Temperature Dependence of Aqueous-Phase Decomposition of α-Hydroxyalkyl-Hydroperoxides

Ozonolysis of unsaturated organic species with water produces a-hydroxyalkylhydroperoxides (alpha-HHs), which are reactive intermediates that lead to the formation of H2O2 and multifunctionalized species in atmospheric condensed phases. Here, we report temperaturedependent rate coefficients (k) for the aqueous-phase decomposition of alpha-terpineol alpha-HHs at 283-318 K and terpinen-4-ol alpha a-HHs at 313-328 K. The temporal profiles of alpha-HH signals, detected as chloride adducts by negative-ion electrospray mass spectrometry, showed single-exponential decay, and the derived first-order k for alpha-HH decomposition increased as temperature increased, e.g., k(288 K) = (4.7 +/- 0.2) X 10(-5), k(298 K) = (1.5 +/- 0.4) X 10(-4), k(308 K) = (3.4 +/- 0.9) X 10(-4), k(318 K) (1.0 +/- 0.2) X 10(-3) s(-1) for alpha-terpineol alpha-HHs at pH 6.1. Arrhenius plot analysis yielded activation energies of 17.9 +/- 0.7 (pH 6.1) and 17.1 +/- 0.2 kcal mol(-1) (pH 6.2) for the decomposition of alpha-terpineol and terpinen-4-ol alpha-HHs, respectively. Activation energies of 18.6 +/- 0.2 and 19.2 +/- 0.5 kcal mol(-1) were also obtained for the decomposition of alpha-terpineol alpha-HHs in acidified water at pH 5.3 and 4.5, respectively. Theoretical kinetic and thermodynamic calculations confirmed that both water-catalyzed and proton-catalyzed mechanisms play important roles in the decomposition of these alpha-HHs. The relatively strong temperature dependence of k suggests that the lifetime of these alpha-HHs in aqueous phases (e.g., aqueous aerosols, fog, cloud droplets, wet films) is controlled not only by the water content and pH but also by the temperature of these media.