction coordinate, representing the first quantitative estimates for the three TAs under investigation. The results demonstrate that aqueous oxidation of semi-volatile terpenoids can efficiently generate low-volatility products contributing to SOA formation. These findings highlight the importance of multiphase processing of oxygenated terpenoids and provide new mechanistic and quantitative data for atmospheric models.