Water Dramatically Accelerates the Decomposition of α-Hydroxyalkyl-Hydroperoxides in Aerosol Particles

α-Hydroxyalkyl-hydroperoxides (α-HHs), from the addition of water to Criegee intermediates in the ozonolysis of olefins, are reactive components of organic aerosols. Assessing the fate of α-HHs in such media requires information on the rates and products of their reactions in aqueous organic matrixes. This information, however, is unavailable due to the lack of analytical techniques for the detection and identification of labile α-HHs. Here, we report the mass spectrometric detection (as Cl^- adducts) of the α-HH produced in the ozonolysis of a C₁₅ diolefin in water (W):acetonitrile (AN) mixtures of variable composition containing inert NaCl. α-HH decays into a gem-diol + H₂O₂ within τ_1/e ≈ 52 min in 50% (v:v) water, but persists longer than a day in ≤10% water mixtures. The strong nonlinear dependence of τ_1/e on solvent composition reveals that water content is a major factor controlling the fate of α-HHs in atmospheric particles. It also suggests that α-HH decomposes while embedded in WAN clusters rather than randomly dissolved in molecularly homogeneous W:AN mixtures.