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 O3, NO3/N2O5, and OH. 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 gammaO3 approximately 10(-5) calculated from the observed loss rate of oleate in the particle phase. The best fit Langmuir-Hinshelwood parameters are kImax=0.05+/-0.01 s-1 and KO3=4(+/-3)x10(-14) cm3molec-1. These parameters 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. We observed efficient processing of oleate by OH (0.1<or=gammaOH<or=1), and we suggest an upper bound of gammaNuOmicron3<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 organics at the aerosol surface is approximately 10 min 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 KO3.