Stability of Monoterpene-Derived α-Hydroxyalkyl-Hydroperoxides in Aqueous Organic Media: Relevance to the Fate of Hydroperoxides in Aerosol Particle Phases

Abstract

The α-hydroxyalkyl-hydroperoxides [R–(H)C(−OH)(−OOH), α-HH] produced in the ozonolysis of unsaturated organic compounds may contribute to secondary organic aerosol (SOA) aging. α-HHs’ inherent instability, however, hampers their detection and a positive assessment of their actual role. Here we report, for the first time, the rates and products of the decomposition of the α-HHs generated in the ozonolysis of atmospherically important monoterpenes α-pinene (α-P), d-limonene (d-L), γ-terpinene (γ-Tn), and α-terpineol (α-Tp) in water/acetonitrile (W/AN) mixtures. We detect α-HHs and multifunctional decomposition products as chloride adducts by online electrospray ionization mass spectrometry. Experiments involving D₂O and H₂¹⁸O, instead of H₂¹⁶O, and an OH-radical scavenger show that α-HHs decompose into gem-diols + H₂O₂ rather than free radicals. α-HHs decay mono- or biexponentially depending on molecular structure and solvent composition. e-Fold times, τ_(1/e), in water-rich solvent mixtures range from τ_(1/e) = 15–45 min for monoterpene-derived α-HHs to τ_(1/e) > 10³ min for the α-Tp-derived α-HH. All τ_(1/e)’s dramatically increase in <20% (v/v) water. Decay rates of the α-Tp-derived α-HH in pure water increase at lower pH (2.3 ≤ pH ≤ 3.3). The hydroperoxides detected in day-old SOA samples may reflect their increased stability in water-poor media and/or the slow decomposition of α-HHs from functionalized terpenes.