(593f) The Oxidation of Oleate in Submicron Aqueous Salt Aerosols: Evidence of a Surface Process

We have studied the oxidation of submicron aqueous aerosols consisting of internal mixtures of sodium oleate (oleic acid proxy), sodium dodecyl sulfate and inorganic salts by ozone (O₃), the hydroxyl radical (OH), and the nitrate radical (NO₃). Experiments were performed using an aerosol flow tube and a continuous flow photochemical reaction chamber coupled to a chemical ionization mass spectrometer (CIMS). The CIMS was fitted with a heated inlet for volatilization and detection of organics in the particle phase simultaneously with the gas phase. A differential mobility analyzer/condensation particle counter was used for determining aerosol size distributions. The oxidation of oleate by O₃ follows Langmuir-Hinshelwood kinetics, with γ ~ 10^-5 calculated from the observed loss rate of oleate in the particle phase. The kinetics showed no dependence on the ionic composition of the aerosols or on the presence of alkyl surfactants. Several ozone oxidation products were observed to be particle bound at ambient temperature, including nonanoic acid. Oxidation of the film by OH was observed to be efficient (0.1≤γ_OH≤1), and we suggest an upper bound for the reactive uptake coefficient of NO₃ on oleic acid of γ_NO3 ~ 10^-3. We conclude that for the aerosol compositions studied, oxidation occurs near the gas-aerosol interface and that the 1 e-fold lifetime of unsaturated organic surfactants at the aerosol surface is ~10 minutes due to O₃ oxidation under atmospheric conditions. In the context of a Langmuir-Hinshelwood mechanism, different underlying aerosol compositions may extend the lifetime of oleic acid at the surface by reducing K_O3.